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

Answers (1)
R Rakesh

Demineralized water means free from all types of cations and anions and also soluble mineral salts.

It can be obtained by passing water successively through a cation exchange (in the H^{+} ion form) and anion exchange (in the form of OH^{-}ion) resins:

Cation exchange process- 

2RH(s)+M^{2+}\rightleftharpoons MR_{2}(s)+2H^{+}(aq)   {  exchange for Na^{+}, Ca^{2+},Mg{2+} and other cation present in water.}

Anion exchange process-

OH^{-}  exchanges for anions like Cl^{-},HCO_{3}^{-},SO_{4}^{2-} etc present in the water    \\RNH_{2}(s)+H_{2}O(l)\rightleftharpoons RNH_{3}^{+}.OH^{-}(s)\\RNH_{3}^{+}.OH^{-}(s)+X^{-}(aq)\rightleftharpoons RNH_{3}^{+}X^{-}(s)+OH^{-}(aq)

OH^{-} ions liberated in anion exchange neutralizes the  H^{+} ions liberated in cation exchange, thereby forming water