For a chemical reaction of base, $\mathrm{OH}^{-}$present in its aqueous solutior is responsible.
(i) Reaction of base with acid:
Reaction of a base with an acid forms salt and water. This reaction is called neutralisation.
For example,
(1) $\mathrm{NaOH}(a q)+\mathrm{HCl}(a q) \rightarrow \mathrm{NaCl}(a q)+\mathrm{H}_{2} \mathrm{O}(l)$
(2) $2 \mathrm{KOH}^{\mathrm{O}}(a q)+\mathrm{H}_{2} \mathrm{SO}_{4}(a q) \rightarrow \mathrm{K}_{2} \mathrm{SO}_{4}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l)$
In short, Base + Acid $\rightarrow$ Salt + Water
(ii) Reaction of base with non-metal oxide :
Reaction of base with oxides of non-metal forms salt and water.
For example,
(1) $\mathrm{Ca}(\mathrm{OH})_{2}(a q)+\mathrm{CO}_{2}(g) \rightarrow \mathrm{CaCO}_{3}(s)+\mathrm{H}_{2} \mathrm{O}(l)$
(2) $2 \mathrm{NaOH}(a q)+\mathrm{SO}_{3}(g) \rightarrow \mathrm{Na}_{2} \mathrm{SO}_{4}(a q)+\mathrm{H}_{2} \mathrm{O}(l)$
In short, Base $+$ Oxide of non-metal $\rightarrow$ Salt $+$ Water
(iii) Reaction of a base with some metals:
By the reaction of a strong base like sodium hydroxide and potassium hydroxide with certain amphoteric metals $(Zn, Al)$ gives salt and hydrogen gas. For example,
(1) $2 \mathrm{NaOH}(a q)+\mathrm{Zn}(s)+2 \mathrm{H}_{2} \mathrm{O}(l) \rightarrow$ $\mathrm{Na}_{2} \mathrm{Zn}(\mathrm{OH})_{4}(a q)+\mathrm{H}_{2}(g)$
(2) $2 \mathrm{KOH}(a q)+2 \mathrm{Al}(s)+6 \mathrm{H}_{2} \mathrm{O}(l) \rightarrow 2 \mathrm{KAl}(\mathrm{OH})_{4}(a q)+$ $3 \mathrm{H}_{2}(g)$
In short, Base $+$ Metal $+$ Water $\rightarrow$ Salt $+H_{2}(g)$