Softening of Water :

The process of the removal of hardness from water is called softening of water.

(a) Removal of temporary hardness: It can be removed by the following methods,

• By boiling : During boiling, the bicarbonates of Ca and Mg decompose into insoluble carbonates and give CO₂. The insoluble carbonates can be removed by filtration.

• Clark’s method : Thisprocess is used on a commercial scale. Inthis process, calculated amount of lime [Ca(OH)₂] is added to temporary hard water.

(b) Removal of permanent hardness: Permanent hardness can be removed by the following methods,

• By washing soda method: In this method, water is treated with a calculated amount of washing soda (Na₂CO₃) which converts the chlorides and sulphates of Ca and Mg into their respective carbonates which get precipitated.

• Permutit method: This is a modern method employed for the softening of hard water. hydrated sodium aluminium silicate (Na₂Al₂Si₂O₈.xH₂O) is called permutit. These complex salts are also known as zeolites.

The permutit as loosely packed in a big tank over a layer of coarse sand. Hard water is introduced into the tank from the top. Water reaches the bottom of the tank and then slowly rises through the permutit layer in the tank. The cations present in hard water are exchanged for sodium ions. Therefore this method is also called ion exchange method.

where Z = Al₂Si₂O₈.xH₂O