Solution
Reason : An alum is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula XAl(SO4)2.12H2O, where X is a monovalent cation such as potassium or ammonium.They are soluble in water.Alum which contains aluminium and sulphate ions is an example of double salt.As Double Salts are salts containing more than one cation or anion, and are obtained by combination of two different salts which were crystallized in the same regular ionic lattice.When it dissolve in water, a double salt completely dissociates into simple ions.(B) double salt
