Question

How does the addition of an alum purify water?

Answer 1

Alum purifies water by the process of coagulation. When alum (K₂SO₄·Al₂(SO₄)₃·24H₂O) is added to water, it dissolves and reacts with the natural alkalinity (like bicarbonates) present in water to form insoluble gelatinous aluminium hydroxide [Al(OH)₃].

This gelatinous precipitate traps and settles down suspended impurities, colloidal particles, and microorganisms, making the water clear.

Chemical reaction:

\[\ce{Al^{3+} + 3HCO^-3 -> Al(OH)3 ↓ + 3CO2}\]