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

Best Answer

Answer: The process of treating permanent hardness of water using synthetic resins is based on the exchange of cations (e.g., Na+ , Ca2+, Mg2+ etc) and anions (e.g., Cl-, SO2-4, HC, HCO3) present in water by H+ and OH– ions respectively.

Synthetic resins are of two types:

l) Cation exchange resins

2) Anion exchange resins

Cation exchange resins are large organic molecules that contain the –SO3H group. The resin is firstly changed to RNa (from RSO3H) by treating it with NaCl. This resin then exchanges Na ions with Ca2+ and Mg2+ ions, thereby making the water soft.

2RNa + M2+(aq) → R2M(s) + 2Na+(aq)


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