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

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

Solution:

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 Na+, Mg2+, Ca2+, and other cations present in water.

In the anion exchange process, $\mathrm{OH}^{-}$exchanges for anions such as $\mathrm{CO}_{3}^{2-}, \mathrm{SO}_{4}^{2-}, \mathrm{Cl}^{-}, \mathrm{HCO}_{3}^{-}$etc. present in water.

OH ions liberated in reaction (2) neutralize H+ ions liberated in reaction (1), thereby forming water.