What is meant by disproportionation? Give one example of disproportionation reaction in aqueous solution. 

Disproportionation reaction- A reaction in which the same species is simultaneously oxidised as well as reduced is called a disproportionation reaction. Thus we can say that disproportionation reaction is a special type of redox reaction in which an element in one oxidation state simultaneously undergoes both oxidation and reduction. For example:

in the above reaction, oxygen is undergoing both reduction (reduced from -1 to -2 oxidation state in water) and oxidation (oxidised to 0 oxidation state from -1 in oxygen). Thus, this oxidation and reduction is with respect to the oxidation state of the element in the reactant.