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.
