Question

Why do we add alum to purify water?

Answer 1

The water obtained from natural sources often contains suspended impurities.

Alum $\left(K_{2}S{O}_{4}A{l}_4\right(S{O}_4{)}_3.24H_{2}O)$ is added to such water to coagulate the suspended Impurities.

So, that these impurities settle down and can removed by decantation or filtration. It makes water fit for drinking purposes .