Neutralisation – IGCSE Chemistry Definition
IGCSE Chemistry definition of neutralisation: the reaction of H⁺ with OH⁻ to form water. Covers equations, applications, and exam tips for Cambridge 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Neutralisation links acids, bases, salts, pH, and titration together — it is the single most connected topic in the acids and bases unit. Every paper tests it in some form, whether through equations, salt preparation, or real-world applications like treating acid indigestion.
The 0620 definition
Neutralisation is the reaction between an acid and a base to form a salt and water.
The Supplement ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l)
This ionic equation shows that every acid–alkali neutralisation is fundamentally the same reaction — hydrogen ions combine with hydroxide ions to form water. The metal cation and acid anion are spectator ions.
Types of neutralisation reactions
| Reaction | Products | Example |
|---|---|---|
| Acid + alkali | Salt + water | HCl + NaOH → NaCl + H₂O |
| Acid + base (insoluble) | Salt + water | H₂SO₄ + CuO → CuSO₄ + H₂O |
| Acid + carbonate | Salt + water + CO₂ | 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂ |
Note: the acid + carbonate reaction produces CO₂ as well — some mark schemes still classify this as neutralisation, but the strict definition requires salt and water only.
pH changes during neutralisation
When alkali is added to acid, pH rises from below 7 toward 7. At the equivalence point of a strong acid–strong alkali reaction, pH = 7. Adding excess alkali pushes pH above 7. This is exactly what a titration measures.
Real-world applications
- Indigestion tablets: contain bases such as CaCO₃ or Mg(OH)₂ to neutralise excess stomach acid (HCl)
- Treating acidic soil: farmers add lime (CaO or Ca(OH)₂) to raise soil pH
- Treating acidic lakes: powdered limestone (CaCO₃) is added to lakes affected by acid rain
Worked exam question
A student adds dilute sulfuric acid to sodium hydroxide solution until the solution is neutral. (a) Write the balanced equation. [1] (b) Write the ionic equation. [1] (c) Explain why the temperature of the mixture increases. [2]
Mark scheme
(a) H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O [1]
(b) H⁺(aq) + OH⁻(aq) → H₂O(l) [1]
(c) Neutralisation is exothermic [1]; energy is released to the surroundings when bonds form in water molecules / when H⁺ and OH⁻ combine [1]
Common exam mistakes
- Forgetting to balance the equation when sulfuric acid is used — H₂SO₄ provides two H⁺ ions, so you need 2NaOH.
- Writing the ionic equation without state symbols. Omitting (aq) and (l) loses the mark.
- Describing the reaction as endothermic — neutralisation always releases energy.
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