Neutralisation

Summary: The reaction between an acid and a base to form salt and water: H⁺(aq) + OH⁻(aq) → H₂O(l). Tags: igcse chemistry definition acids-bases Created: 2026-07-14 Last Updated: 2026-07-14

Neutralisation is the chemical reaction between an acid and a base that produces a salt and water, with the essential ionic process being the combination of hydrogen ions (H+) from the acid with hydroxide ions (OH-) from the base to form water: H+(aq) + OH-(aq) → H2O(l). This ionic equation is the same for any neutralisation between a strong acid and a strong base, because the spectator ions (e.g., Na+ and Cl- when HCl reacts with NaOH) remain unchanged in solution. Neutralisation has widespread real-world applications: antacid tablets containing magnesium hydroxide or calcium carbonate neutralise excess stomach acid, lime (calcium oxide or hydroxide) is spread on acidic soils to raise pH for crop growth, and industrial effluents are treated to neutralise acidic or alkaline waste before discharge. In the laboratory, neutralisation is precisely measured using titration — a technique in which an acid of known concentration is added from a burette to a measured volume of alkali (with an indicator such as phenolphthalein or methyl orange) until the endpoint is reached, enabling the unknown concentration to be calculated. IGCSE students should be able to write both the full balanced equation and the net ionic equation for any acid-base neutralisation, and they should recall that the reaction between ammonia (a weak base, no OH- ion) and an acid is also classified as neutralisation despite following a different ionic pathway.

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