Question

Hardness of water can be removed by :

• A. adding calcium hydroxide
• B. ion-exchange method
• C. both a and b
• D. none of above

Answer 1

Answer: (C)

Hardness of water can be removed by the following two methods:
CLARK'S METHOD
In Clarks method a calculated amount of calcium hydroxide $Ca(OH{)}_2$ is added to hard water. Due to reaction, insoluble carbonates are obtained which are separated by filtration.
$Ca(HC{O}_3{)}_2+Ca(OH{)}_{2}2CaC{O}_3+2H_{2}O$
$Mg(HC{O}_3{)}_2+Ca(OH{)}_{2}CaC{O}_3+MgC{O}_3+2H_{2}O$
ION-EXCHANGE METHOD
In this method sodium zeolite is used as ion-exchanger. Na-zeolite is passed through the pipes containing hard water. Sodium zeolite is converted into calcium-zeolite or magnesium-zeolite. These are insoluble in water and are separated from water by filtration.
$Ca+2+Na2-zeoliteCa-zeolite+2Na+1$
$Mg+2+Na2-zeoliteMg-zeolite+2Na+1$