Question

Alum helps in purifying water by:
(A) forming a silicon complex with clay particles.
(B) sulphate part which combines with the dirt and removes it.
(C) aluminium which coagulates the mud particles.
(D) making mud water-soluble.

Answer 1

Hint: Alum is a type of compound which is very useful in daily life that you find in the grocery store for pickling and in baking powder. It is also used in leather tanning, as an ingredient in aftershave and as a treatment for fire-proof textiles.

Complete step by step solution:
To understand how alum helps in purifying water, we have to have some basic knowledge about it, so let’s go ahead.
Alum is referred for a group of hydrated double salts, usually consisting of aluminium sulfate, water, and the sulfate of another element. A series of hydrated double salts results from the hydration of the sulfate of a singly charged cation and the sulfate of any one of triply charged cations. Aluminium sulfate can thus form alums with sulfates of the singly charged cations of potassium, sodium, ammonium and other elements and compounds. In a similar way, sulfates of the triply charged cations of iron, chromium, manganese and other metals may take the place of aluminium sulfate. The most important alums are potassium aluminium sulfate, ammonium aluminium sulfate, sodium aluminium sulfate etc. Potassium aluminium sulfate, also known as potash alum.
Now, let’s check how alum purifies the water.
Alum is added to water in order to destabilize the mud particles suspended in it. This process is known as coagulation. Alum helps in purifying water by aluminium which helps to coagulate the mud particles. Because of the high charge on the aluminium ion (+3), it helps the small particles of suspended solids to group together until they are of such large size that they sink and can be removed.

Therefore, the correct answer of the above question is (C)- aluminium which coagulates the mud particles.

Note: Remember that alum is a type of chemical compound, usually a hydrated double sulfate salt. Alum often refers to potassium alum. Other alums are named after the monovalent ion, like sodium alum and ammonium alum.