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

Which among the following statement$(s)$ is(are) true? Exposure of silver chloride to sunlight for a long duration turns grey due to:
$(i)$ The formation of silver by decomposition of silver chloride
$(ii)$ Sublimation of silver chloride
$(iii)$ Decomposition of chlorine gas from silver chloride
$(iv)$ Oxidation of silver chloride

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

State three applications of a decomposition reaction.

On heating blue coloured powder of copper $(II)$ nitrate in a boiling tube,copper oxide (black),oxygen gas and a brown gas $X$ is formed.
$(a)$ Identify the type of reaction.
$(b)$ What could be the $pH$ range of an aqueous solution of the gas $X$?

The coating formed on the metals such as iron, silver, and copper after corrosion will be: