Alum is an example of:
A.Single salt
B.Double salt
C.Acids
D.None of the above

Hint: A salt is formed as a product of acid-base neutralization reaction. A single salt contains one anion and one cation.A double salt is a salt that contains more than one cation or anion. Acidic salts have an ionizable proton along with other cations.

Complete step by step answer:
The term ‘alum’ is used to refer to a kind of chemical compound and is usually a hydrated double sulfate salt of aluminium. It has the chemical formula ${\text{X}}{\text{.Al}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_2}{\text{.12}}{{\text{H}}_{\text{2}}}{\text{O}}$ , where X is a monovalent cation like potassium or aluminium. One of the most important alums is potash alum which is simply called alum. It has the chemical formula ${\text{K}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_2}{\text{.Al}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_{\text{3}}}{\text{.24}}{{\text{H}}_{\text{2}}}{\text{O}}$ .
Thus, the name alum is used to represent all the hydrated double salts with the same formula and structure. The only difference is that aluminium is replaced by another trivalent metal cation like chromium (III). Sulfur may also be replaced by another chalcogen, i.e., by another group 16 element like selenium. The most common alum which is an analog of these is the chrome alum which has the chemical formula of ${\text{KCr}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_2}{\text{.12}}{{\text{H}}_{\text{2}}}{\text{O}}$ .
Potash alum also goes by the name potassium aluminium sulfate. It is a double sulfate of potassium and aluminium. It most commonly occurs in the form of the dodecahydrate ${\text{KAl}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_2}{\text{.12}}{{\text{H}}_{\text{2}}}{\text{O}}$ .

Hence, the correct answer is B.

Note:
-Potash alum or alum is found to crystallize in regular octahedra with flattened corners. It is very soluble in water.
-The potash alum solution is found to be slightly acidic. If the potash alum is neutralized with potassium hydroxide, then alumina or aluminium hydroxide will start to separate.
-When heated in red heat, a porous mass known as ‘burnt alum’ is formed.