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.
⚙️ How It Works: H⁺ + OH⁻ → H₂O — The Core Reaction
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.
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).