Q. Muddy water is treated with alum in purification process, it is termed as:
Answer: coagulation
Notes: Natural and wastewater contain small particulates that are suspended in water forming a colloid. These particles carry the same charges, and repulsion prevents them from combining into larger particulates to settle. Historically, dirty water is cleaned by treating with alum, Al2(SO4)3.12 H2O, and lime, Ca(OH)2. The phenomenon is known as coagulation:

• Al2(SO4)3.12 H2O Al(aq)3+ + 3SO4(aq)2- + 12H2O
• SO4(aq)2- + H2O HSO4(aq)- + OH- (causing pH change)
• Ca(OH)2 Ca(aq)2+ + 2 OH- (causing pH change)

The slightly basic water causes Al(OH)3, Fe(OH)3 and Fe(OH)2 to precipitate, bringing the small particulates with them and the water becomes clear.