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

Principle: Synthetic ion-exchange resins are large, insoluble organic polymers bearing acidic (–COOH, –SO3H) or basic (–NH2, quaternary ammonium) functional groups that can exchange their own ions for the hardness-causing ions in water.

Method (two-stage process):

1. Cation exchange: Hard water is first passed through a bed of a cation-exchange resin (e.g., RSO3H or RCOOH, where R is the polymer backbone). The resin's H⁺ ions are exchanged for the Ca²⁺ and Mg²⁺ ions in the water:

The water leaving this stage is free of Ca²⁺/Mg²⁺ but now contains an equivalent amount of H⁺ (and the original anions).

2. Anion exchange: This water is then passed through an anion-exchange resin (bearing basic groups, pre-treated with NaOH), which exchanges the anions present (Cl⁻, SO4²⁻, HCO3⁻) for OH⁻ ions:

3. The H⁺ (from step 1) and OH⁻ (from step 2) released into the water then simply combine:

The net result is water free of all dissolved mineral ions — deionised/demineralised water. …

Explain the two-stage exchange (cation resin removes Ca²⁺/Mg²⁺ releasing H⁺; anion resin removes anions releasing OH⁻), then note the H⁺/OH⁻ …

• Describing only the cation-exchange step and forgetting the anion-exchange step — both are needed to get fully ion-free water, not just hardness-free water. …