Some of the waters produce lather easily with soap whereas others produce lather with difficulty. A water which produces lather with difficulty is known as hard water and which produces lather readily with soap is known as soft water.
In short, hardness of water is defined as soap destroying power of water. Hardness of water is due to the presence of soluble salts i.e. bicarbonates, chlorides and sulphates of calcium and magnesium. Since all these are soluble salts so they remain dissolved in water and are not removed by filtration.
Disadvantages of Hardness of Water:
(i) There is wastage of soap and detergents.
(ii) It is unsuitable for cooking certain vegetables, dal and meat. They take very long time to cook in hard water.
(iii) With hard water clothes are not cleaned properly and they do not have a long life.
(iv) Temporary hard water on boiling leads to deposit of a layer of calcium carbonate on inside walls of boilers and kettles which is known as scaling or furring of boilers.
This layer interferes with smooth action of boilers resulting in loss of their efficiency. Sometimes scaling of boilers may lead to bursting of boilers resulting in serious accidents and deaths.
(v) It is harmful for industrial purposes and also shortens the life of pipes and fixtures in the industries.
(vi) It is harmful to the health as in certain cases it may lead to diarrhea and other digestive disorders.
Types of Hardness of Water:
Hardness of water is of two types:
(a) Temporary hardness.
(b) Permanent hardness.
(a) Temporary Hardness:
Temporary hardness is due to the presence of bicarbonates of calcium and magnesium.
(i) It can be removed by boiling. On boiling the carbon dioxide is expelled out of water and precipitates of calcium carbonate and magnesium carbonate are deposited at the bottom.
Boiling is an expensive method of removing hardness of water on a large scale.
(ii) Temporary hardness can also be removed by adding lime or calcium hydroxide (Clark’s process) to water. Lime absorbs carbon dioxide and precipitates of calcium carbonate are formed which settle at the bottom and separated by filtration.
Ca (HC03)2 + Ca (OH)2 — 2CaC03 + 2H20.
(b) Permanent Hardness:
Permanent hardness is due to the presence of chlorides and sulphates of calcium and magnesium. It is not removed by boiling. However it can be removed by following methods:
(i) When sodium carbonate or soda ash is added to water, it removes both temporary and permanent hardness of water. The sulphate of calcium and magnesium is converted into sodium sulphate.
CaSO4 + Na2CO3———>Na2SO4 + CaCo3
(ii) Base Exchange or Permutit Process:
This process is used for removal of hardness of water on large scale. Sodium permutit is a loose compound of sodium, aluminium and silica. When hard water is passed through it the calcium and magnesium ions are exchanged with sodium ions thus hardness is removed. Therefore this process is also called “base exchange process”.
With continuous use the sodium ions in the permuted get exhausted and become unfit for further removal of hardness of water. Therefore at this stage it is regenerated by adding strong solution of sodium chloride whereby calcium and magnesium is displaced by sodium and it again becomes sodium permuted fit to remove the hardness of water.