Explain the given reactions with examples:
$(a)$ Hydrogenation reaction
$(b)$ Oxidation reaction
$(c)$ Substitution reaction

(N/A) Hydrogenation reaction: Unsaturated hydrocarbons add hydrogen in the presence of a nickel $(Ni)$ catalyst to form saturated hydrocarbons. Example: Ethene to Ethane.
$CH_2=CH_2 + H_2 \xrightarrow{Ni} CH_3-CH_3$
$(b)$ Oxidation reaction: Alcohols can be oxidized to carboxylic acids in the presence of oxidizing agents like alkaline potassium permanganate $(KMnO_4)$ or acidified potassium dichromate $(K_2Cr_2O_7)$ upon heating. Example: Ethanol to Ethanoic acid.
$CH_3CH_2OH \xrightarrow[Heat]{Alk. KMnO_4} CH_3COOH$
$(c)$ Substitution reaction: Saturated hydrocarbons are largely unreactive,but in the presence of sunlight $(hv)$,chlorine can replace hydrogen atoms one by one. Example: Methane to Chloromethane.
$CH_4 + Cl_2 \xrightarrow{hv} CH_3Cl + HCl$