(a) Oxidation: A chemical reaction in which gain of oxygen or loss of hydrogen takes place.

Example: (i)

Here, magnesium is oxidised to form magnesium oxide.

(ii)

Here, copper is oxidised to form copper oxide.

(b) Reduction: A chemical reaction in which loss of oxygen or gain of hydrogen takes place.

Example: (i) When zinc oxide is heated with carbon, then zinc metal and carbon monoxide are formed:

In this reaction, Zinc oxide (ZnO) is reduced to zinc by the loss of oxygen.

(ii) When copper oxide is heated with hydrogen, then copper metal and water are formed:

In this reaction, copper oxide is losing oxygen to form copper metal. Therefore, copper oxide is reduced to copper.