Question

Alum helps in purifying water by

Answer 1

Heyy !
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Alum has a negative charge and tends to disperse in water very fast and very well (Because it's a colloid).

Alum acts as an electrolyte and helps in coagulation
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Answer 2

hlo dear
Aluminium Sulfate, Shortly known asAlum, when added to raw water reacts with the bicarbonate alkalinities present in water and forms a gelatinous precipitate. This floc attracts other fine particles and suspended material in raw water, and settles down at the bottom of the container.
hope it helped :,)