The substance which gains oxygen or loses hydrogen during the reaction, it is said to be oxidised and the substance which loses oxygen or gains hydrogen during the reaction, it is said to be reduced.

i) CuO + H₂→ Cu + H₂O

In the following reaction copper oxide loses oxygen and forms copper so copper is reduced and hydrogen gains oxygen and forms water so hydrogen is oxidised.

ii) MnO₂ +4HCl → MnCl₂ + Cl₂+ 2H₂O

In the following reaction HCl is oxidised to Cl₂ as it loses hydrogen and MnO₂ is reduced to MnCl₂ as it loses oxygen.