Define the following as per ionic theory with examples and ionic equations wherever relevant :

(i) acid (ii) base (iii) alkali (iv) neutralization

(i) Acid — An acid is a compound which when dissolved in water yields hydronium ions [H₃O⁺] as the only positively charged ion.

HCl (aq) ⇌ H⁺ + Cl^-

H⁺ + H₂O ⇌ H₃O⁺ [hydronium ion]

HCl + H₂O ⇌ H₃O⁺ + Cl^-

(ii) Base — A base is a compound which reacts with hydronium ions of an acid to give salt and water only.

CuO + 2HCl ⟶ CuCl₂ + H₂O

Cu(OH)₂ + H₂SO₄ ⟶ CuSO₄ + 2H₂O

• Bases are oxides or hydroxides of a metal [including ammonium hydroxide]

• Examples of insoluble bases [i.e., not alkalis] — ZnO, PbO, CuO, Fe(OH)₂, Pb(OH)₂, Cu(OH)₂

(iii) Alkali — An alkali is a compound which when dissolved in water yields hydroxyl ions [OH^-] as the only negatively charged ions.

NaOH [aq.] ⇌ Na⁺ + OH^- [Hydroxyl or hydroxide ion]

Alkali is a base, soluble in water. [All alkalis are bases, but all bases are not alkalis.]

Examples of soluble bases [i.e., alkalis] — KOH, NaOH [strong alkalis] , Ca(OH)₂, NH₄OH (weak alkalis).

(iv) Neutralization — It is the process due to which [H⁺] ions of an acid react completely or combine with [OH^-] ions of a base to give salt and water only.

Acid + Base ⟶ Salt + Water

HCl + NaOH ⟶ NaCl + H₂O

H⁺Cl^- + Na⁺OH^- ⟶ Na⁺Cl^- + H₂O

[H⁺ (aq) + OH^- (aq) ⇌ H₂O (l)]