Water free from cations ($\mathrm{Ca}^{2+}, \mathrm{Mg}^{2+}$, etc.) and anions ($\mathrm{Cl}^{-}, \mathrm{SO}_4^{2-}, \mathrm{HCO}_3^{-}$, 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 : $\underset{\text{in hard water}}{\mathrm{Ca}^{2+}+2\mathrm{Cl}^{-}}$ + $\underset{\text{cation exchange resin}}{2\mathrm{H}^{+}-\text{resin}}$ → $\mathrm{Ca}(\text{resin})_2+2\mathrm{H}^{+}+2\mathrm{C}^{-}$ $\underset{\text{in hard water}}{\mathrm{Mg}^{2+}+\mathrm{SO}_4^{2-}}$ + $\underset{\text{cation exchange resin}}{2\mathrm{H}^{+}-\text{resin}}$ → $\mathrm{Mg}(\text{resin})_2+2\mathrm{H}^{+}+\mathrm{SO}_4^{2-}$ 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 $\mathrm{OH}^{-}$ with $\mathrm{Cl}^{-}$ ions and $\mathrm{SO}_4^{2-}$ ions present in hard water as : $\underset{\text{in hard water}}{\mathrm{Cl}^{-}}$ + $\underset{\text{anion exchange resin}}{\mathrm{HO}^{-}-\text{resin}}$ → $\mathrm{Cl}^{-}$ - resin + $\mathrm{OH}^{-}$ $\underset{\text{in hard water}}{\mathrm{SO}_4^{2-}}$ + $\underset{\text{anion exchange resin}}{2\mathrm{HO}^{-}-\text{resin}}$ → $\mathrm{SO}_4^{2-}$ - $(\text{resin})_2$ + $2\mathrm{OH}^{-}$ Therefore, the H+ ions (formed in the first tank) combine with the $\mathrm{OH}^{-}$ ions (formed in the second tank) to produce water. $\mathrm{H}^{+}+\mathrm{OH}^{-}$ → $\mathrm{H_2O}$ Thus, water obtained by this method is free from all types of cations as well as anions and is called de-ionised water.