Question

Explein the procces to remove temperary gardness of water

Answer 1

Removal of temporary hardness:

(1) By boiling, carbon dioxide isn't driven off and the soluble hydrogen carbonates are converted into insoluble carbonates and could be removed by filtration or decantation.

Ca(HCO3) 2------------>CaCO3 + H2O + CO2

Calcium carbonate and magnesium carbonate are precipitated leaving the water soft.

This is however, not very practical when large quantities of water are concerned.

(2) By addition of lime (Clark's process)

A more practical method is to add a calculated quantity of slaked lime. The following reaction takes place.

Ca(HCO3) 2 + Ca (OH) 2-------->2CaCO3 + 2H2O
Mg(HCO3) 2 + Ca (OH) 2-------->MgCO3 + 2H2O

Lime is first thoroughly mixed with water in a tank and then fed into another tank containing the hard water. Revolving paddles thoroughly mix the two solutions. Most of the calcium carbonate settles down. If there is any solid left over is removed by a filter. The process goes by the name of Clark's process.

(3) By the addition of washing soda.

When washing soda or soda ash is added to hard water, the corresponding insoluble carbonates settle down and can be removed by filtration.

Ca(HCO3) 2 + Na2CO3---------->CaCO3 + 2NaHCO3
Mg(HCO3) 2 + Na2CO3--------->MgCO3 + 2NaHCO3