$i.$ Hydrogen is sparingly soluble in water. Therefore, as it is collected over water, it picks up moisture. Dry hydrogen gas is obtained when it is collected over mercury.
$ii.$ Since hydrogen is lighter than air, it is possible to collect the gas by downward displacement of air. But it is not safe to do so since a mixture of hydrogen and air can lead to an explosion.
Those reactions in which hydrogen combines with a substance or oxygen is removed from a substance, are known as reduction reactions. Example: $2 \mathrm{HgO} \xrightarrow{\text { heat }} 2 \mathrm{Hg}+\mathrm{O}_2$
A reaction in which a substance combine with oxygen or in which hydrogen is removed is called oxidation reaction.
Example: $\ce{H_2S} + Cl \rightarrow \ce{2HCl} + S$
catalytic hydrogenation is a process by which hydrogen gas is passed through vegetable oils in the presence of catalyst like $Ni, Pt$ or $Pd$ to convert them into solid vanaspati ghee.