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

⚙️ 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.