Define disproportionation reaction and explain it with the help of a reaction.

Hint: The reaction in which one reactant gets oxidized and the same reactant gets reduced is known as disproportionation reaction. The disproportionation reaction is also known as dismutation reaction.

Complete step by step answer:
The reaction in which one reactant gets oxidized and the same reactant gets reduced is known as disproportionation reaction. The disproportionation reaction is also known as dismutation reaction. The same element is oxidized and reduced in the disproportionation reaction.
The example of disproportionation reaction is as follows:
When ${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{3}}}$ is heated it undergoes a disproportionation reaction. During the disproportionation reaction, ${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{3}}}$ undergoes oxidation as well as reduction. ${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$ and ${\text{P}}{{\text{H}}_{\text{3}}}$ are formed during the reaction.
The reaction when ${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{3}}}$ is heated is as follows:
${\text{4}}{{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{3}}}\xrightarrow{{{\text{heat}}}}3{{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}} + {\text{P}}{{\text{H}}_{\text{3}}}$
The oxidation state of phosphorus in ${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{3}}}$ is ${\text{ + 3}}$, in ${{\text{H}}_{\text{3}}}{\text{P}}{{\text{O}}_{\text{4}}}$ it becomes ${\text{ + 5}}$ and in ${\text{P}}{{\text{H}}_{\text{3}}}$ it becomes $ - {\text{3}}$. The oxidation state of phosphorus increases from ${\text{ + 3}}$ to ${\text{ + 5}}$ thus, oxidation reaction occurs. The oxidation state of phosphorus decreases from ${\text{ + 3}}$ to $ - {\text{3}}$ thus, reduction reaction occurs.

Note:
The reverse of disproportionation reaction is called comproportionation reaction. In a comproportionation reaction, a compound in an intermediate oxidation state is formed from the lower or higher oxidation state.
Other examples of disproportionation reaction are as follows:
1.Disproportionation of mercury(I)chloride due to irradiation with UV; ${\text{H}}{{\text{g}}_{\text{2}}}{\text{C}}{{\text{l}}_{\text{2}}} \to {\text{Hg}} + {\text{HgC}}{{\text{l}}_2}$.
2.Thermal degradation of bicarbonate; ${\text{2HCO}}_3^ - \to {\text{CO}}_3^{2 - } + {{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$. The oxidation numbers are constant in this reaction. This reaction is also known as autoionization reaction.
3.Reaction of phosphorus with sodium hydroxide: ${{\text{P}}_{\text{4}}} + {\text{3NaOH}} + {\text{3}}{{\text{H}}_{\text{2}}}{\text{O}} \to {\text{3Na}}{{\text{H}}_{\text{2}}}{\text{P}}{{\text{O}}_{\text{2}}} + {\text{P}}{{\text{H}}_{\text{3}}}$.
4.Reaction of chlorine gas with water: ${\text{C}}{{\text{l}}_{\text{2}}} + {{\text{H}}_{\text{2}}}{\text{O}} \to {\text{HCl}} + {\text{HClO}}$.