Water that does not form an immediate lather with soap is called hard water. Hardness of water is due to the presence of soluble calcium, magnesium or iron compounds. The most common compounds are calcium bicarbonate Ca(HCO3)2, magnesium bicarbonate Mg(HCO3)2, calcium sulphate CaSO4 and magnesium sulphate MgSO4. The addition of soap forms an insoluble scum. The scum consists of insoluble salts of these metals. Removal of these salts from the solution makes the water soft. Water that forms an immediate lather with soap is called soft water. Such water does not have dissolved salts of calcium, magnesium and iron.

Types of Hardness:

Depending upon the behaviour of water towards soap, hardness is divided into two types.

1. Temporary hardness:

Hardness of water due to the presence of soluble bicarbonates of calcium and Magnesium is called temporary hardness. When water containing dissolved carbon dioxide passes over solid carbonates (chalk or limestone deposits etc.), these compounds get dissolved in water. Rainwater and distilled water are always soft because they do not have dissolved (soluble) salts.

Temporary hardness is removed in the following ways:

  1. By boiling the water: On boiling, the soluble bicarbonate is decomposed into insoluble carbonate.
  2. Chemical methods.


By adding slaked lime [Ca(OH)2] to hard water, insoluble carbonates are formed. The insoluble calcium carbonate is the ‘fur’ (or scale) formed in kettles, boilers, pipes, etc.

2. Permanent hardness:

This is due to the presence of chlorides and sulphates of calcium and magnesium. Such a hardness can be removed by the addition of washing soda. This removes both the temporary and the permanent hardness of water.