Question

Alums purify muddy water by :
A.Dialysis
B.Absorption
C.Coagulation
D.Forming true solution

Answer 1

Firstly we have to know what kind of a compound an alum is ( its physical properties, chemical structure and the formulas ). It is colourless, hydrated in structure and has a general formula of $XAl{(S{O_4})_2}\cdot12{H_2}O$ . By this we will get the chemical aspect of the compound. Now we just have to define the process which alum follows for the purification. And then we would get the answer we needed.

Complete step by step answer:
An alum is a colourless astringent compound which is a hydrated double sulphate of aluminium and potassium, used in solution in dyeing and tanning.
When talked in the chemical terms,
An alum can be defined as a type of chemical compound which is usually a hydrated double sulfate salt of aluminium. It has the general formula as: $XAl{(S{O_4})_2}\cdot12{H_2}O$ .
In this formula the X can be termed as a monovalent cation. For eg. potassium or ammonium. By itself, "alum" often refers to potassium alum, with the chemical formula as : $KAl{(S{O_4})_2}\cdot12{H_2}O$ . Similarly the other alums are also named after the monovalent cation in them. For eg. sodium alum and ammonium alum.
Now for the given question:
We know that the stability of the given lyophobic sols is due to the presence of charge on colloidal particles as mentioned in their physical nature.
Now, If the charge is removed from them then the particles will also come together and hence settle down. This type of spontaneous destabilization of solution is ageing process. It in turn also destabilizes the solution by the artificial means, which is called coagulation.
Alums also do this process and hence purify muddy water by coagulation.

So, the correct answer is Option C .

Note:
The name "alum" is also used, more generally, for salts with the same formula and structure, except that aluminium is replaced by another trivalent metal ion like chromium(III), and/or sulfur is replaced by another chalcogen like selenium. The most common of these analogs is chrome alum $KCr{(S{O_4})_2}\cdot12{H_2}O$ .