ICSE Class 8 Answered

Removal of temporary hardness:

1. By boiling, carbon dioxide is driven off and the soluble hydrogen carbonates are converted into insoluble carbonates and can be removed by filtration or decantation.

    Ca(HCO₃)₂  → CaCO₃ ↓ + H₂O + CO₂ ↑

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

This method is not useful for large quantity of water.

In this method calculated quantity of slaked lime is added. Lime is first added to water in a tank and then mixed with hard water in another tank.

The two the solutions are mixed thoroughly by revolving paddles. The calcium carbonate settles down and left over solid is removed by filtration.

 Ca(HCO₃)₂ +  Ca(HO)₂ → 2CaCO₃ ↓ + H₂O

This process goes by the name of Clark’s process.

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

  Ca(HCO₃)₂ + Na₂CO₃  → 2CaCO₃ ↓ + NaHCO₃