Question

What is meant by ‘demineralised’ water and how can it be obtained?

Answer 1

Demineralised water is free from all soluble mineral salts. It does not contain any anions or cations.
Demineralised water is obtained by passing water successively through a cation exchange (in the $H^{+}$ form) and an anion exchange (in the $O{H}^{-}$ form) resin.
During the cation exchange process, $H^{+}$ exchanges for $N{a}^{+},M{g}^{2+},C{a}^{2+}$ and other cations present in water.
$2R{H}_{\left(s\right)}+M_{\left(aq\right)}^{2+}\rightleftharpoons M{R}_{2\left(s\right)}+2H_{\left(aq\right)}^{+}\cdots \left(1\right)$
In the anion exchange process, $O{H}^{-}$ exchanges for anions such as $C{O}_3^{2-},S{O}_4^{2-},C{l}^{-},HC{O}_3^{-}$ etc. present in water.
$RN{H}_{2\left(s\right)}+H_2{O}_{\left(l\right)}\rightleftharpoons RN{H}_3^{+}.O{H}_{\left(s\right)}^{-}$
$RN{H}_3^{+}O{H}_{\left(s\right)}^{-}+X_{\left(aq\right)}^{-}\rightleftharpoons RN{H}_3^{+}.X_{\left(s\right)}^{-}+O{H}_{\left(aq\right)}^{-}\cdots \left(2\right)$
$O{H}^{-}$ ions liberated in reaction (2) neutralize $H^{+}$ ions liberated in reaction (1), thereby forming water.
$H_{\left(aq\right)}^{+}+O{H}_{\left(aq\right)}^{-}\to H_2{O}_{\left(l\right)}$