Question

Permanent hardness of water can be removed by:

• A. Boiling
• B. Addition of washing soda
• C. Addition of potassium permanganate
• D. Addition of slaked lime

Answer 1

The correct option is B

Addition of washing soda

The permanent hardness of water is due to the dissolved salts of chlorides and sulphates of magnesium and calcium. This can be removed by addition of washing soda which reacts with salts of magnesium and calcium to form insoluble carbonates. The insoluble precipitates are then filtered out. The reactions occur as follows:

${\text{Na}}_2{\text{CO}}_3\text{(s)}+{\text{CaSO}}_4\text{(aq)}\to {\text{CaCO}}_3\text{(s)}\downarrow +{\text{Na}}_2{\text{SO}}_4\text{(aq)}$

${\text{Na}}_2{\text{CO}}_3\text{(s)}+{\text{MgCl}}_2\text{(aq)}\to {\text{MgCO}}_3\text{(s)}\downarrow +\text{2NaCl(aq)}$