Discuss the principle and method of softening of hard water by synthetic ion-exchange resins.

Synthetic resins are used for the treatment of permanent hardness of water. Cations and anions present in water are exchanged by hydrogen and hydroxide ions of the resin. Synthetic resins are classified into cation exchange resins and anion exchange resins.
Cation exchange resins contain large organic molecules having $-S{O}_{3}H$ group. Initially, the resin is treated with $NaCl$ which changes it from $RS{O}_{3}H$ to $RS{O}_{3}Na$.
Hard water is then passed through the resin and $N{a}^{+}$ ions of the resin are exchanges with $C{a}^{2+}$ and $M{g}^{2+}$ ions from hard water thereby making water soft.
In other type of cation exchange resins, the $H^{+}$ ions from resin are exchanged with $N{a}^{+},M{g}^{2+}$ and $C{a}^{2+}$ ions from hard water.
In the anion exchange resins, $O{H}^{-}$ ions from resin are exchanged with $C{l}^{-},HC{O}_3^{-}$ and $S{O}_4^{2-}$ ions from hard water.
Hard water is passed through cation exchange resin followed by anion exchange resin. The water is demineralized and neutral in nature.