Question

Washing soda is used for treating .

• A. temporary hardness
• B. permanent hardness
• C. soft water

Answer 1

The correct option is B permanent hardness

Permanent hardness in water is due to the presence of chlorides and sulphates of calcium and magnesium.

$N{a}_{2}C{O}_3$ (washing soda) when added to hard water, precipitates out $C{a}^{+2}$ and $M{g}^{+2}$ ions as insoluble carbonates.

${\mathrm{Na}}_2{\mathrm{CO}}_3\left(\mathrm{aq}\right)+{\mathrm{CaCl}}_2\left(\mathrm{aq}\right)\to {\mathrm{CaCO}}_3\left(\mathrm{ppt}.\right)+2\mathrm{NaCl}\left(\mathrm{aq}\right)$
These precipitates are then easily removed by filtration.