Water free from cations (Ca2+,Mg2+, etc.) and anions (Cl–,SO42–,HCO3–, etc.) is called de-ionised or demineralised water. It is formed by passing water repeatedly through cation and anion exchangers. All the cations will exchange with H+ ions of the resin as :

Ca2++2Cl−

(in‌hard‌water)

+

2H+−resin

(cation‌exchange‌resin)

→ Ca(resin)2+2H++2C−

Mg2++SO42−

(in‌hard‌water)

+

2H+−resin

(cation‌exchange‌resin)

→ Mg(resin)2+2H++SO42−
Therefore, water which comes out from the bottom of the first tank will be richer in H+ ions. This water is then passed through a bed of anion exchange resin which exchanges its OH– with Cl– ions and SO42– ions present in hard water as :

Cl−

(in‌hard‌water)

+

HO−−resin

(anion‌exchange‌resin)

→ Cl− - resin + OH−

SO42−

(in‌hard‌water)

+

2HO−−resin

(anion‌exchange‌resin)

→ SO42− - (resin)2 + 2OH−
Therefore, the H+ ions (formed in the first tank) combine with the OH– ions (formed in the second tank) to produce water.
H++OH− → H2O
Thus, water obtained by this method is free from all types of cations as well as anions and is called de-ionised water.