Question

Permanent hardness is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water. Permanent hardness of water can be removed by:

• A. Boiling
• B. Clark's method
• C. Treatment with sodium carbonate
• D. None of these

Answer 1

The correct option is C Treatment with sodium carbonate

Water softeners which are used to remove permanent hardness are washing soda (sodium carbonate), permutit, calgon etc.
Insoluble carbonates are formed when washing soda reacts with the sulfide and chloride salts of magnesium and calcium and thus hard water is converted to soft water.
$C{a}^{2+}(aq.)+N{a}_{2}C{O}_3(aq.)\to CaC{O}_3\left(s\right)+2N{a}^{+}(aq.)$

$M{g}^{2+}(aq.)+N{a}_{2}C{O}_3(aq.)⟶MgC{O}_3\left(s\right)+2N{a}^{+}(aq.)$