Ncert Exercises & Solutions

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

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 OH^- form) resin.

During the cation exchange process, H⁺ exchanges for $N a^{+}$ , $M g^{2 +}$ , $C a^{2 +}$ , and other cations present in water.

$2 R H_{(s)} + {M^{2 +}}_{(a q)} ⇄ M R_{2 (s)} + 2 {H^{+}}_{(a q)} . . . . . . . . (1)$

In the anion exchange process, OH^- exchange for anions such as $C o_{3}^{2 -}, S O_{4}^{2 -}, C I^{-}, H C O_{3}^{-}$ etc. present in water.

$R N H_{2 (s)} + H_{2} O_{(I)} ⇄ R N {H_{3}}^{+} . O {H^{-}}_{(s)}$
$R H {N_{3}}^{+} O {H^{-}}_{(s)} + {X^{-}}_{(a q)} ⇄ R N {H_{3}}^{+} . {X^{-}}_{(s)} + O {H^{-}}_{(a q)} . . . . . . . . . (2)$

OH^- ions liberated in reaction (2) neutralize H⁺ ions liberated in reaction (1)

Thereby Forming water,

${H^{+}}_{(a q)} + O {H^{-}}_{(a q)} \to H_{2} O_{(I)}$

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