Answer: A. Acidity of water
Addition of alum in water treatment increases acidity of water.
The following chemical reaction takes place when alum is added to water and reacts with the alkalinity of water:
Al2(SO4)3.18H2O + 3Ca(HCO3)2 → 2Al(OH)3 + 3CaSO4 + 6CO2
Alum reacts with the alkalinity present in water and leads to the formation of gelatinous aluminium hydroxide, and temporary hardness is eliminated.
Addition of alum in water treatment increases the acidity of water as carbon dioxide gas is released, which leads to the formation of carbonic acid, thereby, increasing the acidity of water and decreasing the pH.
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