Explain the given reactions with the examples
(a) Hydrogenation reaction
(b) Oxidation reaction
(c) Substitution reaction
(d) Saponification reaction
(e) Combustion reaction

(a) Hydrogenation reaction can be defined as an addition reaction between the hydrogen and other compounds in the presence of a catalyst.

$CH_2 = CH_2 → CH_3CH_3$

(b) An oxidation reaction is a reaction in which oxygen combines with an element or a compound. In other words, it can also be defined as a molecule loses its electrons.

$2Mg + O_2→ 2MgO$

(c) Substitution reaction can be defined as the reaction in which a more reactive element displaces a less reactive element from its aqueous salt solution.

$Zn + CuSO_4 → ZnSO_4 + Cu$

(d) Saponification reaction is the reaction in which the hydrolysis of an ester takes place with NaOH or KOH to give alcohol and sodium or potassium salt of the acid.

$CH_3COOC_2H_5 + NaOH → C_2H_5OH + CH_3COONa$

(e) Combustion reaction is the reaction in which oxygen combines with a compound to form carbon dioxide and water. These reactions are exothermic, meaning they give off heat.

$CH_4 + 2O_2 → 2H_2O + CO_2$