Whenever a reaction between an oxidising agent and a reducing agent is carried out, a compound of lower oxidation state is formed if the reducing agent is in excess and a compound of higher oxidation state is formed if the oxidising agent is in excess. This can be illustrated as follows:

(i) are reducing and oxidising agents respectively.

If an excess of is treated with , then will be produced, wherein the oxidation number (O.N.) of P is +3.

However, if is treated with an excess of , then will be produced, wherein the O.N. of P is +5.

(ii)K acts as a reducing agent, whereas is an oxidising agent.

If an excess of K reacts with , then will be formed, wherein the O.N. of O is -2.

However, if K reacts with an excess of , then will be formed, wherein the O.N. of O is -1.

(iii)C is a reducing agent, while acts as an oxidising agent.

If an excess of C is burnt in the presence of insufficient amount of , then CO will be produced, wherein the O.N. of C is +2.

On the other hand, if C is burnt in an excess of O2, then CO2will be produced, wherein the O.N. of C is +4.