133. What causes the temporary and permanent hardness of water?
Or
Distinguish clearly between temporary hardness and permanent hardness.
Or
What is hardness of water due to? What are its types?

 

The hardness of water is due to the presence of bicarbonates, chlorides and sulphates of calcium and magnesium dissolved in water. It is of two types:

(i) Temporary hardness: It is due to the presence of bicarbonates of calcium, magnesium and iron dissolved in water, it is described as temporary hardness because it is easily removed by simply boiling the water. This type of hardness is also known as carbonate hardness.

(ii) Permanent hardness: It is due to the presence of chlorides and sulphates of calcium, magnesium and iron in the water. This type of hardness is called permanent hardness since it cannot be removed simply by boiling the water. It is also called non-carbonate hardness.




Excess of lime must be avoided because a solution of slaked lime shows hardness again due to absorption of CO₂ from the air.