Strong Acid

Summary: An acid that completely dissociates (ionises) in water. HCl, H₂SO₄, and HNO₃ are strong acids. Tags: igcse chemistry definition acids-bases Created: 2026-07-14 Last Updated: 2026-07-14

A strong acid is an acid that undergoes complete dissociation (ionisation) when dissolved in water, meaning that every dissolved acid molecule releases its hydrogen ion to form H⁺ (or more accurately, H₃O⁺) in solution, and the dissociation is represented by a single forward arrow indicating the reaction goes to completion. The three strong acids encountered at IGCSE level are hydrochloric acid (HCl → H⁺ + Cl⁻), sulfuric acid (H₂SO₄ → 2H⁺ + SO₄²⁻), and nitric acid (HNO₃ → H⁺ + NO₃⁻) — these are the acids you must recognise as strong. Compared to a weak acid of equal concentration, a strong acid exhibits a lower pH (due to a higher concentration of H⁺ ions), a faster rate of reaction with metals and carbonates (more H⁺ ions available to collide), and greater electrical conductivity because more free ions are present to carry charge through the solution. It is crucial to distinguish between the terms “strong” and “concentrated” — strength refers to the degree of dissociation (a chemical property describing what proportion of molecules release H⁺), while concentration refers to the amount of acid dissolved per unit volume of water (a physical property), and it is perfectly possible to have a dilute strong acid or a concentrated weak acid. In IGCSE practical and theory exams, students should be able to compare the relative strength of acids experimentally by measuring the rate of hydrogen evolution with magnesium ribbon or by comparing electrical conductivity under identical conditions of temperature and concentration.

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