Question

# Permanent hardness of water can be removed by:(a) Heating(b) Treating with sodium acetate $(C{H_2}C{O_2}Na)$(c) Treating with $Ca{\left( {HC{O_3}} \right)_2}$(d) Treatment with sodium hexametaphosphate $(N{a_6}{P_6}{O_{18}})$

Calgon process:In this process, $C{a^{2 + }}$ and $M{g^{2 + }}$ present in hard water are removed by the treatment with sodium hexametaphosphate $(N{a_6}{P_6}{O_{18}})$. The trade name of sodium hexametaphosphate is Calgon (which means calcium gone). Therefore, from above we can conclude that option D is the correct option to the given question.
Note: It should be remembered that when Calgon is added to hard water, then $C{a^{2 + }}$ and $M{g^{2 + }}$ present in it combine with sodium hexametaphosphate to form soluble complex of calcium and magnesium salts.