(a) Corrosion: When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion. Rusting of iron is most common example of iron. When iron articles are left in air for some time, they get coated with a radish brown powder called rust. The corrosion of iron is a continuous process, it not prevented in time, whole iron object gets damage.

(b) Rancidity. When the food materials containing oil and fats are kept for a long time, they get oxidised and become rancid due to which their smell, taste and colour change. This process is known as rancidity. Rancidity makes the food unfit for eating. This is why antioxidants are added to foods containing oil and fat to prevent oxidation of food. For example-if potato chips prepared in oil are exposed to air for a long time, they start giving unpleasant smell and taste due the oxidation of oil present in them.