The strength of acids and bases depends on the number of H⁺ produced.

i. Acids that give rise to more H⁺ ions are said to be strong acids and acids that give less H⁺ ions are said to be weak acids.

ii. Hydrochloric acid (HCl) gives more number of H⁺ on dissociation:

HCl → H⁺ + Cl^-

iii. On the other hand, acetic acid (CH₃COOH) gives H⁺ ions but it is a reversible reaction and thus the ions formed gets to form acetic acid and reaction proceeds in backward direction through a reversible reaction. Due to reversibility, less number of H⁺ ions are produced:

CH₃COOH ↔ CH₃COO^- + H⁺

iv. Hence, hydrochloric acid is called strong acid and acetic acid is called a weak acid.