A weak acid is an acid that does not fully ionize in solution; that is, if the acid was represented by the general formula AH, then in aqueous solution a significant amount of undissolved AH still remains.

HA_(aq) H⁺_(aq) + A^-_(aq)

The equilibrium concentrations of reactants and products are related by the Acidity constant expression, (K_a): K_a = [H⁺][A^-]/[HA].

The greater the value of K_a, the more the formation of H⁺ is favored, and the lower the pH of the solution.

The vast majority of acids are weak acids.

Examples

• Phosphoric acid, H₃PO₄
• Acetic acid, CH₃COOH
• Hydrofluoric acid, HF
• Acetylsalicylic acid, C₆H₄OCOCH₃CO₂H
• Nicotinic acid, C₅H₅NCOOH
• Pyruvic acid, CH₃COCOOH
• Bromothymol blue, C₂₁H₁₅NaBr₂O₃S