Water
Sources of water: The common sources of water are ponds, lakes, rivers,
wells, and reservoirs.
Water cycle: Water undergoes different processes in the environment and is
found in different states during these processes. This cyclic process through
which water circulates in the environment is called the water cycle.
Importance of water
Water is necessary for germination of seeds, transportation of nutrients
from soil and food from the leaves to different parts of the plant, in
preparation of food through photosynthesis.
Aquatic animals and plants get their nutrients as well as oxygen supply
from the water. These substances are present in water in dissolved form.
Water is used for many other purposes such as in cooking, cleaning,
industrial work, running hydroelectric and thermal electric power plants.
Sea water is also used as a medium of transportation.
A solution has two components, namely the solvent and the solute.
Solvent is that part of the solution in which the other component is dissolved.
In other words, solvent is that component of a mixture that is present in large
amounts.
Solute is that part of the solution that is dissolved in the solution. This is
present in a lesser quantity as compared to the solvent. Also, more than one
solute can be present in a solution.
Properties of a solution
It is a homogeneous mixture of solutes and solvents
The solute particles in a solution are extremely small in size. They are
less than 1 nm (10-9 m) in diameter.
Solute particles are not visible to the naked eye.
As a result of the small size of the solute particles, a solution does not
scatter light.
Solute particles being small in size get dissolved in the solvent. Hence,
the solute cannot be separated from the solution by filtration.
Solute particles do not settle down when left undisturbed.
The addition of harmful substances to water which causes its physical,
chemical and biological properties to change is called water pollution.
Uses of Water
Water is used for many purposes like drinking, washing clothes and utensils,
generating electricity, bathing, irrigation etc.
Water is essential for life.
About 71% of the earth’s surface is covered with water. It is present as ground
water, in seas, oceans, rivers, lakes, ice caps, and in atmosphere.
Wells, rivers, ground water reservoirs or lakes are sources of fresh water.
Ocean and sea water is not potable because they contain large amount of
dissolved salts.
Water is a necessity for every form of life. It is used for domestic activities,
irrigation, industrial purposes, etc.
22nd March is celebrated as World Water day.
Water Cycle: The circulation of water between water bodies, lands and clouds
is known as water cycle.
The change of water into its vapour is known as evaporation.
The change of vapour back into water is known as condensation.
Fall of water from clouds is known as precipitation or rainfall.
Runoff is the process by which rain water returns back to the water
bodies.
Clouds are formed during the process of condensation.
Rain, snow, hail, etc. replenish rivers, lakes, and wells and recharge the ground
water.
1. Water is present in three forms.
2. Solid form − snow and ice
3. Liquid form − oceans, lakes, rivers, and underground water
4. Gaseous form − water vapour in the atmosphere
5. The three forms of water circulate through the water cycle and keep the total
amount of water constant on the earth.
In free state, water occurs in solid, liquid, and gaseous state while in combined
state, it is found in proteins, carbohydrates, etc.
It is colourless, odourless, and tasteless.
It boils at 100°C and freezes at 0°C.
Pure water is a poor conductor of electricity. It has high latent heat of
vaporisation and fusion and specific heat capacity.
Water is a universal solvent as it dissolves maximum number of substances.
It is stable to heat.
Water has anomalous expansion property, i.e. it expands on cooling below 4 0C
and it has maximum density at 4 0C
1. Solubility is defined as the dissolution of solute in particular amount of solvent.
2. Concentration is defined as the amount of solute dissolved in particular amount of
solvent. The solution in which greater amount of solute is dissolved is called as
concentrated solution and the solution in which amount of solute dissolved is less is
called dilute solution.
3. The maximum amount of solute dissolved in particular amount of solvent forms a
saturated solution. For example 35.7 g of salt dissolved in 100 mL of water forms a
saturated solution.
4. The solubility of substance in water can be increased on heating the solution.
5. When the temperature of saturated solution is increased, more quantity of solute
can be added. If this solution is allowed to cool at room temperature and there is no
precipitation of solute, then the resulting solution is called supersaturated solution.
6. Supersaturated solution tends to get disturbed when seed of the crystal is added in it
as it is returned back to its saturated state. So when a crystal is added in such solution,
then deposition of extra solute on the crystal starts. This process is known as
crystallisation.
Water of crystallisation : It refers to a fixed number of water molecules
present in one formula unit of salt.
Example - In gypsum, the water of crystallisation is 2.
Hydrated substances: Substances containing water of crystallisation for
example, hydrated copper sulphate (CuSO4.5H2O).
Anhydrous substances: Substances either not containing water of
crystallisation or from which water of crystallisation is removed, for example,
sodium chloride (NaCl) and anhydrous copper sulphate (CuSO4).
Drying agents: Substances that absorb moisture without undergoing a chemical
reaction, for example, anhydrous calcium chloride (CaCl2).
Dehydrating agents: Substances the remove chemically bonded water from a
compound, for example, concentrated sulphuric acid (H2SO4).
Experiment to Show the Presence of Dissolved Solids in Tap Water
Take some tap water in a beaker and heat it. Place a watch glass over the
beaker and raise the edge of the watch glass from one side by placing a folded
paper. Then pour about 10 mL of tap water into the watch glass. The steam
produced from the boiling beaker water starts evaporating the water in the
watch glass slowly. As the water evaporates slowly, the dissolved solids
deposit and as a result, the concentric rings of the solid materials are formed.
The dissolved salts in water are important because they provide taste to water
and some of them are also required by our body.
Experiment to Show the Presence of Dissolved Gases in Tap Water
Take some tap water in a round bottom flask. Fix a cork fitted with a delivery
tube in mouth of the flask. The lower end of the delivery tube should be in line
with under-surface of the cork. Its other end should be in the beehive shelf,
placed in a trough of water. Heat the water in the round bottom flask. Water
starts boiling and the gas bubbles start coming out of the beehive shelf. Invert
a graduated tube completely filled with tap water over the beehive shelf. The
boiled-off air starts collecting in the tube by replacing water.
Collect the tube filled with boiled off air and introduce a glowing splinter in it.
The glowing splinter will burst into flame. It can be concluded that oxygen is
dissolved in water. Similarly, gases such as nitrogen, carbon dioxide are
dissolved in water. Dissolved oxygen and carbon dioxide in water is important
because of the following reasons:
Oxygen is required by animals living in water during their respiration. It helps
to keep water purified by killing germs and bacteria.
Carbon dioxide is required by water plants during photosynthesis.
Dissolved carbon dioxide reacts with limestone to form soluble calcium
carbonate, which is used by animals living in water to form hard shells for the
protection of their soft bodies.
Pure water can be obtained by the process known as distillation.
Water obtained form distillation process is known as distilled water and is free
from any salts and minerals.
Distilled water should not be used in drinking purpose.
(i) Water containing Ca2+ and Mg2+ ions and their salts is considered to be hard
water.
(ii) Water free from Ca2+ and Mg2+ ions is called soft water.
(iii) Soft water forms lather with soap, while hard water forms insoluble scum with
soap.
1.Types of hardness
(i) Hardness due to the presence of bicarbonate salts of calcium and magnesium is
known as temporary hardness.
(ii) Hardness due to the presence of chloride and sulphate salts of calcium and
magnesium is known as permanent hardness.
(iii) Temporary hardness can be removed by simple methods like boiling, while
permanent hardness is removed by special methods.
2. Disadvantages of using hard water
(i) Causes dryness and leaves whitish residue on skin
(ii) Leaves yellow stain on clothes if used for washing clothes
(iii) Causes scales on inner walls of containers used for boiling and cooking food
3. Advantages of using hard water
(i) Enhances the taste of beverages and wines
(ii) Helps in strengthening of bones and teeth
(iii) Checks lead poisoning of water in lead water pipelines
4. Removal of hardness of water
(i) Temporary hardness can be removed by simple boiling or with Clark’s method
using Ca(OH)2.
(ii) Permanent hardness can be removed by distillation, soda process and permutit
method.
(a) Permutit is a sodium aluminium silicate, also known as zeolite.
(b) The principle of the permutit method is exchange of basic ions, Ca2+ and Mg2+,
with Na+ ion of the permutit.
Removal of hardness of water
(i) Boiling of hard water converts soluble bicarbonate into insoluble carbonate, which
can be removed by simple filtration.
(ii) Certain metal ions like lead, iron and manganese cannot be removed by permutit
method.
(iii) Water obtained from permutit method is soft water and not pure water.
(iv) Water that is free from any ions is known as distilled water.
The addition of harmful substances to water, as a result of which its physical,
chemical, and biological properties get altered, is called water pollution.
Types of water pollutants
Domestic sewage
It is composed of food wastes, detergents, and disease-causing pathogens.
The bacteria present in faecal matter of mammals indicate the pollution
levels in a river and if such water is consumed, it may cause various
diseases.
Industrial waste
It is rich in toxic chemicals such as arsenic, fluorides, and lead.
It causes toxicity in plants and animals.
It affects the soil by causing changes in its acidity and growth of worms.
Agricultural waste
It is rich in agricultural pesticides and weedicides.
It causes ground water pollution.
It causes an increase in the population of algae in water.
When these algae die, they are acted upon by decomposers, which use
lots of oxygen dissolved in water leading to depletion of dissolved
oxygen.
This results in the death of fish and other aquatic organisms.
Release of Superheated Water
The release of superheated water from some industries and nuclear power
plants causes thermal pollution of the water bodies.
The abrupt change in the temperature of water body can kill the fish and
other organisms adapted to particular temperature range.
Methods of preventing water pollution
Industrial waste must be chemically treated to remove harmful substances
before dumping into the water bodies.
Disposal of human and animal excreta into water should be avoided.
Sewage water must be treated before releasing into the rivers.
Water resources
Basic need of life
Most of the Indian agriculture is dependent on monsoons
Local people have adopted traditional methods to conserve water
Traditional water-harvesting systems
Khadins and nadis in Rajasthan
Bandharas and tals in Maharashtra
Bundhis in Madhya Pradesh and Uttar Pradesh
Ahars and Pynes in Bihar
Kulhs in Himachal Pradesh
Ponds in Jammu
Eris in Tamil Nadu
Surangamo in Kerala
Kattas in Karnataka
Traditional water-harvesting structures use crescent-shaped earthen
embankments or low-strait check dams, built across seasonal flooded gullies,
for storing monsoon water and allowing it to seep into the ground.
After the arrival of Britishers, large dams and canal system were introduced for
irrigation purposes.
Although canal system has benefits, it causes inequitable distribution of
resources.
Major river-valley projects like Sardar Sarover Dam on Narmada has resulted
in social crises due to relocation of large number of people and has also resulted
in environmental degradation.
