Alum is a water purifier because it:

coagulates the impurities

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softens hard water

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gives taste

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destroys the pathogenic bacteria

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Solution

The correct option is B coagulates the impurities
Alum is a water purifier because it coagulates the impurities. The colloidal particles are negatively charged and alum is added as a coagulant to create positively charged ions. Once the repulsive charges have been neutralized, the van der Waals force will cause the particles to combine together and will result in coagulation.

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