A base is a metallic oxide or hydroxide which neutralizes an acid to form salt and water only.
The term was originally used to describe substances that turned red litmus blue.
(i) ZnO(g) + 2HCl(aq) → ZnCl2(s) + H2O
(ii) 2NaOH(aq) + H2SO4(aq) → Na2SO4(s) + 2H2O(l)
1. Arrhenius definition of a Base:
A base is a substance that produces hydroxyl ion (OH–) when dissolved in water or in aqueous solution e.g NaOH, KOH.
NaOH(s) + H2O → Na+(aq) + OH–(aq)
NaOH → Na+(aq) + OH–(aq)
2. Brønsted–Lowry Concept of Bases:
A base is a proton acceptor. Example,
(i) NH3 + HCl → NH4+ + Cl–
(ii) H2O + HCl → H3O+ + Cl–
In examples (i) and (ii), NH3 and H2O molecules are bases, they accept proton from the acids.
3. Lewis Concept of Base:
Lewis base is an electron-pair donor. They are species that possess a Lone-pair of electrons that can be shared with an empty valence orbital.
(i) NH3 + H3O+ → NH4+ + H2O
(ii) Cl– + AlCl3 → AlCl4–
In (i), NH3 donates the Lone-pair of electrons on the nitrogen atom to the proton in H3O+, and in (ii) Cl– donates a Lone-pair electron to the empty valence orbital on the aluminum atom in AlCl3.