Question

Question 15

Which of the following ions will cause hardness in water sample?

(a) $C{a}^{2+}$ (b) $N{a}^{+}$ (c) $C{l}^{-}$ (d) $K^{+}$

Answer 1

(b)

Bicarbonates, chloride and sulphates of Ca and Mg are responsible for the hardness of water.

Note Hard water forms scum/precipitate with soap. Soap containing sodium stearate $\left(C_{17}{H}_{35}COONa\right)$ reacts with hard water to precipitate out Ca/Mg sterarate.

$2C_{17}H35COONa\left(aq\right)+M^{2+}\left(aq\right)\to (C_{17}{H}_{35}COO{)}_{2}M\downarrow +2N{a}^{+}(aq)$ (where, M=Ca/Mg)

It is unsuitable for laundry and boilers.