Home All Posts CHEMISTRY FORM TWO STUDY NOTES TOPIC 3: WATER & TOPIC 4: FUEL...

CHEMISTRY FORM TWO STUDY NOTES TOPIC 3: WATER & TOPIC 4: FUEL AND ENERGY

TOPIC 3: WATER
Occurrence and Nature of Water
The Occurrence and Nature of Water
Describe the occurrence and nature of water
Water
is the most abundant liquid in nature. It is a compound of hydrogen and
oxygen. It occurs on land as seas, oceans, rivers, springs, wells, etc.
It also occurs in the atmosphere as rain, water vapour, clouds, etc.
Water is the essential constituent of animal and plant life. Without
water, no life could exist on earth. All living things need water to
survive. About 60% of the human body by mass is made of water. A human
being needs to drink about 2 litres of water per day to replace the
water lost from the body via sweat, urine, breath, faeces, etc. If you
did not replace this by eating and drinking, you would die in a matter
of days.
Water
is more important than food. A human being can survive without food for
many weeks, but will die in a few days without water. So without water,
no life can be sustained.
Water
is the main constituent of the earth’s surface. 70% of the earth’s
surface is covered by water. The remaining 30% is covered by land.
Types of water
There
are four kinds of natural water namely, rain water, spring and well
water, river water, and lake and sea water. Natural water is never pure.
Water from difference natural sources contains substances dissolved in
it.
Rain water
This
is naturally distilled water. It is almost pure and it contains only
gases and dust dissolved from the air. If the dissolved gases are
acidic, e.g. sulphur dioxide, carbon dioxide or nitrogen dioxide, they
may form “acid rain”. In heavily industrialized countries where emission
of these gases is very great, acid rains have been experienced. Rain
water in non-industrial areas is fairly pure. It is safe to drink though
it is tasteless. The taste in water is due to dissolved substances in
it.
Spring and well water
When
the rain falls, some water sinks into the ground to form ground water.
This water percolates down the earth until it meets layers of impervious
or impermeable (non-porous) rocks, which stop it from percolating or
seeping any further. The ground water may reach the earth’s surface as a
spring. When a whole deep enough is dug to reach the ground water, a
well results. Spring or well water is supposed to be clean, although it
contains dissolved substances. As water passes through the earth, it is
naturally filtered.
River water
River
water contains dissolved and suspended solid materials. The water in
some rivers is very muddy or sandy depending on the nature of the land
from which the river originates and on which it flows. Most of the water
we drink or use at home and industries is from rivers. To make the
river water fit for use, all the substances dissolved and suspended in
it must be removed or filtered.
Lake and sea water
Lakes
and seas receive water from rivers. River water contains dissolved
salts. As it flows through the land, some of its water evaporates into
the air. When it reaches the sea or lake, more water still evaporates.
As a result, sea and lake water will necessarily contain vast quantities
of dissolved substances. Sea water contains about 3.6% by mass of the
dissolved solids. Most of the dissolved solids compose largely of sodium
chloride that can be obtained from sea water in large quantities. Three
quarters of the ocean salts is sodium chloride (common salt).
The Water Cycle
Describe the water cycle
Water
is always on move, travelling a never-ending, cyclical journey between
the earth and the sky. This journey is referred to as the water cycle or hydrological cycle.
The water cycle describes the continuous movement of water on, above
and below the surface of the earth. During its movement, water is
continuously reused and recycled. It also changes its physical state or
form (liquid, vapour, and ice) at various stages in the water cycle.
Figure 3.1 is a diagrammatic representation of the water cycle. It shows
how the water moves around the earth’s environment, changing its form
through the process of evaporation, transpiration (loss of water from
plants), condensation and precipitation (rainfall, snow, hail, fog,
smog, etc.) Stages of the water cycle are described below:
  1. Heat
    from the sun causes water to evaporate from exposed water bodies such
    as oceans, seas, lakes, rivers dams, etc. This causes huge amounts of
    water vapour to float (laden) in the air. The vapour rises up. In the
    cooler upper parts of the atmosphere, the vapour cools and condenses to
    form tiny water droplets. The droplets form clouds.
  2. The clouds
    are drifted by wind. They cool further, and the droplets join to form
    larger drops of water which fall down as rain due to gravitation pull.
    On the other hand, if the air is very cold, they fall as hail, sleet or
    snow. The whole process is called precipitation.
  3. Some rain water
    soaks, and reappears as springs. Some flows over the ground as streams.
    The springs and streams feed rivers. The rivers flow to the ocean, sea
    or lake. The whole cycle starts again.

The water cycle
Water Cycle and Environmental Conservation
Relate water cycle to environmental conservation
Everyone
understands why it is so important to keep our water clean. The fresh
water that is available for use by people, plants and animals must be
clean and safe.
Sometimes
human carelessness pollutes the water system, loading harmful and
unhealthy substances into the system at a rate that exceeds its natural
restorative capabilities. When harmful substances are discarded
(disposed off; dumped) into the environment, they may very well end up
as part of the water cycle. An example of these acts may happen when
untreated municipal and industrial wastes are directed into the water
bodies such as rivers, lakes and seas. These substances are toxic and
may harm human, marine, animal and plant life.
When
chemicals are released into the air, they might well return to the
earth with rain and snow or by simply settling. For example in
industrial areas, sulphur dioxide dissolves in water from the clouds and
with oxygen from the atmosphere to form sulphuric acid.
Sulphur dioxide + water + oxygen gives sulphuric acid = “acid rain”
This
then falls as “acid rain”. The acid rain washes salts from the top
soil. Acidic water and metal salts run into the lakes or rivers. The
introduction of these new substances consequently increases the acidity
and concentration of metal salts in the lake, river or stream. As a
result, fish and other marine life die.
Nitrogen oxides, NOx,
can also cause acid rain. When nitrogen dioxide gas reacts with water
and oxygen in the atmosphere, the result is a weak solution of nitric
acid.
Carbon dioxide also reacts with water in the atmosphere to form a weak carbonic acid (rain water).
Pure
water has a pH of 7.0. Normal rain is slightly acidic because of the
carbon dioxide gas dissolved into it. It has a pH of about 5.5.
It has been confirmed that carbon dioxide (CO2), sulphur dioxide (SO2) and nitrogen oxides (NOx) are the primary causes of acid rain.
When
harmful substances are dumped on land or buried in the ground, they
might well find their way into ground water or surface water. These
substances contaminate the water, which may be someone’s or some
community’s drinking water.
Water
plays an important role in the conservation of the environment and in
determining human settlement and development. It also governs plant and
animal distribution. Animals and plants, as components of the
environment, are mainly concentrated in water or in areas where water is
found.
Plant
roots bind the soil particles together, making the soil compact and
less susceptible to erosion. However, vegetation will only grow and
flourish on land that receives sufficient rainfall. This is possible
only if the water cycle is properly maintained by conserving natural
forests and planting more trees to attract rainfall. So it is obvious
that there is a strong relationship between rainfall (as a crucial stage
of the water cycle) and the vegetation and soil (as components of the
environment).
We
use water from the lakes, rivers, wells or springs to irrigate crop and
non-crop plants. So, when we distort the water cycle in some way or the
other we may not have enough rainfall to fill up rivers or springs from
which we obtain the water we use to conserve our environment
(vegetation).
Properly
watered soils support more plants. We all know that plants absorb
carbon dioxide from the atmosphere, therefore, helping to purify the air
naturally. In addition, plants produce oxygen gas, which is needed by
all living organisms. If there is not enough rainfall, most plants will
die, hence resulting to excessive accumulation of carbon dioxide, which
may rise to toxic levels.
Excessive carbon dioxide in the atmosphere leads to intense heating of the earth’s surface, a phenomenon described as global warming.
The consequence of global warming include encroachment and extension of
desert and arid lands, prolonged droughts, changes in rainfall
patterns, etc.
These
few facts show that there is a strong relationship and correlation
between environmental conservation and the water cycle. Environmental
degradation can lead to serious and irreparable aftermath to the water
cycle.

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