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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