Water which does not contain cations and anions is called ‘demineralised water’ Demineralised water is obtained the same way as soft water is obtained from hard water.It is obtained by first passing through a cation Ex- change Resin (RCOOH or RS${{O}{3}}$H) which removes ${{Ca}^{2+}}$ and ${{Mg}^{2+}}$ ions from hard water by exchanging them with H+.It is then passed through anion exchange resin (R${{N}^{+}}$${{H}^{3}}$O${{H}^{-}}$) which removes Cl" and S${{O}{4}}$ by exchanging them with O${{H}^{-}}$ ions.