Explain the following reactions with chemical equations:
$(i)$ Oxidation
$(ii)$ Hydrogenation
$(iii)$ Substitution

(N/A) $(i)$ Oxidation: Alcohols are converted into carboxylic acids in the presence of an oxidizing agent such as acidified $K_2Cr_2O_7$.
$CH_3CH_2OH \xrightarrow[Acidified]{K_2Cr_2O_7} CH_3COOH$
$(ii)$ Hydrogenation: Unsaturated hydrocarbons (like ethene) react with hydrogen in the presence of a catalyst like $Ni$ or $Pd$ to form saturated hydrocarbons.
$CH_2 = CH_2 + H_2 \xrightarrow[\Delta]{Ni} CH_3 - CH_3$
$(iii)$ Substitution: Saturated hydrocarbons (like methane) react with chlorine in the presence of sunlight to form chloroalkanes.
$CH_4 + Cl_2 \xrightarrow{Sunlight} CH_3Cl + HCl$