Explanation:

Alum is generally used to purify muddy water. This type of process is done by the process called coagulation in which stability of the lyophobic sols is due to presence of charge on colloidal particles. Lyophobic sols are those sols in which the dispersed phase particles have no affinity or we can say that there is no bond for the dispersion medium and these are generally irreversible in nature. If charge is removed then particles come together and settle down. Spontaneous destabilization of solution is ageing and destabilization of solution by artificial means is called coagulation. Thus we can say that alums purify muddy water by coagulation. Historically, dirty water is cleaned by treating with alum, Al2(SO4)3.12 H2O, and lime, Ca(OH)2. The phenomenon is known as coagulation.