Question

Assertion (A): Temporary hardness of water can be removed by boiling.

Reason (R): Bicarbonates of calcium and magnesium undergo thermal decomposition.

• A. Both A and R are true and R is the correct explanation of A.
• B. Both A and R are true, but R is not the correct explanation of A.
• C. A is true and R is false.
• D. A is false and R is true.

Answer 1

The correct option is A

Both A and R are true and R is the correct explanation of A.

Explanation for Assertion:

• Boiling removes only temporary hardness because soluble Bicarbonates of Calcium ($Ca{\left({\mathrm{HCO}}_3\right)}_2$) and Magnesium ( $\mathrm{Mg}{\left({\mathrm{HCO}}_3\right)}_2$) undergo decomposition on heating and insoluble carbonates are formed which can be filtered out.

Explanation for Reason:

• When Magnesium bicarbonate is heated, it breaks down to produce Magnesium carbonate $\left(\mathrm{Mg}{\mathrm{CO}}_3\right)$, Carbon dioxide $\left({\mathrm{CO}}_2\right)$and water $\left({\mathrm{H}}_2\mathrm{O}\right)$
• $\underset{\left(\mathrm{Magnesium}\mathrm{bicarbonate}\right)}{\mathrm{Mg}{\left({\mathrm{HCO}}_3\right)}_2\left(\mathrm{s}\right)}\stackrel{∆}{\to }\underset{\left(\mathrm{Magnesium}\mathrm{carbonate}\right)}{{\mathrm{MgCO}}_3\left(\mathrm{s}\right)}+\underset{(\mathrm{Carbon}\mathrm{dioxide})}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}+\underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$.
• When Calcium carbonate is heated, it breaks down to produce Calcium carbonate $\left({\mathrm{CaCO}}_3\right)$, Carbon dioxide and water.
• $\underset{\left(\mathrm{Calcium}\mathrm{bicarbonate}\right)}{Ca{\left({\mathrm{HCO}}_3\right)}_2\left(\mathrm{s}\right)}\stackrel{∆}{\to }\underset{\left(\mathrm{Calcium}\mathrm{carbonate}\right)}{{\mathrm{MgCO}}_3\left(\mathrm{s}\right)}+\underset{(\mathrm{Carbon}\mathrm{dioxide})}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}+\underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$

Hence Both A and R are true and R is the correct explanation of A.

Hence the correct option is (A)