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How does alum aid in the purification of water?

1. by forming a silicon complex with clay particles
2. by its sulphate portion, which combines with impurities and removes them
3. by aluminium, which causes mud particles to coagulate
4. by making muddy water dissolve

Correct answer: by aluminium, which causes mud particles to coagulate

Solution

Alum supplies Al3+ ions which neutralize the negative charge on suspended clay/mud particles, causing them to coagulate and settle.

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