(a) Give an example for a combination reaction which is exothermic.
(b) Identify the oxidising agent and reducing agent in the following reaction.
`H_(2)S + Cl_(2) to 2HCl + S`
(c) Name the phenomenon due to which the taste and smell of oily food changes when kept for a long time in open. Suggest one method to prevent it.

Explanation

(a) When calcium oxide and water combine to form a single product calcium hydroxide, a large amount of heat is also released. Therefore it is a combination reaction which is exothermic.

(c ) The phenomenon is rancidity. When facts and oils are oxidised, they become rancid and their smell and taste changes. This phenomenon is called rancidity.
Prevention from rancidity. Antioxidants are added to food containing fats and oils to prevent them from oxidation. For example, An inert gas such as nitrogen is added to prevent the packed chips from getting oxidised.