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

The process of treating permanent hardness of water using synthetic resins generally based on exchange of anions and cations present in water by OH^–  and H⁺ ions, respectively.

Two types of synthetic resins are

1.  Anion exchange resins
2. Cation exchange resins

Cation exchange resins are large organic molecules that contain the –SO₃H group. Firstly the resin get changed into RNa by treating it with NaCl. This resin exchanges Na⁺ ions with Ca²⁺ ions and Mg²⁺ ions, thereby making the water soft.

\( 2RNa+M^{2+}_{(aq)}\rightarrow R_{2}M_{(s)}+2Na^{+}_{(aq)} \)

There are cation exchange resins in H⁺ form. The resins exchange H⁺ ions for Na⁺, Ca²⁺, and Mg²⁺ ions.

\( 2RH+M^{2+}_{(aq)}\rightleftharpoons MR_{2(s)}+2H^{+}_{(aq)} \)

Anion exchange resins exchange OH^–  ions for anions like Cl^– , \( HCO^{-}_{3}\)and \( SO_{4}^{2-}\)​ present in water.

\( RNH_{2(s)}+H_{2}O_{(l)} \rightleftharpoons \)

\( RNH^{+}_{3}.OH^{-}_{(s)}​ RNH^{+}_{3}.X^{-}_{(s)}+OH^{-}_{aq)}​\)

During the whole process, first the water passes through the cation exchange process. The water which is obtained after this process is free from mineral cations and naturally acidic. This acidic water is then passed through the anion exchange process where OH^– ions neutralize the H⁺ ions and de-ionize the water obtained.