Which is the reducing agent in the following reaction?
$MnO_{2}(s) + 4HCl(aq) \longrightarrow MnCl_{2}(s) + 2H_{2}O(l) + Cl_{2}(g)$

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.

On adding a drop of barium chloride solution to an aqueous solution of sodium sulphite,a white precipitate is obtained.
$(a)$ Write a balanced chemical equation of the reaction involved.
$(b)$ What other name can be given to this precipitation reaction?
$(c)$ On adding dilute hydrochloric acid to the reaction mixture,the white precipitate disappears. Why?

Explain the type of reactions represented by the following equations:
$(i)$ $Mg + CuSO_4 \longrightarrow MgSO_4 + Cu$
$(ii)$ $NH_4NO_2 \longrightarrow N_2 + 2H_2O$

If copper metal is heated over a flame, it develops a coating. What is the colour and composition of this coating?

Give reasons for the following:
$(i)$ Keeping food in airtight containers helps in preventing rancidity.