Answer:
The distilled water free from
all soluble minerals is termed demineralised water. Demineralisation is
done by ion exchange resins which are synthetic substances. These are of two
types:
Cation exchange resins and
(i) Anion exchange resins.
(ii) The cation exchange resins
are granular insoluble organic acid. Resins have giant molecules with \[-S{{O}_{3}}H\]
or\[COOH\]groups, while anion exchange resins contain giant organic molecules
with basic groups derived from amines.
The hard water is first passed
through a bed of cation exchanger which removes the cations like \[N{{a}^{+}},M{{g}^{2+}}\],\[C{{a}^{2+}}\]
etc., by exchanging with \[{{H}^{+}}\] ions.
\[\underset{\operatorname{Re}\sin
}{\mathop{2RCOOH}}\,+C{{a}^{2+}}\to {{(RCOO)}_{2}}Ca+2{{H}^{+}}\]
The water coming from cation
exchanger is acidic. This water is then passed through anion exchanger which removes
anions like\[C{{l}^{-}},SO_{4}^{2-},NO_{3}^{-}\], etc., by exchanging with \[O{{H}^{-}}\]
ions.
\[R-N{{H}_{3}}OH+C{{l}^{-}}\to
R-N{{H}_{3}}Cl+O{{H}^{-}}\]The \[O{{H}^{-}}\] ions combine with \[{{H}^{+}}\]
ions to form water.
\[{{H}^{+}}+O{{H}^{-}}\to
{{H}_{2}}O\]
This process gives demineralised
water.
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