Weak Acid

Summary: An acid that only partially dissociates in water. Ethanoic acid (CH₃COOH) and carbonic acid (H₂CO₃) are weak acids. Tags: igcse chemistry definition acids-bases Created: 2026-07-14 Last Updated: 2026-07-14

A weak acid is an acid that only partially dissociates (ionises) when dissolved in water, meaning that only a small fraction of the acid molecules release their hydrogen ions, and a dynamic equilibrium is established between the undissociated acid molecules and the ions produced — this is represented by the reversible equilibrium arrow (⇌) in the dissociation equation. The key weak acids encountered at IGCSE level include ethanoic acid (CH₃COOH ⇌ H⁺ + CH₃COO⁻), carbonic acid (H₂CO₃ ⇌ H⁺ + HCO₃⁻), and all other carboxylic acids that contain the —COOH functional group — organic acids are almost always weak. Compared to a strong acid of the same concentration, a weak acid has a higher pH (lower H⁺ concentration), reacts more slowly with metals and carbonates because fewer H⁺ ions are available to collide and react, and has lower electrical conductivity because fewer charge-carrying ions are present in the solution. The strength of an acid is entirely distinct from its concentration — strength describes the proportion of dissolved molecules that dissociate (a chemical property), while concentration describes how many moles of acid are dissolved per dm³ (a physical property), and it is entirely possible to have concentrated ethanoic acid or dilute hydrochloric acid. In IGCSE exams, students should be able to identify a weak acid from its partial dissociation equation using the equilibrium arrow, compare and explain the differences in properties between strong and weak acids at the same concentration, and recognise the carboxylic acid group as the structural indicator of a weak organic acid.

This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.