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Hydrogen — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistryHydrogen5 Marks
Question
What is meant by ‘demineralised’ water and how can it be obtained?
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Answer

Demineralised water is free from all soluble mineral salts which is obtained by passing water successively through a cation exchange (in the form of $\mathrm{H}^{+}$) and an anion exchange in the form of $\mathrm{OH}^{-}$resins.
$2 \mathrm{RH}(\mathrm{~s})+\mathrm{M}^{2+}(\mathrm{aq}) \rightleftharpoons \mathrm{MR}_2(\mathrm{~s})+2 \mathrm{H}^{+}(\mathrm{aq})$
$\mathrm{H}^{+}$exchanges for $\mathrm{Na}^{+}, \mathrm{Ca}^{2+}, \mathrm{Mg}^{2+}$ and other cations present in water. This process results in release of proton which makes the water acidic.
$\mathrm{OH}^{-}$exchanges, for anions like $\mathrm{Cl}^{-}, \mathrm{HCO}_3-\mathrm{SO}_4{ }^{2-}$ etc.
$\mathrm{OH}^{-}$ions thus liberated neutralize the $\mathrm{H}^{+}$ions set free in the cation exchange process.
$\mathrm{H}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq}) \longrightarrow \mathrm{H}_2 \mathrm{O}(\mathrm{l})$

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