Water is the most important substance needed for survival of living beings. Living beings cannot live long without water. Water is available in plenty on earth. More than three-fourth of the earth’s surface is covered with water in the form of seas, rivers and lakes. It is also found inside the earth’s crust Most of the water that we get from the wells comes from this source.

Sources of Water

The natural sources of water are rain, springs, wells, rivers and seas.

Rain water

Rain water is considered to be the purest form of natural water (distilled water) free from impurities. Water from sea and rivers get evaporated into water vapour by the heat of sun. During this process of evaporation, impurities are left behind. When the water vapours go high up in the air they condense to form clouds. The water drops come down as rain.

Spring water

Springs are formed by percolation of rain water into soil. Springs supply water to wells and lakes.

Well water

The rain water seeps through the soil and goes down and is stored over rocks or hard earth crust. On digging the well this underground water becomes available to us. This is known as well water. This water may not be pure and may contain impurities such as suspended particles, bacteria and other microorganisms.

River water

Rivers are formed by melting of snow on the mountain, and also sometimes from the rain water. River water is also not pure and is not fit for drinking.

Sea water

Out of all the sources, sea water is the largest natural source of water. However, it is also the source of common salt and other important chemicals. It is the most impure form of water. All the impurities dissolved in river water are carried into the sea. As such, sea water cannot be used for drinking purpose because of high salinity and impurities.

Potable and Non-potable water

Potable water means water which is fit for drinking by humans and other animals. It can be consumed with low risk of immediate or long term harm.

Non-potable water is that which is not safe for drinking. It may carry disease causing microbes, and high levels of dissolved salts and minerals, heavy metals and suspended solids. Drinking or using such water for cooking leads to illnesses and may even cause death. Contaminated or non-potable water can be treated to turn it into potable or drinking water.

Purification of Water


By decantation, insoluble impurities can be removed. Decantation is the process of separation of solid from the liquid by allowing the former to settle down and pouring off the latter. Water is kept in a vessel for some time. The suspended insoluble impurities settle down at the bottom. Clean water can now be carefully poured into another clean vessel without disturbing the settled impurities which are left behind. But, this water has to be made fit for drinking through further treatment.


By filtration, the insoluble impurities can be removed. It is a more effective method than decantation and can remove even very fine particles of insoluble impurities. A piece of clean and very fine cloth can be used as a cheap and easily available filter. When water is poured through it, the insoluble impurities are stopped by the filter and clean water passes through it.

Commercially available water filters use "candles" made of porous material. Pure water passes through it leaving the impurities on its outer surface. These candles must be cleaned and washed periodically to maintain their effectiveness.


By boiling, bacteria and other germs in the water get killed. When boiled water is allowed to cool, heavy impurities collect at the bottom and dissolved salts form a thin layer on the surface called scum. Now if we filter the water, it becomes safe for drinking.

Chlorine Treatment

By chlorine treatment small living organisms and bacteria are killed. If required, treated water may be filtered to remove insoluble impurities.

Water as Universal Solvent

Water has a unique property of dissolving a large number of substances starting from solids such as common salt, sugar, to gases like oxygen, carbon dioxide, etc. As so many substances dissolve in water, it is called a universal solvent. This property of water is useful for plants to take their food materials and minerals from the soil. It helps to absorb food that we eat. Many chemical reactions also take place only in aqueous solution.

Hard water and Soft water

Water forms lather with soap which is used for cleaning purposes. It is called soft water. Sometimes water from some sources like rivers or hand pumps does not produce any lather with soap. It is called hard water.

Water, which we get from taps, contain lesser amounts of dissolved salts in it than water that we get from hand pumps. The dissolved salts are usually bicarbonates, sulphates and chlorides of calcium and magnesium. Their presence prevents formation of soap lather.

Soap is a sodium salt called sodium stearate. It is soluble in water. When soap is added to hard water, which contains calcium and magnesium ions, a precipitate of Ca or Mg stearate is formed. These calcium and magnesium steartes are insoluble in water and appears as a greasy scum. The formation of scum in place of lather makes it more difficult to clean things.

Hard water can be converted into soft water, by removal of Ca and Mg ions which are responsible for hardness. This is called softening of water. Hardness of water is of two types:

  • Temporary hardness
  • Permanent hardness

Temporary hardness

Temporary hardness of water is due to the presence of soluble bicarbonates of calcium and magnesium. It is also called carbonate hardness. It can be removed by boiling and by soda lime process.

  • By boiling: Upon boiling hard water, calcium or magnesium bicarbonate present in it are decomposed to give magnesium or calcium carbonate. These carbonate salts are insoluble in water. They settle down easily and water can be decanted.
  • By soda lime process (Clark’s method): When a calculated amount of lime is added to hard water, then the soluble bicarbonates are converted to insoluble carbonates.

Permanent hardness

Permanent hardness of water is due to the presence of soluble chlorides and sulphates of calcium and magnesium. It is also known as non-carbonate hardness. It can be removed by addition of washing soda or by the ion exchange method.

By addition of washing soda: The hard water is treated with the ‘calculated’ quantity of washing soda (sodium carbonate). Washing soda reacts with chloride and sulphate of calcium and magnesium to form precipitate of calcium and magnesium carbonate. The precipitate settles down and can be removed by decantation.

By ion exchange method: Two types of ion exchangers can be used - inorganic ion exchanger and organic ion exchanger. In inorganic ion exchange process, complex compounds known as Zeolite are used to soften the hard water. The salts causing hardness of water are precipitated as insoluble zeolite of calcium and magnesium and are replaced by soluble sodium salts. After using it for sometime the zeolite is regenerated by soaking it in 10% solution of NaCl (brine) and then washing away chlorides. The washings are removed and are replaced by soluble sodium salts.

By using organic ion exchanger, water obtained is free from cations and anions and is known as deionized water or demineralized water.

Polar Nature of Water

Water is a very effective solvent for ionic compounds. Although water is an electrically neutral molecule, it has a small positive charge (on the H atoms) and a negative charge (on the O atom), Therefore, it is polar in nature and can dissolve ionic compounds.

Surface Tension

Surface tension is the property of all the liquids. Due to this tension water drops try to occupy a minimum surface area. 

Hence, water droplets always tend to take the shape of a sphere. The tension exerted by molecules of water present on the surface layer is called surface tension.

Capillarity - Rise of Water

When a capillary tube with a fine bore is dipped in water, water rises in the capillary. The extent to which water rises depends on the diameter of the capillary. The smaller the diameter of the capillary, the higher will be the rise of water in the capillary tube. This property of rise of water inside a capillary is called capillarity or capillary action.

This is the property, by which water from the soil enters the leaves and branches of the plants through the stems. When a piece of cloth or blotting paper is placed in water, it soaks the water by this process of capillary action. The thread strands in the cloth and cellulose of the blotting paper serves like very fine capillaries for the water to rise.

Density of Water

Water behaves in an unusual way when it is heated from 0°C. As the temperature rises from 0°C to 4°C it actually contracts. However, from 4°C upwards it expands like any other liquid. This means that water takes up the least space at  4°C. It has the highest density at this temperature and will sink through warmer or colder water around it.

The surface water cools down to 4°C and sinks to the bottom of the lake due to its high density and hotter water comes up to the surface. Gradually the whole water cools down to 4°C. Further cooling decreases the temperature of surface water which finally freezes. Ice being lighter than water keeps floating on the surface. It acts as an insulator and slows down the cooling and freezing of the lower layers of water.

Water Pollution

Water pollution is the contamination of water bodies like lakes, rivers, ground water and oceans. It occurs due to the discharge of untreated pollutants into water bodies. It not only affects plants and organisms living near the location of discharge but also travels to other locations through transportation of polluted water.