Question

Assertion (A): Hardness of water is removed by treatment with sodium carbonate.
Reason (R): Sodium carbonate reacts with soluble magnesium and calcium chloride to form water soluble compounds.

• A. (A) and (R) both are correct and (R) is the correct explanation of (A).
• B. (A) and (R) both are correct but (R) is not the correct explanation of (A).
• C. (A) is correct but (R) is not correct.
• D. (A) is not correct but (R) is correct.

Answer 1

The correct option is C (A) is correct but (R) is not correct.

Hardness of water is due to the presence of chlorides and sulphates of calcium and magnesium. It can be removed by adding sodium carbonate (${\text{Na}}_2{\text{CO}}_3$).

Hard water forms insoluble compound (either calcium carbonate or magnesium carbonate) on reaction with sodium carbonate. This insoluble compound is later filtered out. The reaction involved is:

${\text{CaCl}}_2\text{(aq)}+{\text{Na}}_2{\text{CO}}_3\text{(s)}\to {\text{CaCO}}_3\text{(s)}+\text{NaCl(aq)}$

Hence, the assertion is correct but the reason is not correct.