TOPIC 5: MATTER
Concept of matter
States of matter
The Three States of Matter
Describe the three states of matter
Any chemical substance we study exists in any of the three forms (or physical states). The three different states of matter are
- solid state
- liquid state and
- gaseous states
So,
each of the many millions of substances around us can be classified as a
solid, a liquid or gas. Look around you and name substances that are
solids, liquids and gases. The state in which any matter exists depends
on temperature and sometimes pressure conditions. One substance may
exist as a solid in one condition and as a liquid or gas under a
different condition. Water is an example of such substances. This change
is called a change in the state of matter.
each of the many millions of substances around us can be classified as a
solid, a liquid or gas. Look around you and name substances that are
solids, liquids and gases. The state in which any matter exists depends
on temperature and sometimes pressure conditions. One substance may
exist as a solid in one condition and as a liquid or gas under a
different condition. Water is an example of such substances. This change
is called a change in the state of matter.
The
three physical states of matter differ in the way they respond to
temperature and pressure. All three states can increase in volume
(expansion) when the temperature is increased. They decrease in volume
(contraction) when the temperature is decreased. Gases are easily
compressed. Liquids are only slightly compressible. Solids are
incompressible. They are not affected by change in pressure.
three physical states of matter differ in the way they respond to
temperature and pressure. All three states can increase in volume
(expansion) when the temperature is increased. They decrease in volume
(contraction) when the temperature is decreased. Gases are easily
compressed. Liquids are only slightly compressible. Solids are
incompressible. They are not affected by change in pressure.
Investigation of the compressibility of solids, liquids and gases
Procedure
- Take three new syringes and fill them with sand, water and air respectively (figure 5.1).
- Try to push in the end of each syringe.
- Observe what happens.
Figure
Compressibility of solids, liquids and gases
Observation
Which of the substances under investigation can compress into a smaller volume?
Findings
You should have found that a solid (sand) and a liquid (water) cannot be compressed but a gas (air) is easily compressed.
The three states of matter differ in their physical properties. These differences in properties are summarized in table bellow
Differences in properties of the three states of matter
| Property | Physical state | ||
| Solid | Liquid | Gas | |
| Shape | has a definite shape | no definite shape, takes shape of the container | no definite shape, occupy whole container |
| Volume | has a fixed volume | has a fixed volume | variable (depending on temperature and pressure) |
| Fluidity | does not flow | generally flows easily | flows easily |
| Expansion on heating | low | medium | high |
| Compressibility | incompressible | almost incompressible | highly compressible |
| Motion of particles | slow | high | very high |
| Density | high | moderate to high | low |
| Tangibility | tangible | tangible | intangible |
| Visibility | visible | visible | invisible |
One State of Matter to Another
Change one state of matter to another
We
have seen that matter exists in three different states – solids,
liquids and gases. We can use the kinetic theory of matter to explain
how a substance changes from one state to another. Basically, changes
from one state to another are caused by alterations in temperature and
pressure. Normally molecules, ions or atoms of a substance move faster
when the temperature is increased.
have seen that matter exists in three different states – solids,
liquids and gases. We can use the kinetic theory of matter to explain
how a substance changes from one state to another. Basically, changes
from one state to another are caused by alterations in temperature and
pressure. Normally molecules, ions or atoms of a substance move faster
when the temperature is increased.
Melting and freezing
Melting
is a change from solid to liquid state. When solids are heated, their
constituent particles (atoms, molecules or ions) get energy and vibrate
more violently. Vibrations of these particles overcome (exceed) their
binding forces. The particles become mobile. The crystalline structure
of solid is destroyed. A liquid state is reached and the particles are
free to move. The temperature at which this happens is called melting point of the solid.
is a change from solid to liquid state. When solids are heated, their
constituent particles (atoms, molecules or ions) get energy and vibrate
more violently. Vibrations of these particles overcome (exceed) their
binding forces. The particles become mobile. The crystalline structure
of solid is destroyed. A liquid state is reached and the particles are
free to move. The temperature at which this happens is called melting point of the solid.
The
melting point of a solid tells us something about the strength of
forces holding its constituent particles together. Substances with high
melting points have strong forces between their particles. Those with
low melting points have weak forces between their particles.
melting point of a solid tells us something about the strength of
forces holding its constituent particles together. Substances with high
melting points have strong forces between their particles. Those with
low melting points have weak forces between their particles.
Change in state from solid to liquid
Freezing
is a change from liquid to solid state. Freezing is the opposite of
melting. The process is reversed at the same temperature if a liquid is
cooled. The temperature at which a substance turns to a solid is called freezing point.
The melting point and freezing point of any given substance are both
the same. For example, the melting and freezing of pure water takes
place at 0°C. Melting is not affected by any changes in atmospheric
pressure.
is a change from liquid to solid state. Freezing is the opposite of
melting. The process is reversed at the same temperature if a liquid is
cooled. The temperature at which a substance turns to a solid is called freezing point.
The melting point and freezing point of any given substance are both
the same. For example, the melting and freezing of pure water takes
place at 0°C. Melting is not affected by any changes in atmospheric
pressure.
Evaporation and boiling
Boiling
is a change from liquid to vapour state at a particular temperature.
Evaporation is the change from liquid to vapour state at any given
temperature. If a liquid is exposed to open air, it evaporates.
Splashes of water evaporate at room temperature. After rain, small pools
of water dry up. When a liquid changes into a gas at any temperature,
the process is called evaporation. Evaporation takes places
from the surface of the liquid. The larger the surface area, the faster
the liquid evaporates. The warmer the liquid is, the faster it
evaporates. Thus, surface area and temperature affects the rate of
evaporation of a liquid.
is a change from liquid to vapour state at a particular temperature.
Evaporation is the change from liquid to vapour state at any given
temperature. If a liquid is exposed to open air, it evaporates.
Splashes of water evaporate at room temperature. After rain, small pools
of water dry up. When a liquid changes into a gas at any temperature,
the process is called evaporation. Evaporation takes places
from the surface of the liquid. The larger the surface area, the faster
the liquid evaporates. The warmer the liquid is, the faster it
evaporates. Thus, surface area and temperature affects the rate of
evaporation of a liquid.
When
a liquid is heated, its molecules get more energy and move faster. They
knock into each other violently and bounce further apart. As the
heating goes on, its molecules vibrate even faster. Bubbles of gas (due
to air dissolved in water) appear inside the liquid. The whole process
is called boiling. The temperature at which a liquid boils is called boiling point.
a liquid is heated, its molecules get more energy and move faster. They
knock into each other violently and bounce further apart. As the
heating goes on, its molecules vibrate even faster. Bubbles of gas (due
to air dissolved in water) appear inside the liquid. The whole process
is called boiling. The temperature at which a liquid boils is called boiling point.
The
molecules at the surface of the liquid gain enough energy to overcome
the forces holding them together. They break away from the liquid and
from a gas (vapour). As more of the liquid molecules escape to form a
gas, a liquid is said to evaporate. This occurs at the boiling point of a
liquid.
molecules at the surface of the liquid gain enough energy to overcome
the forces holding them together. They break away from the liquid and
from a gas (vapour). As more of the liquid molecules escape to form a
gas, a liquid is said to evaporate. This occurs at the boiling point of a
liquid.
Change in state from liquid to gas
The
temperature at which a liquid boils explains how strong the forces
holding its particles (molecules) together are. Liquids with high
boiling points have strong forces of attraction between their molecules
than those liquids with low boiling points.
temperature at which a liquid boils explains how strong the forces
holding its particles (molecules) together are. Liquids with high
boiling points have strong forces of attraction between their molecules
than those liquids with low boiling points.
The
boiling point of a liquid can change if the surrounding pressure
changes. If the surrounding pressure falls, the boiling point also
falls. The boiling point of water at standard pressure (760 mmHg) is
100°C. On a high mountain, where pressure is low, it is lower than
100°C. If the surrounding pressure is increased, the boiling point
rises. The same behaviour is experienced by a gas when the pressure is
either increased or decreased.
boiling point of a liquid can change if the surrounding pressure
changes. If the surrounding pressure falls, the boiling point also
falls. The boiling point of water at standard pressure (760 mmHg) is
100°C. On a high mountain, where pressure is low, it is lower than
100°C. If the surrounding pressure is increased, the boiling point
rises. The same behaviour is experienced by a gas when the pressure is
either increased or decreased.
The melting and boiling points of some common chemical substances at standard temperature and pressure (s.t.p)
| Substance | Physical state at room temperature (20°C) | Melting point (°C) | Boiling point(°C) |
| Oxygen | gas | -219 | -183 |
| Nitrogen | gas | -210 | -196 |
| Ethanol (alcohol) | liquid | -117 | 78 |
| Water | liquid | 0 | 100 |
| Sulphur | solid | 115 | 444 |
| Common salt (sodium chloride) | solid | 801 | 1465 |
| Copper | solid | 1083 | 2600 |
| Carbon dioxide | gas | sublimation point (°C): -78 |
|
From the above explanation, obvious differences between evaporation and boiling can be detected. See table bellow
| Evaporation | Boiling |
| 1.Occurs at all temperatures | Occurs at one particular temperature (boiling point) |
| 2.Occurs on the surface of the liquid | Occurs both inside and on the surface of the liquid |
| 3.Takes place slowly | Takes place faster |
| 4.Bubbles are not necessarily formed | Bubbles are formed |
Therefore, the two terms can be defined as follows: Evaporation is a change in state of a substance from liquid to gas (vapour) state at any temperature.
Boiling is a change in state of a substance from liquid to gas at a particular temperature and pressure.
Condensation and solidification
The
reverse of evaporation is condensation. This is brought about by
cooling. When a gas is cooled down, its particles lose energy. They move
more and more slowly. When they knock into each other, they do not have
enough energy to bounce away again. They stay close together and a
liquid forms. This process is called condensation. When the
liquid is cooled further, the movement of the particles slows down even
more. Eventually, they stop moving and a solid forms. This is called solidification.
reverse of evaporation is condensation. This is brought about by
cooling. When a gas is cooled down, its particles lose energy. They move
more and more slowly. When they knock into each other, they do not have
enough energy to bounce away again. They stay close together and a
liquid forms. This process is called condensation. When the
liquid is cooled further, the movement of the particles slows down even
more. Eventually, they stop moving and a solid forms. This is called solidification.
Condensation can be defined as a change in state of a substance from gas (vapour) to liquid. Solidification is a change from liquid to solid state of a substance. Solidification is the same as freezing.
Sublimation
A
few solids do not melt when they are heated. Instead, they change
directly from the solid to gaseous state without passing through the
liquid state. This change in state is called sublimation. When a
solid changes directly into gas, it is said to sublime. Iodine, solid
carbon dioxide (“dry ice”) and ammonium chloride are examples of solids
that sublime. Like melting, sublimation also occurs at one particular
temperature for each pure solid.
few solids do not melt when they are heated. Instead, they change
directly from the solid to gaseous state without passing through the
liquid state. This change in state is called sublimation. When a
solid changes directly into gas, it is said to sublime. Iodine, solid
carbon dioxide (“dry ice”) and ammonium chloride are examples of solids
that sublime. Like melting, sublimation also occurs at one particular
temperature for each pure solid.
The Importance of Changing One State of Matter to Another
Explain the importance of changing one state of matter to another
The following points summarize the importance of change in state:
1. Separation of mixtures
Different
mixtures can be separated through such processes as distillation,
sublimation, evaporation and condensation. Let us have a look at an
example of distillation. This process involves boiling, evaporation and
condensation. Distillation as a process can be applied in separation of a
mixture (solution) of two or more substances. A mixture of two or more
substances with different boiling points e.g. water and alcohol can be
separated by this means. In such a case, a container with the mixed-up
liquids is heated. The liquid with a low boiling point evaporates and
condenses first, leaving the one with a high boiling point in the
container. The distillate (liquid with low boiling point) is collected,
cooled down and transferred into another container.
mixtures can be separated through such processes as distillation,
sublimation, evaporation and condensation. Let us have a look at an
example of distillation. This process involves boiling, evaporation and
condensation. Distillation as a process can be applied in separation of a
mixture (solution) of two or more substances. A mixture of two or more
substances with different boiling points e.g. water and alcohol can be
separated by this means. In such a case, a container with the mixed-up
liquids is heated. The liquid with a low boiling point evaporates and
condenses first, leaving the one with a high boiling point in the
container. The distillate (liquid with low boiling point) is collected,
cooled down and transferred into another container.
2. Industrial manufacture of products
Industrially,
the process of distillation is applied in the production of pure
substances such as beer and other alcoholic drinks such as wine, vodka, konyagi, etc. The manufacturing process involves boiling, evaporating and condensation.
the process of distillation is applied in the production of pure
substances such as beer and other alcoholic drinks such as wine, vodka, konyagi, etc. The manufacturing process involves boiling, evaporating and condensation.
3. Refining of petroleum (crude oil)
Crude
oil contains organic liquid components, each with a different boiling
point. In the refinery, the components with lower boiling points
evaporate first and get separated out, leaving those with higher boiling
points behind. In this way, we get various types of oil components
(fractions) such as petrol, diesel, kerosene, lubricating oil, etc.
oil contains organic liquid components, each with a different boiling
point. In the refinery, the components with lower boiling points
evaporate first and get separated out, leaving those with higher boiling
points behind. In this way, we get various types of oil components
(fractions) such as petrol, diesel, kerosene, lubricating oil, etc.
4. Drying of crops and clothes
When
you suspend your clothing on a cloth line to dry, the moisture in it is
lost through evaporation. Likewise, farmers in the village often spread
crops on the ground to dry. They do this in order to reduce moisture
content and hence prevent decaying. The moisture contained in crops
leave by evaporation. Therefore, you can notice how evaporation, as a
change in state, is important in everyday lives.
you suspend your clothing on a cloth line to dry, the moisture in it is
lost through evaporation. Likewise, farmers in the village often spread
crops on the ground to dry. They do this in order to reduce moisture
content and hence prevent decaying. The moisture contained in crops
leave by evaporation. Therefore, you can notice how evaporation, as a
change in state, is important in everyday lives.
5. Cooling of our bodies in hot weather
You
all like to drink cold water or beverages especially during hot
weather. You can use a refrigerator to cool down drinking water or
beverages directly. Alternatively, you can freeze water into ice and
then use the resulting ice for cooling the beverage. Ice blocks are also
saleable. Moreover, one can earn some money if she freezes water into
ice blocks and then sells them to beverage vendors. Perishable products
such as fish, meat, milk, etc are often packed in ice blocks to prevent
them from going bad. Ice, as we studied early, is formed when water
freezes (a change in state from liquid to solid).
all like to drink cold water or beverages especially during hot
weather. You can use a refrigerator to cool down drinking water or
beverages directly. Alternatively, you can freeze water into ice and
then use the resulting ice for cooling the beverage. Ice blocks are also
saleable. Moreover, one can earn some money if she freezes water into
ice blocks and then sells them to beverage vendors. Perishable products
such as fish, meat, milk, etc are often packed in ice blocks to prevent
them from going bad. Ice, as we studied early, is formed when water
freezes (a change in state from liquid to solid).
6. Ice formation in refrigerators
You
all like to drink cold water or beverages especially during hot
weather. You can use a refrigerator to cool down drinking water or
beverages directly. Alternatively, you can freeze water into ice and
then use the resulting ice for cooling the beverage. Ice blocks are also
saleable. Moreover, one can earn some money if she freezes water into
ice blocks and then sells them to beverage vendors. Perishable products
such as fish, meat, milk, etc are often packed in ice blocks to prevent
them from going bad. Ice, as we studied early, is formed when water
freezes (a change in state from liquid to solid).
all like to drink cold water or beverages especially during hot
weather. You can use a refrigerator to cool down drinking water or
beverages directly. Alternatively, you can freeze water into ice and
then use the resulting ice for cooling the beverage. Ice blocks are also
saleable. Moreover, one can earn some money if she freezes water into
ice blocks and then sells them to beverage vendors. Perishable products
such as fish, meat, milk, etc are often packed in ice blocks to prevent
them from going bad. Ice, as we studied early, is formed when water
freezes (a change in state from liquid to solid).
7. Melting metals to make alloys
In
metallurgical industries, need may arise to mix two or more metals
(alloys) together. This is only possible, where two or more metals are
first melted at high temperatures into liquids. Then the resulting
liquid metals are mixed in appropriate proportions. This is followed by
cooling down the mixture to a solid alloy. Normally alloys have better
qualities than individual metals.
metallurgical industries, need may arise to mix two or more metals
(alloys) together. This is only possible, where two or more metals are
first melted at high temperatures into liquids. Then the resulting
liquid metals are mixed in appropriate proportions. This is followed by
cooling down the mixture to a solid alloy. Normally alloys have better
qualities than individual metals.
8. Testing the purity of substances
The
presence of impurity may lower or raise the boiling point of the
substance. A pure substance melts and boils at definite temperatures
(see table 5.4). The values for the melting point and boiling point are
precise and predictable. This means that we can use them to test the
purity of a sample. They can also be used to check the identity of
unknown substance.
presence of impurity may lower or raise the boiling point of the
substance. A pure substance melts and boils at definite temperatures
(see table 5.4). The values for the melting point and boiling point are
precise and predictable. This means that we can use them to test the
purity of a sample. They can also be used to check the identity of
unknown substance.
A typical example
Sea
water is impure. It freezes at a temperature well below the freezing
point of pure water (0°C) and boils at a temperature above the boiling
point of pure water (100°C). Other substances behave in a similar
manner. So, boiling as a change in state can be used to test for the
purity of a substance.
water is impure. It freezes at a temperature well below the freezing
point of pure water (0°C) and boils at a temperature above the boiling
point of pure water (100°C). Other substances behave in a similar
manner. So, boiling as a change in state can be used to test for the
purity of a substance.
In
addition, the impurity also reduces the exactness of the melting or
boiling point. An impure substance melts or boils over a range of
temperature, not at a particular point.
addition, the impurity also reduces the exactness of the melting or
boiling point. An impure substance melts or boils over a range of
temperature, not at a particular point.
Melting and boiling points of some pure substances
| Substance | Melting point (°C) | Boiling point (°C) |
| Water | 0 | 100 |
| Ethanol | -117 | 78 |
| Oxygen | -219 | -183 |
| Sodium | 98 | 890 |
| Sulphur | 119 | 445 |
| Iron | 1540 | 2900 |
| Diamond | 3550 | 4832 |
| Cobalt | 1492 | 2900 |
| Nitrogen | -210 | -196 |
| Propane | -188 | – 42 |
| Ethanoic acid | 16 | 118 |
9. Formation of rain
Perhaps
the most important of all, as far as change in state is concerned, is
the formation of rain. Rained is mainly formed through the process of
evaporation and condensation. Water vapour, evaporating mostly from
water bodies (oceans, seas, lakes, rivers, ponds, etc), land and plants
rises up to the sky. As it rises, it cools down and condenses into tiny
droplets
the most important of all, as far as change in state is concerned, is
the formation of rain. Rained is mainly formed through the process of
evaporation and condensation. Water vapour, evaporating mostly from
water bodies (oceans, seas, lakes, rivers, ponds, etc), land and plants
rises up to the sky. As it rises, it cools down and condenses into tiny
droplets
On
further cooling as they rise up, these droplets form bigger water
drops. Owing to gravitational force, these drops fall down as rainfall.
Every one of you knows how important rain is to our life. Therefore, you
have noticed how evaporation and condensation, as changes in state,
contribute to rain formation.
further cooling as they rise up, these droplets form bigger water
drops. Owing to gravitational force, these drops fall down as rainfall.
Every one of you knows how important rain is to our life. Therefore, you
have noticed how evaporation and condensation, as changes in state,
contribute to rain formation.
Rain formation
Kinetic nature of matter
We
already know that matter is composed of atoms, ions or molecules. We
have not yet considered the reason why the same substance, say water,
can exist in more than one form, for example as solid ice, liquid water,
and gaseous steam. But does matter behave like that?
already know that matter is composed of atoms, ions or molecules. We
have not yet considered the reason why the same substance, say water,
can exist in more than one form, for example as solid ice, liquid water,
and gaseous steam. But does matter behave like that?
The kinetic theory of matter
has been used to explain the way in which the arrangement of the
particles of a substance can determine the properties of that substance,
and particularly the state in which it is likely to be found under a
given set of conditions. The idea is that all matter is made up of tiny
moving particles. The main points of the theory are as follows:
has been used to explain the way in which the arrangement of the
particles of a substance can determine the properties of that substance,
and particularly the state in which it is likely to be found under a
given set of conditions. The idea is that all matter is made up of tiny
moving particles. The main points of the theory are as follows:
- All matter is made up of tiny particles (atoms and molecules) that are invisible to the naked eye and to most microscopes.
- The particles are moving all the time. The higher the temperature is, the higher the average energy of the particles.
- Heavier particles move more slowly than lighter particles at the same temperature.
- Each substance has unique particles that are different from the particles of other substances.
- The particles of matter are held together by strong electrostatic forces.
- There are empty spaces between the particles of matter that are very large compared to the particles themselves.
The solid state
In
the solid state, the particles are so closely packed (see figure
bellow. The particles are held together by strong forces of attraction
that act like a chemical glue. Free movement of particles cannot take
place. They cannot move around freely in this arrangement. Instead, they
vibrate about a fixed position. They are arranged in a fixed pattern
which form a cluster of vibrating masses. This makes a solid to have a
fixed shape, which cannot be changed except by applying strong external
forces.
the solid state, the particles are so closely packed (see figure
bellow. The particles are held together by strong forces of attraction
that act like a chemical glue. Free movement of particles cannot take
place. They cannot move around freely in this arrangement. Instead, they
vibrate about a fixed position. They are arranged in a fixed pattern
which form a cluster of vibrating masses. This makes a solid to have a
fixed shape, which cannot be changed except by applying strong external
forces.
The liquid state
The
particles of a liquid are also closely packed but the forces of
attraction between them are weaker than of a solid. These forces of
attraction tend to bind them together. The particles have more kinetic
energy and they can move around each other. The binding forces are
strong when particles come close to one another. It is thought that the
particles of a liquid are fairly randomly arranged but consist of
“clusters” closely packed together. This property makes a liquid to have
a definite volume. However, since the particles are fairly free to move
a liquid does not have any characteristic shape (see figure 5.5(b).
Thus, a liquid will always take the shape of its container.
particles of a liquid are also closely packed but the forces of
attraction between them are weaker than of a solid. These forces of
attraction tend to bind them together. The particles have more kinetic
energy and they can move around each other. The binding forces are
strong when particles come close to one another. It is thought that the
particles of a liquid are fairly randomly arranged but consist of
“clusters” closely packed together. This property makes a liquid to have
a definite volume. However, since the particles are fairly free to move
a liquid does not have any characteristic shape (see figure 5.5(b).
Thus, a liquid will always take the shape of its container.
The gaseous state
The
gaseous state is one in which the particles are moving independently of
each other in all directions and at great speeds. The particles of a
gas are relatively far apart. They exert no force of attraction on each
other. They have more energy than the particles of solids and liquids.
They move rapidly and randomly, colliding with each other and with the
walls of the container. A typical speed for a molecule of hydrogen in
air at ordinary temperature and pressure has been found to be
approximately 500 ms-1
gaseous state is one in which the particles are moving independently of
each other in all directions and at great speeds. The particles of a
gas are relatively far apart. They exert no force of attraction on each
other. They have more energy than the particles of solids and liquids.
They move rapidly and randomly, colliding with each other and with the
walls of the container. A typical speed for a molecule of hydrogen in
air at ordinary temperature and pressure has been found to be
approximately 500 ms-1
It
has been estimated that a nitrogen molecule makes collisions each
second. Thus, a gas will rapidly spread out to fill any container in
which it is placed. A gas cannot have any shape of its own.
has been estimated that a nitrogen molecule makes collisions each
second. Thus, a gas will rapidly spread out to fill any container in
which it is placed. A gas cannot have any shape of its own.
Three states of matter
Physical and chemical changes
Depending on the nature of change, all changes that matter undergoes can be classified as either physical or chemical.
The Characteristics of a Physical Change
Describe the characteristics of a physical change
Physical change
Substances
may undergo changes in their physical properties e.g. changes in
colour, shape (or form), state, density, structure and texture, etc. If
you take a stone and break it down into small particles, you will have
only changed its form, but it will remain as a stone. Likewise, melting
ice to water or freezing water to ice does not change it, but it is
still water. The same case happens when you dissolve salt in water to
get a solution of salt in water. You can still get back the original
salt by evaporation, except that the crystals of the salt obtained will
not look exactly the same as those of the original salt.
may undergo changes in their physical properties e.g. changes in
colour, shape (or form), state, density, structure and texture, etc. If
you take a stone and break it down into small particles, you will have
only changed its form, but it will remain as a stone. Likewise, melting
ice to water or freezing water to ice does not change it, but it is
still water. The same case happens when you dissolve salt in water to
get a solution of salt in water. You can still get back the original
salt by evaporation, except that the crystals of the salt obtained will
not look exactly the same as those of the original salt.
These
changes of state are examples of physical changes. Physical changes
such as melting and boiling do not result in new substances being formed
changes of state are examples of physical changes. Physical changes
such as melting and boiling do not result in new substances being formed
For example, ice and water still contain the same particles whether in solid (ice) liquid (water) or gaseous (vapour) state.
Changes in states
Characteristics
In
the explanation above, we find that in a physical change it is only the
physical form, and not the actual nature, of a substance that changes.
The changes are brought about by a mere addition or removal of heat, as
in the case with water or ice. Such a change is called a physical
change. It can be distinguished by the following characteristics:
the explanation above, we find that in a physical change it is only the
physical form, and not the actual nature, of a substance that changes.
The changes are brought about by a mere addition or removal of heat, as
in the case with water or ice. Such a change is called a physical
change. It can be distinguished by the following characteristics:
- There
is no formation of a new substance. Consider an example given above.
The ice, liquid water and steam are the solid, liquid and gaseous forms
of the same substance (water). - There is no change in weight of
the substance undergoing the change. If you start with 50g of ice, you
will still get the same mass of water and steam (vapour) upon melting
and boiling respectively. - The changes are readily reversible.
You can easily change water back to ice and vapour to water by a mere
subtraction of heat (cooling). - It is not accompanied by a great heat change. Just a little heat is required to change ice to water, and water to steam.
Physical Changes of Matter Experimentally
Demonstrate physical changes of matter experimentally
Experiment
- Add
some common salt (sodium chloride) to distilled water in a beaker. Stir
the mixture until the salt disappears and forms a solution with water.
Transfer the water into a porcelain dish. Heat the content until all the
water has evaporated off. The salt reappears in its original white
solid form. - Grind some roll sulphur in a mortar to powder. Put
the resultant powder in a test-tube and heat gently, shaking all the
time. The sulphur melts to an amber-coloured liquid. On cooling, this
liquid returns to its original condition as a yellow solid. - Put a
block of ice in a beaker. Heat gently until the whole block melts to
form water. Pour the water formed in a cup and place it in a deep
freezer overnight. The water will freeze back to ice.
You
will have seen that all the above changes involve only changes in
physical forms of the substances. The chemical nature of substances
remained unchanged. Therefore, we can define a physical change as a
change that does not involve formation of a new substance but involves a
change in state or physical form of the substance and that such a form
can be reversed.
will have seen that all the above changes involve only changes in
physical forms of the substances. The chemical nature of substances
remained unchanged. Therefore, we can define a physical change as a
change that does not involve formation of a new substance but involves a
change in state or physical form of the substance and that such a form
can be reversed.
The Characteristics of a Chemical Change
Describe the characteristics of a chemical change
Chemical change
Some
changes that materials undergo are permanent. Such changes usually
involve changes in chemical properties of a substance. For example, when
you burn a piece of wood in fire, you get ash. The properties of wood
and ash are very different. There is no way you can change ash back to
wood. It is practically impossible. A permanent change in chemical
properties of a substance is called a chemical change. In a chemical
change, a substance losses all its physical and chemical properties.
changes that materials undergo are permanent. Such changes usually
involve changes in chemical properties of a substance. For example, when
you burn a piece of wood in fire, you get ash. The properties of wood
and ash are very different. There is no way you can change ash back to
wood. It is practically impossible. A permanent change in chemical
properties of a substance is called a chemical change. In a chemical
change, a substance losses all its physical and chemical properties.
Characteristics
Includes
- A
chemical change results in the formation of a new substance. The new
substance has different chemical and physical properties as compared to
the original substance. - It is generally not reversible. For example, you cannot turn the ash back to wood.
- There
is a change in weight or mass of the substance undergoing the change.
When you burn wood weighing 5 kg, you cannot expect to get the same
weight of ash. - The change is accompanied by a considerable heat change. For wood to burn to ash a lot of heat must be supplied.
Chemical Changes of Matter Experimentally
Demonstrate chemical changes of matter experimentally
Experiment
- Strongly
heat some roll sulphur on a deflagrating spoon until it melts and
begins to burn with a blue flame. If you continue heating, it gradually
decreases in amount and finally the spoon will be left empty. The
disappearance of sulphur is due to the formation of a new gaseous
substance that is invisible. The presence and existence of a gas in air
can be defected by its irritating smell. The gas can also be detected by
burning the sulphur in a gas jar to which some blue litmus solution has
been added. The gas formed, sulphur dioxide, will turn the blue litmus
paper into a red one. - With the aid of tongs, subject a piece of
magnesium ribbon to a Bunsen burner flame. The ribbon burns to produce a
new substance, white ash of magnesium oxide. - Wrap a wet cotton wool around an iron nail. Keep it in a test tube for 3 days. By the 3rd
day, some brown marks of rust will appear on the surface of the nail.
Rust is hydrated iron (III) oxide. This is quite a new substance
compared to iron nails.
Table
Differences between physical and chemical changes
| Physical change | Chemical change |
| 1. Produces no new kind of matter | Always produces a new kind of matter |
| 2. There is no change is mass or weight of the substance | 2. There is a substantial change in the weight of the substance |
| 3. The change can be reversed | 3. The change cannot be reversed |
| 4. Little heat is absorbed or evolved | 4. Heat changes may be large |
| 5. The change involves only a change in physical properties of a substance | 5. Both physical and chemical properties are changed. |
Elements and symbols
Depending on the nature of change, all changes that matter undergoes can be classified as either physical or chemical.
The Characteristics of a Physical Change
Describe the characteristics of a physical change
Physical change
Substances
may undergo changes in their physical properties e.g. changes in
colour, shape (or form), state, density, structure and texture, etc. If
you take a stone and break it down into small particles, you will have
only changed its form, but it will remain as a stone. Likewise, melting
ice to water or freezing water to ice does not change it, but it is
still water. The same case happens when you dissolve salt in water to
get a solution of salt in water. You can still get back the original
salt by evaporation, except that the crystals of the salt obtained will
not look exactly the same as those of the original salt.
may undergo changes in their physical properties e.g. changes in
colour, shape (or form), state, density, structure and texture, etc. If
you take a stone and break it down into small particles, you will have
only changed its form, but it will remain as a stone. Likewise, melting
ice to water or freezing water to ice does not change it, but it is
still water. The same case happens when you dissolve salt in water to
get a solution of salt in water. You can still get back the original
salt by evaporation, except that the crystals of the salt obtained will
not look exactly the same as those of the original salt.
These
changes of state are examples of physical changes. Physical changes
such as melting and boiling do not result in new substances being formed
changes of state are examples of physical changes. Physical changes
such as melting and boiling do not result in new substances being formed
For example, ice and water still contain the same particles whether in solid (ice) liquid (water) or gaseous (vapour) state.
Changes in states
Characteristics
In
the explanation above, we find that in a physical change it is only the
physical form, and not the actual nature, of a substance that changes.
The changes are brought about by a mere addition or removal of heat, as
in the case with water or ice. Such a change is called a physical
change. It can be distinguished by the following characteristics:
the explanation above, we find that in a physical change it is only the
physical form, and not the actual nature, of a substance that changes.
The changes are brought about by a mere addition or removal of heat, as
in the case with water or ice. Such a change is called a physical
change. It can be distinguished by the following characteristics:
- There
is no formation of a new substance. Consider an example given above.
The ice, liquid water and steam are the solid, liquid and gaseous forms
of the same substance (water). - There is no change in weight of
the substance undergoing the change. If you start with 50g of ice, you
will still get the same mass of water and steam (vapour) upon melting
and boiling respectively. - The changes are readily reversible.
You can easily change water back to ice and vapour to water by a mere
subtraction of heat (cooling). - It is not accompanied by a great heat change. Just a little heat is required to change ice to water, and water to steam.
Physical Changes of Matter Experimentally
Demonstrate physical changes of matter experimentally
Experiment
- Add
some common salt (sodium chloride) to distilled water in a beaker. Stir
the mixture until the salt disappears and forms a solution with water.
Transfer the water into a porcelain dish. Heat the content until all the
water has evaporated off. The salt reappears in its original white
solid form. - Grind some roll sulphur in a mortar to powder. Put
the resultant powder in a test-tube and heat gently, shaking all the
time. The sulphur melts to an amber-coloured liquid. On cooling, this
liquid returns to its original condition as a yellow solid. - Put a
block of ice in a beaker. Heat gently until the whole block melts to
form water. Pour the water formed in a cup and place it in a deep
freezer overnight. The water will freeze back to ice.
You
will have seen that all the above changes involve only changes in
physical forms of the substances. The chemical nature of substances
remained unchanged. Therefore, we can define a physical change as a
change that does not involve formation of a new substance but involves a
change in state or physical form of the substance and that such a form
can be reversed.
will have seen that all the above changes involve only changes in
physical forms of the substances. The chemical nature of substances
remained unchanged. Therefore, we can define a physical change as a
change that does not involve formation of a new substance but involves a
change in state or physical form of the substance and that such a form
can be reversed.
The Characteristics of a Chemical Change
Describe the characteristics of a chemical change
Chemical change
Some
changes that materials undergo are permanent. Such changes usually
involve changes in chemical properties of a substance. For example, when
you burn a piece of wood in fire, you get ash. The properties of wood
and ash are very different. There is no way you can change ash back to
wood. It is practically impossible. A permanent change in chemical
properties of a substance is called a chemical change. In a chemical
change, a substance losses all its physical and chemical properties.
changes that materials undergo are permanent. Such changes usually
involve changes in chemical properties of a substance. For example, when
you burn a piece of wood in fire, you get ash. The properties of wood
and ash are very different. There is no way you can change ash back to
wood. It is practically impossible. A permanent change in chemical
properties of a substance is called a chemical change. In a chemical
change, a substance losses all its physical and chemical properties.
Characteristics
Includes
- A
chemical change results in the formation of a new substance. The new
substance has different chemical and physical properties as compared to
the original substance. - It is generally not reversible. For example, you cannot turn the ash back to wood.
- There
is a change in weight or mass of the substance undergoing the change.
When you burn wood weighing 5 kg, you cannot expect to get the same
weight of ash. - The change is accompanied by a considerable heat change. For wood to burn to ash a lot of heat must be supplied.
Chemical Changes of Matter Experimentally
Demonstrate chemical changes of matter experimentally
Experiment
- Strongly
heat some roll sulphur on a deflagrating spoon until it melts and
begins to burn with a blue flame. If you continue heating, it gradually
decreases in amount and finally the spoon will be left empty. The
disappearance of sulphur is due to the formation of a new gaseous
substance that is invisible. The presence and existence of a gas in air
can be defected by its irritating smell. The gas can also be detected by
burning the sulphur in a gas jar to which some blue litmus solution has
been added. The gas formed, sulphur dioxide, will turn the blue litmus
paper into a red one. - With the aid of tongs, subject a piece of
magnesium ribbon to a Bunsen burner flame. The ribbon burns to produce a
new substance, white ash of magnesium oxide. - Wrap a wet cotton wool around an iron nail. Keep it in a test tube for 3 days. By the 3rd
day, some brown marks of rust will appear on the surface of the nail.
Rust is hydrated iron (III) oxide. This is quite a new substance
compared to iron nails.
Table
Differences between physical and chemical changes
| Physical change | Chemical change |
| 1. Produces no new kind of matter | Always produces a new kind of matter |
| 2. There is no change is mass or weight of the substance | 2. There is a substantial change in the weight of the substance |
| 3. The change can be reversed | 3. The change cannot be reversed |
| 4. Little heat is absorbed or evolved | 4. Heat changes may be large |
| 5. The change involves only a change in physical properties of a substance | 5. Both physical and chemical properties are changed. |
Separation of mixtures
To
make use of the materials around us, we need methods for physically
separating the many and varied mixtures that we come across. One of the
distinctive characteristics of a mixture of substances is that it is
usually possible to separate the constituents by physical means. There
are many different physical methods used to separate a wide variety of
mixtures. The particular method employed to separate any given mixture
depends upon the nature of its constituents. The following are some of
the methods in wide use.
make use of the materials around us, we need methods for physically
separating the many and varied mixtures that we come across. One of the
distinctive characteristics of a mixture of substances is that it is
usually possible to separate the constituents by physical means. There
are many different physical methods used to separate a wide variety of
mixtures. The particular method employed to separate any given mixture
depends upon the nature of its constituents. The following are some of
the methods in wide use.
The Different Methods of Separating Mixtures
Describe the different methods of separating mixtures
1. Filtration
This method is best applicable in separation of components of mixtures called suspensions.
A
mixture of chalk dust or flour with water can be separated by filtering
the suspension. The suspended particles get trapped in the filter
paper. The trapped particles are called the residue. The water is called the filtrate
mixture of chalk dust or flour with water can be separated by filtering
the suspension. The suspended particles get trapped in the filter
paper. The trapped particles are called the residue. The water is called the filtrate
Filtration
2. Decantation
This
is another method that can be used to separate mixtures called
suspensions. However, in this case, separation will be successful if the
suspended particles are large enough. Otherwise, the decantation
exercise should be accompanied by filtration if you want to get a clear
liquid.
is another method that can be used to separate mixtures called
suspensions. However, in this case, separation will be successful if the
suspended particles are large enough. Otherwise, the decantation
exercise should be accompanied by filtration if you want to get a clear
liquid.
Once the solid has settled to the bottom of the container (sedimented), the liquid can be carefully poured off. This is called decantation. Decantation can be applied to separate such components as mixtures of mud, sand or gravel in water and so on.
Decantation of muddy water
3. Evaporation
This
method is used to separate substances that form a solution. In such a
mixture, the solute is completely dissolved in a solvent to make a
uniform solution. To separate these substances, the solution is heated
so that the solvent evaporates, leaving the solid residue behind.
method is used to separate substances that form a solution. In such a
mixture, the solute is completely dissolved in a solvent to make a
uniform solution. To separate these substances, the solution is heated
so that the solvent evaporates, leaving the solid residue behind.
A mixture of salt or sugar in water can be separated by applying this method.
Evaporating the solvent
4. Simple distillation
Separating
a liquid from a solution can be carried out by distillation. The
boiling point of a liquid is usually very much lower than that of the
dissolved solid. The liquid can easily be evaporated off in a
distillation flask. It is condensed by passing it down a water-cooled
condenser and then collected as the distillate
a liquid from a solution can be carried out by distillation. The
boiling point of a liquid is usually very much lower than that of the
dissolved solid. The liquid can easily be evaporated off in a
distillation flask. It is condensed by passing it down a water-cooled
condenser and then collected as the distillate
This
method can be used to obtain pure water from impure water or from water
with dissolved impurities. The process may be used to separate a liquid
from a solution or to separate two liquids whose boiling points differ
by an appreciable temperature interval. This is a way of getting a pure
solvent out of a solution.
method can be used to obtain pure water from impure water or from water
with dissolved impurities. The process may be used to separate a liquid
from a solution or to separate two liquids whose boiling points differ
by an appreciable temperature interval. This is a way of getting a pure
solvent out of a solution.
Simple distillation
5. Fractional distillation
Separating
the liquids from a mixture of two (or more) miscible liquids is again
based on the fact that liquids will have different boiling points.
However, the boiling points are closer together than for solid-in-liquid
solutions. It is difficult to separate mixtures of liquids whose
boiling points differ by only a few degrees. In this case, fractional
distillation is used.
the liquids from a mixture of two (or more) miscible liquids is again
based on the fact that liquids will have different boiling points.
However, the boiling points are closer together than for solid-in-liquid
solutions. It is difficult to separate mixtures of liquids whose
boiling points differ by only a few degrees. In this case, fractional
distillation is used.
For
example, ethanol boils at 78°C whereas water boils at 100°C. When a
solution of ethanol and water is heated, ethanol and water vapours
enters the fractionating column. Evaporation and condensation take place
as the vapours rise up the column. Ethanol passes through the condenser
first as the temperature of the column is raised above the boiling
point. Water condenses in the column and flows back into the flask
because the temperature of the column is below its boiling point of
100°C.
example, ethanol boils at 78°C whereas water boils at 100°C. When a
solution of ethanol and water is heated, ethanol and water vapours
enters the fractionating column. Evaporation and condensation take place
as the vapours rise up the column. Ethanol passes through the condenser
first as the temperature of the column is raised above the boiling
point. Water condenses in the column and flows back into the flask
because the temperature of the column is below its boiling point of
100°C.
The
temperature on the thermometer stays at 78°C until the ethanol has
distilled over. Eventually, the thermometer reading rises above 78C°.
This is a sign that all the ethanol has been separated, so heating can
be stopped. By watching the temperature carefully, the two liquids
(fractions) can be collected separately.
temperature on the thermometer stays at 78°C until the ethanol has
distilled over. Eventually, the thermometer reading rises above 78C°.
This is a sign that all the ethanol has been separated, so heating can
be stopped. By watching the temperature carefully, the two liquids
(fractions) can be collected separately.
Various
forms of fractionating column can be used. Their general purpose is to
provide surfaces, e.g. flat discs, on which ascending vapour can
condense. Glass beads in the column provide a large surface area for
condensation.
forms of fractionating column can be used. Their general purpose is to
provide surfaces, e.g. flat discs, on which ascending vapour can
condense. Glass beads in the column provide a large surface area for
condensation.
Fractional distillation
6. Sublimation
This
is a technique used to separate a mixture of solids where one of the
solids sublimes. Examples of solids which sublime are ammonium chloride,
iodine, solid carbon dioxide and naphthalene. A mixture of any of these
solids with another solid can be separated by sublimation.
is a technique used to separate a mixture of solids where one of the
solids sublimes. Examples of solids which sublime are ammonium chloride,
iodine, solid carbon dioxide and naphthalene. A mixture of any of these
solids with another solid can be separated by sublimation.
Let
us consider a mixture of iodine and sodium chloride. The mixture is
placed in a beaker and covered with a filter funnel as shown in the
diagram below. Then, as the mixture is heated, the ammonium chloride
sublimes. The ammonium chloride vapour rises and condenses on the cooler
walls of the filter funnel. The sodium chloride is left in the beaker.
us consider a mixture of iodine and sodium chloride. The mixture is
placed in a beaker and covered with a filter funnel as shown in the
diagram below. Then, as the mixture is heated, the ammonium chloride
sublimes. The ammonium chloride vapour rises and condenses on the cooler
walls of the filter funnel. The sodium chloride is left in the beaker.
Sublimation of ammonium chloride
7. Chromatography
This
method is commonly used to separate a mixture of coloured substances
(solids or dyes). An example of this is the separation of dyes that make
up black ink. Chromatography works better when a solvent is used. The
commonest solvent is water, though other solvents such as ethanol or
ether may be used for those substances that do not dissolve in water.
There are two types of chromatography, namely column chromatography and paper chromatography.
The two types of chromatography follow the same principle, but paper
chromatography is the simplest form to set up, and hence is more
commonly used. On which principle does chromatography work? Let us
consider an example of separating dyes that make up black ink. In this
case, water is used as a solvent.
method is commonly used to separate a mixture of coloured substances
(solids or dyes). An example of this is the separation of dyes that make
up black ink. Chromatography works better when a solvent is used. The
commonest solvent is water, though other solvents such as ethanol or
ether may be used for those substances that do not dissolve in water.
There are two types of chromatography, namely column chromatography and paper chromatography.
The two types of chromatography follow the same principle, but paper
chromatography is the simplest form to set up, and hence is more
commonly used. On which principle does chromatography work? Let us
consider an example of separating dyes that make up black ink. In this
case, water is used as a solvent.
Separating dyes in ink
Procedure
- Put a small spot of the water-soluble ink onto a strip of filter paper as shown in figure bellow
- Place the filter paper in a beaker of water. Make sure the level of the water is below the level of the ink spot.
- Leave the filter paper until the water has risen to the top of the paper.
- Remove the paper and allow it to dry.
- Note the colours the ink contains.
Observation
Separating the components of black ink by paper chromatography
As
the solvent (water) moves up the paper, the dyes are carried with it
and begin to separate. They separate because they have different
solubilities in water and are absorbed to different degrees by the
filter (chromatography) paper. As they rise, they are gradually
separated.
the solvent (water) moves up the paper, the dyes are carried with it
and begin to separate. They separate because they have different
solubilities in water and are absorbed to different degrees by the
filter (chromatography) paper. As they rise, they are gradually
separated.
Findings
The
different colours of the ink make a pattern of colours formed during
the process of chromatography. This pattern of colours is called a
chromatogram.
different colours of the ink make a pattern of colours formed during
the process of chromatography. This pattern of colours is called a
chromatogram.
Figure
aboveshows a chromatogram of black ink. The blue ink has the fastest
speed. This means it is the most soluble in water and least absorbed by
the paper. The green ink has travelled least. This means it is the least
soluble in water and most absorbed by the paper.
aboveshows a chromatogram of black ink. The blue ink has the fastest
speed. This means it is the most soluble in water and least absorbed by
the paper. The green ink has travelled least. This means it is the least
soluble in water and most absorbed by the paper.
Paper chromatography showing the separated components of black ink
Uses of Chromatography
Chromatography is used in many different ways. The following are some of the application of chromatography:
- It can be used to find out the components of a liquid or solid, or even to identify different substances.
- It can be used by security agents and medical personnel to analyse blood and urine samples.
- Causes of pollution in water and in animals that live in water can also be detected using chromatography.
- In chemistry, chromatography is used to test the purity of substances and in separation of mixtures.
8. Layer separation
Mixtures
of two immiscible liquids can be separated with a separating funnel.
The mixture is placed in a separating funnel and allowed to stand. The
liquids separate into two different layers. The lower denser layer is
then “tapped” off at the bottom.
of two immiscible liquids can be separated with a separating funnel.
The mixture is placed in a separating funnel and allowed to stand. The
liquids separate into two different layers. The lower denser layer is
then “tapped” off at the bottom.
For
example, when a mixture of kerosene and water is poured into the
funnel, the kerosene floats to the top as shown in figure above. When
the tap is opened, the water runs out. The tap is closed again when all
water has gone, leaving the kerosene in the funnel.
example, when a mixture of kerosene and water is poured into the
funnel, the kerosene floats to the top as shown in figure above. When
the tap is opened, the water runs out. The tap is closed again when all
water has gone, leaving the kerosene in the funnel.
Separating immiscible liquids
9. Solvent extraction
Solvent
extraction, also known as liquid-liquid extraction, refers to the
separation of materials of different chemical types and solubilities by
selective solvent extraction. That is, some materials are more soluble
in one solvent than in another. The method is used to refine petroleum
products, chemicals, vegetable oils, and vitamins.
extraction, also known as liquid-liquid extraction, refers to the
separation of materials of different chemical types and solubilities by
selective solvent extraction. That is, some materials are more soluble
in one solvent than in another. The method is used to refine petroleum
products, chemicals, vegetable oils, and vitamins.
This
method is used is to separate a solid from a solution in which there is
more than one solid dissolved. An example of this is a water solution
of iodine and sodium chloride.
method is used is to separate a solid from a solution in which there is
more than one solid dissolved. An example of this is a water solution
of iodine and sodium chloride.
EXPERIMENT:Separating iodine from sodium chloride by solvent extraction.
Method
- Put the solution into a separating funnel as shown in figure (a).
- Add ethoxyethane. This forms a layer on top of the solution (b). The ethoxyethane is called the extracting solvent.
- Stopper
the separating funnel and shake well figure (c). The iodine, which is
more soluble in the ethoxyethane, passes into the ethoxyethane layer.
The sodium chloride remains in the water layer. - The water layer
is run off into a beaker followed by the ethoxyethane layer into another
beaker (Caution: Remove the stopper before opening the tap). - The
ethoxyethane is then evaporated off by simple distillation. Similarly,
the water layer can be evaporated to yield sodium chloride.
The solvent extraction works on two principles:
Separating iodine from sodium chloride by solvent extraction
- One solid in the solution must be more soluble in the extracting solvent than the other.
- The
extracting solvent must not be miscible with the solvent in which the
mixture of solids is dissolved. Neither should it react with it.
10. Centrifugation
A
centrifuge is used to separate small amounts of suspension.
Centrifugation is used with insoluble solids where the particles are
very small and spread throughout the liquid. In centrifugation, test
tubes containing suspensions are spun round very fast. The solid gets
thrown to the bottom. Here, it is no longer the force of gravity on the
solid that causes settling.
centrifuge is used to separate small amounts of suspension.
Centrifugation is used with insoluble solids where the particles are
very small and spread throughout the liquid. In centrifugation, test
tubes containing suspensions are spun round very fast. The solid gets
thrown to the bottom. Here, it is no longer the force of gravity on the
solid that causes settling.
Instead,
there is a huge centrifugal force acting on the particles due to the
high speed spinning of the samples. This causes the solid to be
deposited at the bottom of the centrifuge tube.
there is a huge centrifugal force acting on the particles due to the
high speed spinning of the samples. This causes the solid to be
deposited at the bottom of the centrifuge tube.
Separation by centrifugation
After centrifugation, the liquid can be decanted (poured out) from the test tube, or removed with a small pipette.
This makes the solid to be left behind.
The solid after centrifugation
11. Magnetic separation
If
the solid mixture contains iron, the iron can be removed using a
magnet. This method is used to separate scrap iron from other metals.
Magnetic iron ore can be separated from other material in the crushed
ore by using an electromagnet. In the process of recycling metals, iron
objects can be picked out from other scrap metals using electromagnets.
the solid mixture contains iron, the iron can be removed using a
magnet. This method is used to separate scrap iron from other metals.
Magnetic iron ore can be separated from other material in the crushed
ore by using an electromagnet. In the process of recycling metals, iron
objects can be picked out from other scrap metals using electromagnets.
12. Crystallization
This
process involves evaporation but the speed of evaporation is much
slower. In principle the salt solution can be left in the evaporating
basin for a long period until all the water has evaporated but in
practice this takes longer time. The process begins by evaporating away
the liquid. However, because the crystals are needed, evaporation is
stopped after the solution has been concentrated enough. The
concentrated solution is allowed to cool slowly and crystallize. The
crystals so formed can be filtered off and dried. A similar process is
used to extract salt from the sea. Salty sea water is placed in wide
basins and put in the sun. Water evaporates off, leaving the salt
crystals in basins.
process involves evaporation but the speed of evaporation is much
slower. In principle the salt solution can be left in the evaporating
basin for a long period until all the water has evaporated but in
practice this takes longer time. The process begins by evaporating away
the liquid. However, because the crystals are needed, evaporation is
stopped after the solution has been concentrated enough. The
concentrated solution is allowed to cool slowly and crystallize. The
crystals so formed can be filtered off and dried. A similar process is
used to extract salt from the sea. Salty sea water is placed in wide
basins and put in the sun. Water evaporates off, leaving the salt
crystals in basins.
13. Winnowing or threshing
This
is a method used to separate grains from husks or bran. The process
makes use of the differences in density of the constituents in the
mixture. When the winnower is shaken around, grains, being denser than
husks or bran, sink to the bottom of the winnower
is a method used to separate grains from husks or bran. The process
makes use of the differences in density of the constituents in the
mixture. When the winnower is shaken around, grains, being denser than
husks or bran, sink to the bottom of the winnower
The
less dense husks or bran moves to the top. They are then blown off the
winnower by wind or breath, or sometimes picked by hand and separated
from the grains.
less dense husks or bran moves to the top. They are then blown off the
winnower by wind or breath, or sometimes picked by hand and separated
from the grains.
Separating thresh from grains by winnowing
The Significance of Separating Different Mixtures
Explain the significance of separating different mixtures
We
separate mixtures in order to obtain the mixture constituents and put
them into appropriate use. The world around us is made up of mixtures of
different substances. These substances are often of little use when
they are in the form of a mixture. For this reason, separation of the
individual components of mixtures is deemed inevitable.
separate mixtures in order to obtain the mixture constituents and put
them into appropriate use. The world around us is made up of mixtures of
different substances. These substances are often of little use when
they are in the form of a mixture. For this reason, separation of the
individual components of mixtures is deemed inevitable.
1. (i) Fractional distillation
Is
used industrially to separate the various fractions of crude oil such
as natural gas, petrol, kerosene, diesel, lubricating oils, waxes,
asphalt and bitumen. All these fractions have a significant use in man’s
industrial, domestic and commercial activities. The functions or use of
all these fractions is well known to everyone of us. Can you mention
the functions of each fraction? You will learn more about these products
in organic chemistry section.
used industrially to separate the various fractions of crude oil such
as natural gas, petrol, kerosene, diesel, lubricating oils, waxes,
asphalt and bitumen. All these fractions have a significant use in man’s
industrial, domestic and commercial activities. The functions or use of
all these fractions is well known to everyone of us. Can you mention
the functions of each fraction? You will learn more about these products
in organic chemistry section.
(ii) Fractional distillation of liquid air
Separates
the air into its component gases. This is important because these
components have many uses in our everyday life. Some of these fractions
and their functions are summarized below:
the air into its component gases. This is important because these
components have many uses in our everyday life. Some of these fractions
and their functions are summarized below:
| Component | Use |
| Nitrogen | Manufacture of fertilizer |
| Oxygen | Used in hospitals, steel making, diving and space travel |
| Argon | Filling light bulbs |
| Carbon dioxide | Fire extinguishing, used in carbonate drinks, etc. |
| Helium | Filling airships and water balloons |
| Krypton and Xenon | Used in photographic flash lamps |
2. Filtration and purification of drinking water make use of processes such as decantation, filtration and sometimes distillation. The bottled water we drink is prepared by some or a combination of these processes.
3. In mining, an electromagnet is used to separate magnetic iron ore from other materials in the crushed ore.
4. In the manufacture of ethanol by fermentation in breweries, distillation
is used in the final stage to purify ethanol to its purest form
(surgical spirit) in which the ethanol is usually sold. Likewise,
distillation of fermented starch (8-12% ethanol), yields alcoholic
drinks called sprits (whisky, gin, brandy, rum) which contain about
35-40% ethanol.
is used in the final stage to purify ethanol to its purest form
(surgical spirit) in which the ethanol is usually sold. Likewise,
distillation of fermented starch (8-12% ethanol), yields alcoholic
drinks called sprits (whisky, gin, brandy, rum) which contain about
35-40% ethanol.
5. (i) Paper chromatography
is very useful in analysis of substances present in a solution. For
example, it can tell whether a substance has become contaminated or
otherwise. This can be very important, because contamination of food or
drinking water, for instance, may be dangerous to our health.
is very useful in analysis of substances present in a solution. For
example, it can tell whether a substance has become contaminated or
otherwise. This can be very important, because contamination of food or
drinking water, for instance, may be dangerous to our health.
(ii) Chromatography has
proved very useful in the analysis of biologically important molecules
such as sugars, amino acids, and nucleotide bases. Molecules such as
amino acids can be seen if the paper is viewed under ultra- violet
light.
proved very useful in the analysis of biologically important molecules
such as sugars, amino acids, and nucleotide bases. Molecules such as
amino acids can be seen if the paper is viewed under ultra- violet
light.
(iii) Paper chromatography
is the test that can be used to check for the purity of a substance. If
the sample is pure, it should only give one spot when run in several
different solvents.
is the test that can be used to check for the purity of a substance. If
the sample is pure, it should only give one spot when run in several
different solvents.
6.
Other separation methods are also used to check whether purification
has been successful. Samples obtained by distillation can be
re-distilled. The purity of crystals can be improved by
re-crystallisation. A water sample can be tested for amount of dissolved
material by evaporating a certain amount of water to dryness.
The solid waste can be weighed. This would give the amount of dissolved
solid in the water.
Other separation methods are also used to check whether purification
has been successful. Samples obtained by distillation can be
re-distilled. The purity of crystals can be improved by
re-crystallisation. A water sample can be tested for amount of dissolved
material by evaporating a certain amount of water to dryness.
The solid waste can be weighed. This would give the amount of dissolved
solid in the water.
The
process of purification is of crucial importance in many areas of
chemical industry. Medical drugs (pharmaceuticals) must be of highest
possible degrees of purity. Any contaminating substances even in very
small amounts may have harmful side effects.
process of purification is of crucial importance in many areas of
chemical industry. Medical drugs (pharmaceuticals) must be of highest
possible degrees of purity. Any contaminating substances even in very
small amounts may have harmful side effects.
7. (i) Separation of cream from whole milk is done by the process of centrifugation.
As the milk is spun, the heavier contents are forced down and the
lighter cream rises up. After centrifugation, the cream is poured off
the top by decantation. This is the initial stage of milk constituent
separation, after which other components such as milk proteins (cheese)
are separated.
As the milk is spun, the heavier contents are forced down and the
lighter cream rises up. After centrifugation, the cream is poured off
the top by decantation. This is the initial stage of milk constituent
separation, after which other components such as milk proteins (cheese)
are separated.
(ii)
Centrifugation is applicable in blood analysis, where the solid part of
blood is separated from the liquid part by centrifugation. Blood is a
suspension containing microscopic blood cells (corpuscles) in a liquid
called plasma. If blood is centrifuged in a test tube, the blood cells are flung to the bottom, leaving the liquid plasma on top.
Centrifugation is applicable in blood analysis, where the solid part of
blood is separated from the liquid part by centrifugation. Blood is a
suspension containing microscopic blood cells (corpuscles) in a liquid
called plasma. If blood is centrifuged in a test tube, the blood cells are flung to the bottom, leaving the liquid plasma on top.
8.
Knowledge of separation of two immiscible liquids can be applied in the
extraction of metals such as iron from their ores. For example, at the
base of the blast furnace, the molten slug forms a separate layer on top
of the liquid iron. The two can then be “tapped” off separately. The
method is very useful in organic chemistry as part of the process called
solvent extraction.
Knowledge of separation of two immiscible liquids can be applied in the
extraction of metals such as iron from their ores. For example, at the
base of the blast furnace, the molten slug forms a separate layer on top
of the liquid iron. The two can then be “tapped” off separately. The
method is very useful in organic chemistry as part of the process called
solvent extraction.
9. Evaporation
process is used in the extraction of common salt from seawater whereby
the sun evaporates water molecules from salty water, leaving crystals of
the salt behind.
process is used in the extraction of common salt from seawater whereby
the sun evaporates water molecules from salty water, leaving crystals of
the salt behind.
10. Layer separation technique is applied in the recovery of liquids from contaminants.
11. Solvent extraction
process is applied in the extraction of certain edible oils from seeds,
and in the extraction of some metals from sludge mixture.
process is applied in the extraction of certain edible oils from seeds,
and in the extraction of some metals from sludge mixture.
The Components of Different Mixtures using Different Methods
Separate the components of different mixtures using different methods
Activity 1
Separate the components of different mixtures using different methods.
TOPIC 6: AIR COMBUSTION, RUSTING AND FIRE FIGHTING.
Composition of air
Air
is a mixture of different gases. The gases that make up the air include
nitrogen, oxygen, carbon dioxide, noble gases (argon, helium, neon,
krypton and xenon) and a little water vapour. Air may also contain
traces of impurities such as carbon monoxide (CO), sulphur dioxide (SO2), hydrogen sulphide (H2S)
and other gases. The presence of these gases in air results in air
pollution. Table bellow shows the composition of air by volume. The
proportion of water vapour and impurities in air is very variable.
is a mixture of different gases. The gases that make up the air include
nitrogen, oxygen, carbon dioxide, noble gases (argon, helium, neon,
krypton and xenon) and a little water vapour. Air may also contain
traces of impurities such as carbon monoxide (CO), sulphur dioxide (SO2), hydrogen sulphide (H2S)
and other gases. The presence of these gases in air results in air
pollution. Table bellow shows the composition of air by volume. The
proportion of water vapour and impurities in air is very variable.
The Gases Present in Air and their Proportions
Name the gases present in air and their proportions
The
composition of air is not exactly the same everywhere. It changes
slightly from day to day and from place to place. There is more water
vapour in the air on a damp day and in air above water bodies such as
oceans, seas, lakes, rivers, etc. Over busy cities and industrial areas
there is more carbon dioxide. But the uneven heating of the earth’s
surface by the sun causes the air to move continually, resulting in
winds. The resultant winds spread the pollutants around.
composition of air is not exactly the same everywhere. It changes
slightly from day to day and from place to place. There is more water
vapour in the air on a damp day and in air above water bodies such as
oceans, seas, lakes, rivers, etc. Over busy cities and industrial areas
there is more carbon dioxide. But the uneven heating of the earth’s
surface by the sun causes the air to move continually, resulting in
winds. The resultant winds spread the pollutants around.
The percentage composition of air by volume
| Gas | Approximate percentage |
| Nitrogen | 78.00% |
| Oxygen | 21.00% |
| Noble (rare) gases mainly argon | 0.94% |
| Carbon dioxide | 0.03% |
| Water vapour | 0 – 4% |
The Presence of Different Gases in Air
Demonstrate the presence of different gases in air
The determination of air by mass was carried out by Dumas in 1841. The apparatus used consists of three units as shown bellow.
The three parts of the apparatus include the following:
Determination of the composition of air by weight
- Several
U-tubes containing potassium hydroxide pellets to remove carbon dioxide
(only one tube shown in the figure for simplicity). - Another set of U-tubes containing concentrated sulphuric acid to remove water vapour (only one tube shown in the figure).
- A heated, weighed glass tube containing finely divided copper to absorb oxygen.
The
three parts of the apparatus would, therefore, remove all carbon
dioxide, water vapour and oxygen contained in air. The remaining gas
which enters the weighed evacuated flask (globe) will be atmospheric
nitrogen and, of course, plus the rare gases. The copper will have
reacted with all oxygen to form copper (II) oxide. The increase in mass
of the copper will give the mass of oxygen. The increase in weight of
the globe will be due to the weight of nitrogen and the rare gases. If
we neglect the weight of carbon dioxide, the percentage of oxygen by
mass (weight) in dry, pure air is 23.2% and the remaining 76.8% is the
percentage of nitrogen and rare gases.
three parts of the apparatus would, therefore, remove all carbon
dioxide, water vapour and oxygen contained in air. The remaining gas
which enters the weighed evacuated flask (globe) will be atmospheric
nitrogen and, of course, plus the rare gases. The copper will have
reacted with all oxygen to form copper (II) oxide. The increase in mass
of the copper will give the mass of oxygen. The increase in weight of
the globe will be due to the weight of nitrogen and the rare gases. If
we neglect the weight of carbon dioxide, the percentage of oxygen by
mass (weight) in dry, pure air is 23.2% and the remaining 76.8% is the
percentage of nitrogen and rare gases.
The presence of nitrogen in air
In
order to demonstrate the presence of nitrogen in air, we need to carry
out an experiment that will convert the nitrogen of the air into a
chemically recognizable substance. This is easily done by strongly
heating magnesium in the residual gas from the above experiment.
Magnesium and nitrogen will react thus:
order to demonstrate the presence of nitrogen in air, we need to carry
out an experiment that will convert the nitrogen of the air into a
chemically recognizable substance. This is easily done by strongly
heating magnesium in the residual gas from the above experiment.
Magnesium and nitrogen will react thus:
Upon
treatment with water, magnesium nitrite gives ammonia gas. The gas can
be recognized by its characteristic smell and its action of turning red
litmus paper to blue.
treatment with water, magnesium nitrite gives ammonia gas. The gas can
be recognized by its characteristic smell and its action of turning red
litmus paper to blue.
The presence of oxygen in air
Oxygen
is known as the active portion of the air because it supports
combustion and combines with many other substances. Its presence and
composition in air can be determined by using these properties. Any of
the following two (2) experiments can be used to determine the
composition, by volume of oxygen contained in air.
is known as the active portion of the air because it supports
combustion and combines with many other substances. Its presence and
composition in air can be determined by using these properties. Any of
the following two (2) experiments can be used to determine the
composition, by volume of oxygen contained in air.
The Percentage of Oxygen in Air Experimentally
Determine the percentage of oxygen in air experimentally
1. Experiment. Determination of the presence and proportion of oxygen in air by combustion of a candle
Method
- Place
a small candle on a plastic lid or any object that can float. Then set
up the apparatus as shown in figure bellow. Sodium hydroxide is used in
order to absorb the carbon dioxide gas produced by a burning candle. - Light
the candle and place the measuring cylinder over the top. Note the
level of sodium hydroxide solution in the measuring cylinder at the
start. A candle will stop burning (go off) once all the oxygen in the
cylinder is used up. - When the candle goes off, leave the
apparatus to cool to room temperature. The purpose of cooling is to let
the heated and expanded air to return to its normal condition. Then note
the level of sodium hydroxide solution in the measuring cylinder.
.
Determining the presence and percentage composition of oxygen in air by burning a candle
Observation and findings
The
oxygen in air enclosed in the measuring cylinder is used to burn the
candle to produce carbon dioxide gas. The carbon dioxide so produced
dissolves in sodium hydroxide solution. The dissolved carbon dioxide
causes the level of sodium hydroxide solution to rise up. The oxygen gas
used to burn the candle is practically equal to the amount of carbon
dioxide produced. This fact is, therefore, used to calculate the
percentage of oxygen in air.
oxygen in air enclosed in the measuring cylinder is used to burn the
candle to produce carbon dioxide gas. The carbon dioxide so produced
dissolves in sodium hydroxide solution. The dissolved carbon dioxide
causes the level of sodium hydroxide solution to rise up. The oxygen gas
used to burn the candle is practically equal to the amount of carbon
dioxide produced. This fact is, therefore, used to calculate the
percentage of oxygen in air.
Model results
In the experiment, the initial volume of air was found to be 70.5 cm3 and the final volume was 55 cm3. The percentage of oxygen in the air is calculated in two steps:
1.
To find the volume of oxygen used up to burn the candle (which is
practically equal to the volume of carbon dioxide produced and then
absorbed by sodium hydroxide), we subtract the final volume of air from
the initial volume
To find the volume of oxygen used up to burn the candle (which is
practically equal to the volume of carbon dioxide produced and then
absorbed by sodium hydroxide), we subtract the final volume of air from
the initial volume
Volume or oxygen used = Initial volume of air – final volume of air
Therefore, the volume of oxygen used for combustion of the candle = 14.7 cm.
Alternatively,
the volume of oxygen used up can be calculated by subtracting the
initial volume of sodium hydroxide solution from the final volume. That
is: Volume of oxygen used = final volume of sodium hydroxide – initial
volume of sodium hydroxide = Volume of carbon dioxide dissolved in
sodium hydroxide.
the volume of oxygen used up can be calculated by subtracting the
initial volume of sodium hydroxide solution from the final volume. That
is: Volume of oxygen used = final volume of sodium hydroxide – initial
volume of sodium hydroxide = Volume of carbon dioxide dissolved in
sodium hydroxide.
Therefore, the percentage of oxygen =
In practice, it is difficult to get an accurate result with the above experiment.
This is due to a number of reasons such as:
- Not all the carbon dioxide is absorbed by the sodium hydroxide.
- The candle may go out (stop burning) before all the oxygen is used up due to accumulation of carbon dioxide in the cylinder.
- The
heating of the air inside the measuring cylinder causes the gases to
expand. This is why it is essential that the gases be allowed to cool to
room temperature before reading the level.
Experiment withcombustion of copper in airgives
the more accurate results than the combustion of the candle. The copper
reacts with oxygen in the air to give copper (II) oxide.
the more accurate results than the combustion of the candle. The copper
reacts with oxygen in the air to give copper (II) oxide.
2. Experiment . Determination of the presence and proportion of oxygen in air by the combustion of copper in air
Method
- Set up the apparatus as shown in figure bellow. Syringe A should contain 100 cm of air, syringe B should be empty.
- Heat
the copper strongly and pass the air from syringe A back and forth (by
pushing the piston of the syringe inward and outward) over the copper
turnings a few times. Allow the air to cool and measure the volume of
air in syringe A. - Repeat the heating and cooling until the
volume of air that remains in syringe A is constant. The copper is
heated and cooled several times to ensure that it reacts with all oxygen
in the sample of air.
`
Determining the presence and percentage composition of oxygen in air by heating copper
Observations and findings
2.
The final volume of air in the syringe, at the end of the experiment,
is less than that of the original volume. This is because oxygen in the
original air has combined with copper
The final volume of air in the syringe, at the end of the experiment,
is less than that of the original volume. This is because oxygen in the
original air has combined with copper
Model of result
The volume of air in the syringe at different heating and cooling is as shown below:
Initial volume before heating = 100
Volume after first heating and cooling = 82
Volume after third heating and cooling = 79
The volume of oxygen used up = Initial volume of air before cooling – volume of air after the last heating and cooling
= 100 – 79
= 21
The presence of carbon dioxide in air
Carbon
dioxide is present in air to the extent of 0.03% by volume. The gas is
formed during the combustion of all common fuels – wood, coal, coke,
natural gas, petrol, diesel, paraffin oil, etc, all of which contain
carbon.
dioxide is present in air to the extent of 0.03% by volume. The gas is
formed during the combustion of all common fuels – wood, coal, coke,
natural gas, petrol, diesel, paraffin oil, etc, all of which contain
carbon.
It
is breathed out as a waste product of respiration by all animals. All
sorts of combustion and burning produce carbon dioxide. The gas produced
by all these processes accumulates in air. However, the amount of
carbon dioxide in air remains constant instead of the tremendous
quantities released into the atmosphere. This is because plants take up
carbon dioxide. They then convert it into complex starchy compounds
during photosynthesis. The gas also dissolves in ocean water and other
water bodies.
is breathed out as a waste product of respiration by all animals. All
sorts of combustion and burning produce carbon dioxide. The gas produced
by all these processes accumulates in air. However, the amount of
carbon dioxide in air remains constant instead of the tremendous
quantities released into the atmosphere. This is because plants take up
carbon dioxide. They then convert it into complex starchy compounds
during photosynthesis. The gas also dissolves in ocean water and other
water bodies.
The
presence of carbon dioxide in air can be shown by passing air through a
test tube containing some limewater (figure 6.5). After a time, the
limewater turns milky. This shows the presence of carbon dioxide.
presence of carbon dioxide in air can be shown by passing air through a
test tube containing some limewater (figure 6.5). After a time, the
limewater turns milky. This shows the presence of carbon dioxide.
The reaction involved is as follows:
“
Testing for the presence of carbon dioxide in air
The presence of water vapour in air
Water
vapour is present in air in varying quantities. It is given off by
evaporation from the oceans, lakes and rivers. The presence of water
vapour in air can be demonstrated by exposing deliquescent substances to
the air on a watch glass. These are substances which when exposed to
air tend to absorb much moisture from the air, dissolve in that
moisture, and finally form a solution. Examples of deliquescent
substances include calcium chloride, sodium hydroxide and phosphorous
pentoxide.
vapour is present in air in varying quantities. It is given off by
evaporation from the oceans, lakes and rivers. The presence of water
vapour in air can be demonstrated by exposing deliquescent substances to
the air on a watch glass. These are substances which when exposed to
air tend to absorb much moisture from the air, dissolve in that
moisture, and finally form a solution. Examples of deliquescent
substances include calcium chloride, sodium hydroxide and phosphorous
pentoxide.
The
resulting solution is distilled. The colourless liquid obtained from
distillation may be proved to be water by various water tests such as
use of cobalt chloride paper or anhydrous copper (II) sulphate. The
cobalt chloride paper turns from blue to pink in the presence of water.
The white anhydrous copper (II) sulphate turns blue. Any of the two
tests confirms the presence of water.
resulting solution is distilled. The colourless liquid obtained from
distillation may be proved to be water by various water tests such as
use of cobalt chloride paper or anhydrous copper (II) sulphate. The
cobalt chloride paper turns from blue to pink in the presence of water.
The white anhydrous copper (II) sulphate turns blue. Any of the two
tests confirms the presence of water.
Alternatively,
one may expose the anhydrous copper (II) sulphate salt to open air
straight away for quite some time and then observe any change in its
colour and/or form. Upon absorption of water vapour from the air, the
white, powdery and anhydrous copper sulphate salt turns into hydrated
blue crystals.
one may expose the anhydrous copper (II) sulphate salt to open air
straight away for quite some time and then observe any change in its
colour and/or form. Upon absorption of water vapour from the air, the
white, powdery and anhydrous copper sulphate salt turns into hydrated
blue crystals.
The noble (rare) gases
About
1% of the air by volume is made up of the noble gases. The most
abundant of the noble gases is argon. Others are neon, xenon, krypton
and helium. The proportion of these four is very minute. Argon and neon
are used in “gas-filled” electric light bulbs and coloured “neon”
electrical signs. They are obtained from liquefied air.
1% of the air by volume is made up of the noble gases. The most
abundant of the noble gases is argon. Others are neon, xenon, krypton
and helium. The proportion of these four is very minute. Argon and neon
are used in “gas-filled” electric light bulbs and coloured “neon”
electrical signs. They are obtained from liquefied air.
Air pollutants
The
air always contains small quantities of many gases. Such gases include
hydrogen sulphide, sulphur dioxide, as well as dust and other solid
particles, especially in industrial areas. These gases are given off
during the combustion of coal, and the fuels resulting from coal.
air always contains small quantities of many gases. Such gases include
hydrogen sulphide, sulphur dioxide, as well as dust and other solid
particles, especially in industrial areas. These gases are given off
during the combustion of coal, and the fuels resulting from coal.
SEPARATION OF AIR INTO ITS CONSTITUENT GASES
The
air we breathe is necessary to keep us alive. It is also a chemical
resource. Oxygen is used in steel making, and nitrogen is used in making
fertilizers. To use these gases in this way, they must be separated
from the atmospheric air. Air, as we studied in chapter 5, is a mixture
of different gases. The method used to separate its constituent gases is
fractional distillation. The gases have to be liquefied so that the
mixture can be fractionally distilled.
air we breathe is necessary to keep us alive. It is also a chemical
resource. Oxygen is used in steel making, and nitrogen is used in making
fertilizers. To use these gases in this way, they must be separated
from the atmospheric air. Air, as we studied in chapter 5, is a mixture
of different gases. The method used to separate its constituent gases is
fractional distillation. The gases have to be liquefied so that the
mixture can be fractionally distilled.
The
process of separating the air into its constituent gases is difficult.
It cannot be done in the laboratory. It is only done in industry. The
chemical industry needs the gases from the air in their pure form.
process of separating the air into its constituent gases is difficult.
It cannot be done in the laboratory. It is only done in industry. The
chemical industry needs the gases from the air in their pure form.
The fractional distillation of air involves essentially two stages:
- First, the air must be cooled until it turns into a liquid.
- Then, the liquid air is allowed to warm up again. The various gases boil off at different temperatures
Stage 1: Liquefaction of air
- Air is filtered to remove any dust particles (purification).
- The air is cooled to -180°C to remove the water vapour and carbon dioxide.
- The air is then compressed to 100-150 atmospheres. As the compressed air gets very hot, it has to be cooled.
- The
compressed cooled air is allowed to expand rapidly. The rapid expansion
cools the air to very low temperatures, and the liquid drops out. At
-200°C, only helium and neon remain as gases. The cold gases are used to
cool the compressed air.
Stage 2: Fractional distillation of liquid air
The
air is cooled and compressed to form liquid air. The liquid air is
allowed to warm up. Nitrogen boils off first because it has a low
boiling point, -196°C. Argon follows by boiling at -186°C and finally
oxygen at -183°C
air is cooled and compressed to form liquid air. The liquid air is
allowed to warm up. Nitrogen boils off first because it has a low
boiling point, -196°C. Argon follows by boiling at -186°C and finally
oxygen at -183°C
Figure above illustrates all the steps that take place during the process.
Fractional distillation of liquid air
Combustion
The Concept of Combustion
Explain the concept of combustion
Combustion
of a substance in oxygen or air is so common that it becomes almost a
habit to use the word “combustion” as if it referred to this kind of
reaction alone. In real sense, it may be applied to any chemical
reaction accompanied by light and heat in which one or more of the
reactants are gaseous.
of a substance in oxygen or air is so common that it becomes almost a
habit to use the word “combustion” as if it referred to this kind of
reaction alone. In real sense, it may be applied to any chemical
reaction accompanied by light and heat in which one or more of the
reactants are gaseous.
Many
common substances burn in air. Substances such as coal, wood, kerosene,
petrol, etc, burn in air. Any substance that burns is called a
combustible material. The air or oxygen that supports the combustion is
called a supporter of combustion. This is because we live in an
atmosphere of air that contains oxygen, which is a very reactive gas.
The gas surrounds any burning material. Oxygen is regarded as a
supporter of combustion. However, it can sometimes combine chemically
with the burning substance to produce a new substance, as we shall see
later.
common substances burn in air. Substances such as coal, wood, kerosene,
petrol, etc, burn in air. Any substance that burns is called a
combustible material. The air or oxygen that supports the combustion is
called a supporter of combustion. This is because we live in an
atmosphere of air that contains oxygen, which is a very reactive gas.
The gas surrounds any burning material. Oxygen is regarded as a
supporter of combustion. However, it can sometimes combine chemically
with the burning substance to produce a new substance, as we shall see
later.
Combustion
of a substance involves its reaction with oxygen and the release of
energy. These reactions are exothermic and often produce a flame. An
exothermic reaction is the one that is accompanied by release of heat to
the surrounding environment. Combustion in which a flame is produced is
described as burning. During burning energy is given out in the form of
heat, light and sound.
of a substance involves its reaction with oxygen and the release of
energy. These reactions are exothermic and often produce a flame. An
exothermic reaction is the one that is accompanied by release of heat to
the surrounding environment. Combustion in which a flame is produced is
described as burning. During burning energy is given out in the form of
heat, light and sound.
The Combustion of Different Substances in Air and Analyse the Products
Demonstrate the combustion of different substances in air and analyse the products
Many different substances burn in air to produce different products. Here are examples of combustion of some common substances:
Sulphur
This is a yellow powder. When burnt in air, it gives misty fumes of sulphur dioxide gas.
Copper
When
a piece of copper foil in a pair of tongs is held in a Bunsen flame, it
becomes red-hot. On cooling, a black layer of some substance is
observed. This black substance is copper oxide. The reaction occurs
thus:
a piece of copper foil in a pair of tongs is held in a Bunsen flame, it
becomes red-hot. On cooling, a black layer of some substance is
observed. This black substance is copper oxide. The reaction occurs
thus:
Magnesium
When
one end of a piece of magnesium ribbon in tongs is placed in a Bunsen
flame, it burns with a dazzling flame leaving a white ash. This white
ash is magnesium oxide.
one end of a piece of magnesium ribbon in tongs is placed in a Bunsen
flame, it burns with a dazzling flame leaving a white ash. This white
ash is magnesium oxide.
Hydrocarbons
Candle
wax is a hydrocarbon. When it burns in air, the carbon and hydrogen of
the wax react with the oxygen of the air to give carbon dioxide and
water vapour respectively.
wax is a hydrocarbon. When it burns in air, the carbon and hydrogen of
the wax react with the oxygen of the air to give carbon dioxide and
water vapour respectively.
These
are substances containing carbon and hydrogen only. The burning of
these organic substances produces carbon dioxide and water vapour as the
main products. If oxygen supply is low, combustion is incomplete and
carbon monoxide may be formed.
are substances containing carbon and hydrogen only. The burning of
these organic substances produces carbon dioxide and water vapour as the
main products. If oxygen supply is low, combustion is incomplete and
carbon monoxide may be formed.
Coal
Coal is a solid fuel that will burn in air to give the following products:
The Application of Combustion in Real Life
Describe the application of combustion in real life
1.
The combustion of a natural gas is an important source of energy for
homes and industry. Natural gas is mainly methane. Its complete
combustion produces carbon dioxide and water vapour.
The combustion of a natural gas is an important source of energy for
homes and industry. Natural gas is mainly methane. Its complete
combustion produces carbon dioxide and water vapour.
Substances like methane, which undergo combustion readily and give out large amount of energy, are known as fuels.
2.
There are some reactions where fuels and other substances burn to
produce a flame. These are combustion reactions. There are also other
combustion reactions (exothermic) where no flame is evident. The most
important of these is the crucial biochemical reaction that releases
energy in our body cells called cellular respiration.
There are some reactions where fuels and other substances burn to
produce a flame. These are combustion reactions. There are also other
combustion reactions (exothermic) where no flame is evident. The most
important of these is the crucial biochemical reaction that releases
energy in our body cells called cellular respiration.
Our
bodies need energy to make possible the reactions that take place in
our cells. These reactions allow us to carry out our everyday
activities. We need energy to stay alive. We get this energy from food.
During digestion, food is broken down into simpler substances. For
example, the carbohydrates in rice, potatoes and bread are broken down
to form glucose. The combustion of glucose with oxygen in the cells of
our body provides energy.
bodies need energy to make possible the reactions that take place in
our cells. These reactions allow us to carry out our everyday
activities. We need energy to stay alive. We get this energy from food.
During digestion, food is broken down into simpler substances. For
example, the carbohydrates in rice, potatoes and bread are broken down
to form glucose. The combustion of glucose with oxygen in the cells of
our body provides energy.
The reaction is exothermic and is known as cellular respiration.
3. We combust fuels to heat homes and keep ourselves warm, cook our food, and even burn wastes.
4.
Combustion of fuels in automobile engines produces power (energy). This
energy is supplied to different parts of motor vehicles to make them
move from one point to another or carry out some crucial activities such
as grinding, pumping, hauling etc. The operation of such machines could
be impossible without combustion of fuel that produces energy to make
them work.
Combustion of fuels in automobile engines produces power (energy). This
energy is supplied to different parts of motor vehicles to make them
move from one point to another or carry out some crucial activities such
as grinding, pumping, hauling etc. The operation of such machines could
be impossible without combustion of fuel that produces energy to make
them work.
5. Combustion of fuel in different burners produces heat and light used for different purposes in a chemistry laboratory.
6.
Extraction of metals: Moderately reactive metals such as zinc, iron and
lead are roasted in a special furnace (kiln) to form oxides. The
resulting oxides are then reduced with carbon to get the pure metal.
This process of extracting a metal from its ore by heating is called smelting.
Extraction of metals: Moderately reactive metals such as zinc, iron and
lead are roasted in a special furnace (kiln) to form oxides. The
resulting oxides are then reduced with carbon to get the pure metal.
This process of extracting a metal from its ore by heating is called smelting.
7.
In metallurgical industry, combustion is used during welding. Welding
is the process of joining metals by melting the parts and then using a
filler to form a joint. It can be done using different energy sources,
including a gas flame.
In metallurgical industry, combustion is used during welding. Welding
is the process of joining metals by melting the parts and then using a
filler to form a joint. It can be done using different energy sources,
including a gas flame.
Fire fighting
Firefighting
is the act of extinguishing destructive fires. A fire fighter fights
these fires to prevent destruction of life, property and the
environment. Firefighting is a highly technical profession that requires
training and education in order to become proficient.
is the act of extinguishing destructive fires. A fire fighter fights
these fires to prevent destruction of life, property and the
environment. Firefighting is a highly technical profession that requires
training and education in order to become proficient.
Types of Fires According to their Causes
Classify types of fires according to their causes
Before
starting to fight the fire, it is important to know the size and type
of the fire that you are going to put off. The kind of firefighting
material you are going to use will also depend on the type of fire in
question. Fires are classified based on the type of burning materials.
starting to fight the fire, it is important to know the size and type
of the fire that you are going to put off. The kind of firefighting
material you are going to use will also depend on the type of fire in
question. Fires are classified based on the type of burning materials.
1. Class A fires
These
are the fires in which the burning materials are ordinary combustible
materials such as paper, wood, cardboard, coal, rubber, clothing,
furniture and most plastics. Water is the best extinguisher for these
fires. However, any other type of extinguisher, except carbon dioxide,
may be used.
are the fires in which the burning materials are ordinary combustible
materials such as paper, wood, cardboard, coal, rubber, clothing,
furniture and most plastics. Water is the best extinguisher for these
fires. However, any other type of extinguisher, except carbon dioxide,
may be used.
2. Class B fires
These
fires involve flammable liquids such as petrol, kerosene, oil, alcohol,
ether, vanishes, etc. For small fires, a fire blanket or sand may be
used. If the fire is large, use foam, dry powder or carbon dioxide
extinguisher. Water should not be used on class B fires because the
burning material, being lighter than water, will just float and spread
the fire further.
fires involve flammable liquids such as petrol, kerosene, oil, alcohol,
ether, vanishes, etc. For small fires, a fire blanket or sand may be
used. If the fire is large, use foam, dry powder or carbon dioxide
extinguisher. Water should not be used on class B fires because the
burning material, being lighter than water, will just float and spread
the fire further.
3. Class C fires
The
burning material involves flammable gases e.g. hydrogen, acetylene,
coal gas, butane, methane, propane, etc. The best extinguishers to use
in fighting against these fires are foam, dry powder or carbon dioxide
extinguishers. It is important to turn off the gas supply, and spray
water on the gas tank to cool it down.
burning material involves flammable gases e.g. hydrogen, acetylene,
coal gas, butane, methane, propane, etc. The best extinguishers to use
in fighting against these fires are foam, dry powder or carbon dioxide
extinguishers. It is important to turn off the gas supply, and spray
water on the gas tank to cool it down.
4. Class D fires
The
burning material is a metal. Alkali metals such as sodium or potassium
may catch fire when they come in contact with water and oxygen. At high
temperatures, many metals react with oxygen vigorously. Fires that
involve burning metals should not be extinguished by water. This is
because the burning metal can react with water to give hydrogen (another
potential fuel). The appropriate extinguisher to use is foam or dry
powder extinguisher.
burning material is a metal. Alkali metals such as sodium or potassium
may catch fire when they come in contact with water and oxygen. At high
temperatures, many metals react with oxygen vigorously. Fires that
involve burning metals should not be extinguished by water. This is
because the burning metal can react with water to give hydrogen (another
potential fuel). The appropriate extinguisher to use is foam or dry
powder extinguisher.
5. Class E fires
These
fires involve electrical equipment such as appliances, wiring, circuit
breakers and outlets. You may use carbon dioxide or dry powder
extinguisher to put off these fires. Never use water as it can conduct
electricity and give an electric shock. Also remember to switch off
power from the mains.
fires involve electrical equipment such as appliances, wiring, circuit
breakers and outlets. You may use carbon dioxide or dry powder
extinguisher to put off these fires. Never use water as it can conduct
electricity and give an electric shock. Also remember to switch off
power from the mains.
6. Class F fires
The
burning material is cooking oil or fat. A cooking oil fire in the
kitchen can be extinguished by covering the pan with a fire blanket or
damp cloth. Foam, dry powder or carbon dioxide extinguishers also work
by cutting off the air supply to the fire. For large fires, wet chemical
extinguishers are recommended.
burning material is cooking oil or fat. A cooking oil fire in the
kitchen can be extinguished by covering the pan with a fire blanket or
damp cloth. Foam, dry powder or carbon dioxide extinguishers also work
by cutting off the air supply to the fire. For large fires, wet chemical
extinguishers are recommended.
Different Types of Fire Extinguishers used to Extinguish Different Types of Fire
Identify different types of fire extinguishers used to extinguish different types of fire
Before
choosing the best fire extinguishers for fighting different types of
fires it is crucial to identify the type of burning materials first, and
hence the type of fire such as:
choosing the best fire extinguishers for fighting different types of
fires it is crucial to identify the type of burning materials first, and
hence the type of fire such as:
Class A: Solids such as paper, wood, clothing, rubber, etc
Class B: Flammable liquids such as paraffin, petrol, oil, spirit, alcohol, etc.
Class C: Flammable gases such as propane, butane, methane, hydrogen, etc
Class D: Metals such as aluminium, magnesium, titanium, etc
Class E: Fires involving electrical equipment such as appliances, circuit breakers and outlets, etc.
Types of fire extinguisher to use for each type of fire
Water extinguisher
This
is the cheapest and most widely used fire extinguisher. It is used for
class A fires. It is not suitable for class B (liquid) fires, or where
electricity is involved.
is the cheapest and most widely used fire extinguisher. It is used for
class A fires. It is not suitable for class B (liquid) fires, or where
electricity is involved.
Foam extinguisher
This
is more expensive than water extinguisher, but more versatile. It is
used for classes A and B fires. Foam spray extinguishers are not
recommended for fires involving electricity, but are safer than water if
mistakenly sprayed onto live electrical apparatus.
is more expensive than water extinguisher, but more versatile. It is
used for classes A and B fires. Foam spray extinguishers are not
recommended for fires involving electricity, but are safer than water if
mistakenly sprayed onto live electrical apparatus.
Dry powder extinguisher
This
is often termed as “multi-purpose” extinguisher, as it can be used on
classes A, B and C fires. It is the best for liquid fires (class B). It
will also efficiently extinguish class C (gas) fires. However, take care
because it can be dangerous to extinguish a gas fire without first
isolating the gas supply. Special powders are available for class D
fires.
is often termed as “multi-purpose” extinguisher, as it can be used on
classes A, B and C fires. It is the best for liquid fires (class B). It
will also efficiently extinguish class C (gas) fires. However, take care
because it can be dangerous to extinguish a gas fire without first
isolating the gas supply. Special powders are available for class D
fires.
When
powder-type extinguishers are used indoors, the powder can obscure
vision or damage goods and machinery. It is also very messy.
powder-type extinguishers are used indoors, the powder can obscure
vision or damage goods and machinery. It is also very messy.
Carbon dioxide extinguisher
Carbon
dioxide is ideal for fires involving electrical apparatus (class E). It
will also extinguish class B (liquid) fires. However, the extinguisher
has no post-fire security and the fire could re-ignite.
dioxide is ideal for fires involving electrical apparatus (class E). It
will also extinguish class B (liquid) fires. However, the extinguisher
has no post-fire security and the fire could re-ignite.
Wet chemical extinguisher
This
is a special extinguisher for class F fires. The extinguisher contains
potassium salts. The salts not only help to cool down the flames but
also form a ‘saponification’ blanket that effectively smothers the
flames with thick, soapy foam.
is a special extinguisher for class F fires. The extinguisher contains
potassium salts. The salts not only help to cool down the flames but
also form a ‘saponification’ blanket that effectively smothers the
flames with thick, soapy foam.
Specialist powder extinguisher
This
is a specialist fire extinguisher for use on class D fires (fires on
combustible metals such as sodium, potassium, magnesium, lithium,
titanium, manganese and aluminium), especially in the form of powder or
turnings.
is a specialist fire extinguisher for use on class D fires (fires on
combustible metals such as sodium, potassium, magnesium, lithium,
titanium, manganese and aluminium), especially in the form of powder or
turnings.
The Components Needed to Start a Fire
State the components needed to start a fire
To
extinguish fire, it is necessary to remove one or more of the three
components of combustion. Any fire needs a fuel, oxygen (air) and heat
to keep it going. Remove any one of them and the fire will go out. These
components are as shown in the fire triangle below.
extinguish fire, it is necessary to remove one or more of the three
components of combustion. Any fire needs a fuel, oxygen (air) and heat
to keep it going. Remove any one of them and the fire will go out. These
components are as shown in the fire triangle below.
The fire triangle
A fire will continue or start to burn if these components are present:
(i) Fuel:
This refers to any combustible material be it solid, liquid or gaseous
material provided it can catch fire and burn. You can stop fire by
removing the combustible material from the path of fire.
This refers to any combustible material be it solid, liquid or gaseous
material provided it can catch fire and burn. You can stop fire by
removing the combustible material from the path of fire.
(ii) Oxygen (air):
Oxygen supports combustion. A fuel will only burn if there is
sufficient supply of oxygen. You can extinguish fire by displacing, or
taking away oxygen supply from the fire or by blocking the gas supply to
the fire.
Oxygen supports combustion. A fuel will only burn if there is
sufficient supply of oxygen. You can extinguish fire by displacing, or
taking away oxygen supply from the fire or by blocking the gas supply to
the fire.
(iii) Heat:
The temperature should be at the kindling point of that fuel or above
it. Every fuel has its own kindling point. Below the kindling point, the
fuel will not catch fire. You can put out fire by lowering the
temperature below the kindling point of a particular fuel. Water may be
used to cool down the fuel. The vapourization of water absorbs the heat;
it cools the smoke, air, walls, objects etc, which could be used as
further fuel.
The temperature should be at the kindling point of that fuel or above
it. Every fuel has its own kindling point. Below the kindling point, the
fuel will not catch fire. You can put out fire by lowering the
temperature below the kindling point of a particular fuel. Water may be
used to cool down the fuel. The vapourization of water absorbs the heat;
it cools the smoke, air, walls, objects etc, which could be used as
further fuel.
Fire Extinguishers According to the Chemicals they Contain
Classify fire extinguishers according to the chemicals they contain
Fire extinguishers are classified according to the type of chemicals they contain
1. Liquid carbon dioxide extinguisher
This
extinguisher contains liquid carbon dioxide. The liquid is contained in
a metal container. When the safety pin is removed, carbon dioxide
evaporates as solid “snow” (carbon dioxide sublimes). The snow settles
on the fire and suffocates it.
extinguisher contains liquid carbon dioxide. The liquid is contained in
a metal container. When the safety pin is removed, carbon dioxide
evaporates as solid “snow” (carbon dioxide sublimes). The snow settles
on the fire and suffocates it.
2. Soda-acid extinguisher
This
extinguisher has a metal case containing soda (aqueous sodium carbonate
or sodium hydrogen carbonate). In the metal case there is a glass
bottle containing a concentrated acid (sulphuric or hydrochloric acid).
There is a knob attached to the top of a metal case. Hitting this knob
breaks the acid bottle thus bringing the acid and the soda into contact.
The two react to give carbon dioxide, e.g.
extinguisher has a metal case containing soda (aqueous sodium carbonate
or sodium hydrogen carbonate). In the metal case there is a glass
bottle containing a concentrated acid (sulphuric or hydrochloric acid).
There is a knob attached to the top of a metal case. Hitting this knob
breaks the acid bottle thus bringing the acid and the soda into contact.
The two react to give carbon dioxide, e.g.
The
gas forms bubbles with the solution, thereby forming foam which is
forced out of a jet of the case. The foam is directed to the fire where
it covers the burning liquid, excluding all air from reaching the fire.
gas forms bubbles with the solution, thereby forming foam which is
forced out of a jet of the case. The foam is directed to the fire where
it covers the burning liquid, excluding all air from reaching the fire.
Some
extinguishers are made in such a way that turning them upside down
brings the soda and acid into contact and the reaction proceeds as
stated above.
extinguishers are made in such a way that turning them upside down
brings the soda and acid into contact and the reaction proceeds as
stated above.
Soda-acid fire extinguishers
3. Foam extinguishers
This
is different from the soda–acid type in that it contains sodium
hydrogen carbonate in the metal case, but instead of the concentrated
acid, it contains aluminium sulphate and saponium in the glass bottle.
On mixing the three components, carbon dioxide gas is produced. The gas
is ejected out as foam. The foam here lasts longer than the foam in the
soda–acid extinguisher. The foam so produced also keeps air away from
the burning material.
is different from the soda–acid type in that it contains sodium
hydrogen carbonate in the metal case, but instead of the concentrated
acid, it contains aluminium sulphate and saponium in the glass bottle.
On mixing the three components, carbon dioxide gas is produced. The gas
is ejected out as foam. The foam here lasts longer than the foam in the
soda–acid extinguisher. The foam so produced also keeps air away from
the burning material.
4. Dry chemical extinguisher
This
extinguisher uses powdered sodium hydrogen carbonate and a nitrogen gas
kept at high pressure. When the gas cartridge is broken using the top
cap, the carbon dioxide under pressure propels the powder. The powder
forms a layer over the burning material to keep air away.
extinguisher uses powdered sodium hydrogen carbonate and a nitrogen gas
kept at high pressure. When the gas cartridge is broken using the top
cap, the carbon dioxide under pressure propels the powder. The powder
forms a layer over the burning material to keep air away.
Table
bellow summarizes the types of fire extinguishers, indicating the
chemicals they contain and the classes of fire they are suitable or
unsuitable for.
bellow summarizes the types of fire extinguishers, indicating the
chemicals they contain and the classes of fire they are suitable or
unsuitable for.
Table: Types fire extinguishers and the chemical composition of their extinguishing agents
| Type | Chemical composition of agent | Suitable for | Unsuitable for |
| APW (Air- pressurized water) | Ordinary tap water pressurized with air | Class A | Class B, C, D and E (will spread the flame and make the fire bigger!) |
| Dry chemical(DC) | Fine sodium bicarbonate powder pressurized with nitrogen | Class A, B, C and E | – Class D- Aircraft and electronics ( corrosive to metals such aluminium)Note: -Though it is safe to use indoors it can obscure vision |
| CO2 | Non-flammable carbon dioxide gas under extreme pressure | Class B, C and E | Class A (leaves a flammable substance on the extinguished material which canre-ignite later ) |
| Halon | Bromochloro-difluoro-methane | Class A and E | Class B and C (least suitable) |
| Foam | Proteins and fluoro-proteins | Class A and B | Class E |
| Wet chemical(WC) | Potassium acetate | Class F | Class E |
| ABC | Mono-ammonium phosphate with a nitrogen carrier | Class A, Class B and C | Electronic equipment (leaves a stick y residue that may be damaging to electrical appliances such as a computer) |
| Specialist powder (SP) | Powders of NaCl, Cu or graphite under extreme pressure | Class D | Class A, B, C, E and F |
Precautions on using fire extinguishers
The following are some safety precautions you have to keep in mind when using fire extinguishers:
- Keep a reasonable distance from the fire as it may suddenly change direction.
- Never use a portable extinguisher on people, instead use a fire blanket.
- Do not test a portable extinguisher to see if it works. It may leak and later fail to work during an emergency.
- Do not return a used portable extinguisher to the wall. Make sure it is recharged first.
- When a fire gets out of control, notify the nearest fire brigade.
Extinguishing Small Fires Using the Right Types of Fire Extinguishers
Extinguish small fires using the right types of fire extinguishers
Activity 1
Extinguish small fires using the right types of fire extinguishers
The Concept of Rusting
Explain the concept of rusting
Rusting
is the name given to the oxidation of iron or steel in damp air. It is
also called corrosion. Rust is hydrated iron (III) oxide. It is a soft,
crumbly solid and hence weakens the structure of iron and steel. During
rusting, iron reacts with oxygen to form brown iron (III) oxide
is the name given to the oxidation of iron or steel in damp air. It is
also called corrosion. Rust is hydrated iron (III) oxide. It is a soft,
crumbly solid and hence weakens the structure of iron and steel. During
rusting, iron reacts with oxygen to form brown iron (III) oxide
At the same time the iron (III) oxide reacts with water to form hydrated iron (III) oxide (or rust):
Note: The x in
the equation indicates that the number of water molecules in the
hydrated iron (III) oxide can vary. So, both oxygen and water are
needed to cause rusting of iron.
the equation indicates that the number of water molecules in the
hydrated iron (III) oxide can vary. So, both oxygen and water are
needed to cause rusting of iron.
Rusting
is a serious economic problem. Large sums of money are spent each year
to replace damaged iron and steel structures, or protecting structures
from such damages. Rusting of bridges, corrugated iron sheets on house
roofs, containers, articles, etc. require an expenditure of big sums of
money as well as labour for replacement. Rust weakens structures such as
car bodies, iron railings, and ships’ hulls, and shortens their useful
life. Preventing it can cost a lot of money. All efforts must be made to
stop iron or steel items from rusting. This can be achieved if we know
the conditions necessary for iron to rust.
is a serious economic problem. Large sums of money are spent each year
to replace damaged iron and steel structures, or protecting structures
from such damages. Rusting of bridges, corrugated iron sheets on house
roofs, containers, articles, etc. require an expenditure of big sums of
money as well as labour for replacement. Rust weakens structures such as
car bodies, iron railings, and ships’ hulls, and shortens their useful
life. Preventing it can cost a lot of money. All efforts must be made to
stop iron or steel items from rusting. This can be achieved if we know
the conditions necessary for iron to rust.
The Conditions Necessary for Iron to Rust
Demonstrate the conditions necessary for iron to rust
When
iron is left in contact with both water and oxygen (or air), it reacts
to form hydrated iron (III) oxide. Iron will not rust on exposure to dry
air or air-free water (water that has been boiled to expel all
dissolved air) only. However, iron will easily and readily rust in water
that has dissolved air in it. In figure 6.8, only the iron nail that is
in contact with both water and air rusts. Therefore, rusting will only
occur in the presence of both water and oxygen. If one of the two
conditions is excluded, in one way or another, rusting will not take
place at all.
iron is left in contact with both water and oxygen (or air), it reacts
to form hydrated iron (III) oxide. Iron will not rust on exposure to dry
air or air-free water (water that has been boiled to expel all
dissolved air) only. However, iron will easily and readily rust in water
that has dissolved air in it. In figure 6.8, only the iron nail that is
in contact with both water and air rusts. Therefore, rusting will only
occur in the presence of both water and oxygen. If one of the two
conditions is excluded, in one way or another, rusting will not take
place at all.
Testing for conditions necessary for iron rusting
Findings
Nails in tube 1 will rust. Nails in tubes 2 and 3 will not rust.
Reasons
In
tube 1, nails are in contact with both water and air (oxygen). In tube
2, the water has been boiled to expel the dissolved air. In addition,
any air above the water is prevented from dissolving in boiled water by a
layer of oil. So, the nails are completely shielded away from air.
Therefore, rusting is impossible. In tube 3, nails are in contact with
air only. The moisture present in air is absorbed by anhydrous calcium
chloride. Any moisture that might have been absorbed by the anhydrous
calcium chloride is prevented from reaching the nails by a tuft of
cotton wool. The cotton wool also absorbs some moisture directly from
the air. Therefore, tube 3 will always carry dry air (moisture-free
air). Hence, no rusting of iron nails occurs.
tube 1, nails are in contact with both water and air (oxygen). In tube
2, the water has been boiled to expel the dissolved air. In addition,
any air above the water is prevented from dissolving in boiled water by a
layer of oil. So, the nails are completely shielded away from air.
Therefore, rusting is impossible. In tube 3, nails are in contact with
air only. The moisture present in air is absorbed by anhydrous calcium
chloride. Any moisture that might have been absorbed by the anhydrous
calcium chloride is prevented from reaching the nails by a tuft of
cotton wool. The cotton wool also absorbs some moisture directly from
the air. Therefore, tube 3 will always carry dry air (moisture-free
air). Hence, no rusting of iron nails occurs.
This
experiment demonstrates the fact that for iron to rust, both water and
air (oxygen) must be present. If one of these conditions is controlled,
no rusting can take place.
experiment demonstrates the fact that for iron to rust, both water and
air (oxygen) must be present. If one of these conditions is controlled,
no rusting can take place.
Similarity between rusting and burning
Chemically,
rusting and burning are similar processes in that they both require
oxygen. Consider the burning of magnesium to give magnesium oxide.
rusting and burning are similar processes in that they both require
oxygen. Consider the burning of magnesium to give magnesium oxide.
In this process, magnesium combines with the oxygen of the air to form magnesium oxide.
During rusting, iron combines with oxygen of the air in the presence of water to form brown hydrated iron (III) oxide, “rust.”
In
addition, the two processes, burning and rusting, are exactly similar
in that they both generate heat. The only difference is in the time
required for each of the two processes to take place. During rusting
heat is given out, but without being noticed because of its slower rate
of production. Burning produces noticeable heat and light.
addition, the two processes, burning and rusting, are exactly similar
in that they both generate heat. The only difference is in the time
required for each of the two processes to take place. During rusting
heat is given out, but without being noticed because of its slower rate
of production. Burning produces noticeable heat and light.
The Different Methods of Preventing Iron from Rustin
Describe the different methods of preventing iron from rusting
We
have learned that for iron to rust there must be direct contact between
the iron and both water and oxygen from the air. Therefore, in order to
stop rusting we must protect iron from either water (moisture) or
oxygen (air) or both. The following are some of the methods used to
prevent iron from rusting:
have learned that for iron to rust there must be direct contact between
the iron and both water and oxygen from the air. Therefore, in order to
stop rusting we must protect iron from either water (moisture) or
oxygen (air) or both. The following are some of the methods used to
prevent iron from rusting:
Painting
Painting
the iron article creates a waterproof and airproof cover over the
surface of the iron. This method is widespread for objects ranging in
size from ships and bridges to garden gates. Paints that contain lead or
zinc are mostly used. These paints are especially good for preventing
rusting. For example, “red lead” paints contain an oxide of lead,.
the iron article creates a waterproof and airproof cover over the
surface of the iron. This method is widespread for objects ranging in
size from ships and bridges to garden gates. Paints that contain lead or
zinc are mostly used. These paints are especially good for preventing
rusting. For example, “red lead” paints contain an oxide of lead,.
As
oxygen and water cannot reach the iron, it does not rust. However, if
the paint layer is scratched off rusting may occur. So, regular
repainting is necessary to keep this protection intact.
oxygen and water cannot reach the iron, it does not rust. However, if
the paint layer is scratched off rusting may occur. So, regular
repainting is necessary to keep this protection intact.
Oiling and greasing
The
oiling and/or greasing of the moving parts of machinery forms a
protective film, preventing rusting. Moving parts cannot be painted
since the paint layer can be easily scratched off during movement.
Again, the treatment must be repeated to continue the protection.
oiling and/or greasing of the moving parts of machinery forms a
protective film, preventing rusting. Moving parts cannot be painted
since the paint layer can be easily scratched off during movement.
Again, the treatment must be repeated to continue the protection.
Plastic coating
Steel
is coated with plastic for use in garden chairs, refrigerators, bicycle
baskets, dish racks, etc. The plastic PVC (polyvinyl chloride), a trade
name for polychloroethene, is often used for this purpose. Plastic is
cheap and can be made to look attractive.
is coated with plastic for use in garden chairs, refrigerators, bicycle
baskets, dish racks, etc. The plastic PVC (polyvinyl chloride), a trade
name for polychloroethene, is often used for this purpose. Plastic is
cheap and can be made to look attractive.
Electroplating
Electroplating
is the coating of one metal with a layer of another metal by means of
electrolysis, where the metal to be coated is the cathode and the
coating metal the anode.
is the coating of one metal with a layer of another metal by means of
electrolysis, where the metal to be coated is the cathode and the
coating metal the anode.
An
iron or steel object can be electroplated with a layer of chromium or
tin to protect against rusting. A ‘tin can’ is made of steel coated on
both sides with a fine layer of tin. Tin is used because it is
unreactive and non-toxic. However, if protective layer is broken, then
the steel beneath will begin to rust. So, proper handling of tin-plated
items is needed.
iron or steel object can be electroplated with a layer of chromium or
tin to protect against rusting. A ‘tin can’ is made of steel coated on
both sides with a fine layer of tin. Tin is used because it is
unreactive and non-toxic. However, if protective layer is broken, then
the steel beneath will begin to rust. So, proper handling of tin-plated
items is needed.
Galvanizing
An iron object may be covered with a layer of zinc. This is called galvanizing.
Even if the zinc is scratched to expose the iron, the iron does not
rust. This is because zinc is higher in the reactivity series than iron.
So, zinc reacts with water and oxygen in preference to iron.
Even if the zinc is scratched to expose the iron, the iron does not
rust. This is because zinc is higher in the reactivity series than iron.
So, zinc reacts with water and oxygen in preference to iron.
The
zinc layer can be applied by several different methods. These include
electroplating or dipping the object into molten zinc. When an iron or
steel article is dipped into molten zinc and then removed, it becomes
coated with a thin layer of zinc. The zinc forms a protective coat over
the surface of iron. This process is used for dustbins, car bodies,
barbed wires and motorway crash barriers.
zinc layer can be applied by several different methods. These include
electroplating or dipping the object into molten zinc. When an iron or
steel article is dipped into molten zinc and then removed, it becomes
coated with a thin layer of zinc. The zinc forms a protective coat over
the surface of iron. This process is used for dustbins, car bodies,
barbed wires and motorway crash barriers.
Sacrificial protection
This
is a method of rust protection in which blocks of a metal more reactive
than iron are attached to the iron surface. Zinc and magnesium are more
reactive than iron. When blocks of zinc or magnesium are attached to
the hull of a steel ship or oil rig, it corrodes in preference to iron.
This is called sacrificial protection because the zinc or
magnesium is sacrificed to protect the iron. When the blocks are nearly
eaten away, they can be replaced by fresh blocks. Underground gas and
water pipes are connected by wire to blocks of magnesium to obtain the
same protection.
is a method of rust protection in which blocks of a metal more reactive
than iron are attached to the iron surface. Zinc and magnesium are more
reactive than iron. When blocks of zinc or magnesium are attached to
the hull of a steel ship or oil rig, it corrodes in preference to iron.
This is called sacrificial protection because the zinc or
magnesium is sacrificed to protect the iron. When the blocks are nearly
eaten away, they can be replaced by fresh blocks. Underground gas and
water pipes are connected by wire to blocks of magnesium to obtain the
same protection.
It
is not necessary to cover the whole surface of a steel article with the
more reactive metal for sacrificial protection to work. A ship may have
magnesium blocks riveted to its hull every few metres to prevent
rusting of the whole hull.
is not necessary to cover the whole surface of a steel article with the
more reactive metal for sacrificial protection to work. A ship may have
magnesium blocks riveted to its hull every few metres to prevent
rusting of the whole hull.
Blocks of zinc (or magnesium) attached to the hull of a ship
Alloying
Alloys
are mixtures of metals. For example, iron can be mixed with small
quantities of much less reactive metals to form an alloy called stainless steel.
Stainless steel contains iron mixed with chromium, nickel and
manganese. Stainless steel does not rust. It also has a very attractive
appearance. It is used to make cutlery and kitchen equipment.
are mixtures of metals. For example, iron can be mixed with small
quantities of much less reactive metals to form an alloy called stainless steel.
Stainless steel contains iron mixed with chromium, nickel and
manganese. Stainless steel does not rust. It also has a very attractive
appearance. It is used to make cutlery and kitchen equipment.
Use of silica gel
Silica is a common name for silicon dioxide (SiO2).
Silica gel is a granular, vitreous, highly porous form of silica made
synthetically from sodium silicate. Despite its name, silica gel is a
solid. It is used as a desiccant, which absorbs moisture to prevent
rusting of iron items or articles. Most often, a small bag of silica gel
is put inside bags or boxes used for storing or carrying iron items to
absorb any moisture that may cause rusting.
Silica gel is a granular, vitreous, highly porous form of silica made
synthetically from sodium silicate. Despite its name, silica gel is a
solid. It is used as a desiccant, which absorbs moisture to prevent
rusting of iron items or articles. Most often, a small bag of silica gel
is put inside bags or boxes used for storing or carrying iron items to
absorb any moisture that may cause rusting.
CHEMISTRY FORM ONE ALL TOPICS
CHEMISTRY FORM ONE TOPIC 1 & 2
CHEMISTRY FORM ONE TOPIC 3 & 4.
CHEMISTRY FORM ONE TOPIC 5 & 6.
O’LEVEL CHEMISTRY
CHEMISTRY STUDY NOTES, FORM FOUR.
CHEMISTRY STUDY NOTES, FORM THREE.
CHEMISTRY STUDY NOTES, FORM TWO.
CHEMISTRY STUDY NOTES FORM ONE.
Your notes are very helpful, may God bless you so much.
but how about biology notes for o-level sec-schools? I didn't saw them.
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