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The alum used for purifying water is:
$A)$Ferric alum
$B)$Chrome alum
$C)$Potash alum
$D)$Ammonium alum

Last updated date: 13th Jul 2024
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Hint: Alum helps in purifying water by aluminum and which coagulates the mud particles. Because of the high charge on the aluminum ion, it helps the small particles of suspended solids to group together until they are of such size that they sink and can be removed.

Complete answer:
 Potassium alum, potash alum, or potassium aluminium sulfate is a chemical compound: the double sulfate of potassium and aluminium, with chemical formula $KAI{(S{O_4})_2}$. It is commonly encountered as the dodecahydrate, $KAI{(S{O_4})_2}.12{H_2}O$. It crystallizes in an octahedral structure in neutral solution and cubic structure in an alkali solution and lattice parameter. The compound is the most important member of the generic class of compounds called alums, and is often called simply alum.
Purification occurs due the process known as flocculation. Flocculation refers to the process by which fine particulates are caused to clump together into a floc. The floc may then float to the top of the liquid, settle to the bottom of the liquid, or be readily filtered from the liquid. When potash and aluminium alum are added to water they dissociate and form ions. Suspended colloidal particles with negative surface charges bind together with these ions and become heavy enough to settle down in the water.
So, the correct options are $C)$ and $D)$.

Potassium alum, in its various forms, is commonly used as an astringent and antiseptic. This is the main reason why it is used to inhibit body odour when used as natural deodorant. The astringent and antiseptic properties in Potash Alum also help in reducing bleeding in minor abrasions and cuts, nosebleeds etc.