All neutralisation reactions between acids and alkalis share the same fundamental mechanism, regardless of the specific acid and alkali used. The acid provides H⁺ ions and the alkali provides OH⁻ ions. When these ions meet, they combine to form water molecules:
H⁺(aq) + OH⁻(aq) → H₂O(l)
This is the ionic equation for all acid-alkali neutralisations. The other ions (Na⁺, Cl⁻, K⁺, SO₄²⁻, etc.) are spectator ions — they are present but unchanged throughout the reaction.
As neutralisation proceeds, the H⁺ ions from the acid are progressively consumed by OH⁻ ions. The solution pH rises from acidic toward neutral. At the equivalence point, all H⁺ ions have been exactly matched by OH⁻ ions — the solution is neutral (pH 7) and contains only the salt and water.
If too much alkali is added, the solution becomes alkaline (excess OH⁻ ions remain). The pH does not necessarily reach 7 unless exactly the right amounts are used — which is why the neutralisation is not always "perfectly neutral" unless the moles are matched exactly.
Neutralisation reactions are exothermic — the formation of water releases energy, and the reaction mixture warms up. This energy can be measured in calorimetry experiments.
This how it works covers How It Works: H⁺ + OH⁻ → H₂O — The Core Reaction within Neutralisation Reactions for GCSE Chemistry. Revise Neutralisation Reactions in Chemical Changes for GCSE Chemistry with 20 exam-style questions and 20 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 4 of 13 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Neutralisation Reactions
Which word equation correctly represents a neutralisation reaction?
Explain why the ionic equation for any strong acid-alkali neutralisation is always H⁺(aq) + OH⁻(aq) → H₂O(l).