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प्रश्न
Why do we add alum to purify water?
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उत्तर
River water carries negatively charged colloidal particles of clay and sand; alum \[\ce{(K2SO4Al2(SO4)3 24H2O)}\] is added to river water to coagulate the colloidal particles of clay and sand. Thus, alum helps to purify water.
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