State one industrial application of the reduction process.

$(a)$ Why is the corrosion of iron a serious problem?
$(b)$ How can we prevent corrosion?

What type of coating is formed on copper articles when they get corroded?

When a green iron salt is heated strongly, its colour finally changes to black and the odour of burning sulphur is given out.
$(a)$ Name the iron salt.
$(b)$ Name the type of reaction that takes place during the heating of iron salt.

The substances which get oxidised easily can be kept for longer duration by:

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