Demineralised water is free from all soluble mineral salts. It is obtained by passing water successively through a cation exchange and an anion exchange resin. Resins have giant molecules with -SO₃H or -COOH groups.

First, the hard water is passed through the cation exchanger, which removes cations like Na⁺, Mg²⁺, Ca²⁺,etc. by exchanging them with H⁺ ions.

Now, this water coming out from cation exchanger is acidic, therefore it is passed through the anionic exchanger which removes anions like OH^-, NO₃^-, etc. by exchanging with OH^- ions.

At the end, the H⁺ and OH^- thus produced, combine amongst themselves to produce water

This is demineralisation of water by ion exchange resin.