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This lesson covers the distinction between strong and weak acids as required for the higher tier of the AQA GCSE Combined Science Trilogy specification (8464). You must understand ionisation, write equations showing partial and complete ionisation, and distinguish between strong/weak and concentrated/dilute.
A strong acid is one that completely ionises (dissociates) in water. Every molecule of the acid breaks apart to release H⁺ ions.
| Acid | Formula | Ionisation Equation |
|---|---|---|
| Hydrochloric acid | HCl | HCl(aq) → H⁺(aq) + Cl⁻(aq) |
| Sulfuric acid | H₂SO₄ | H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq) |
| Nitric acid | HNO₃ | HNO₃(aq) → H⁺(aq) + NO₃⁻(aq) |
The → (forward arrow only) shows that ionisation is complete and irreversible.
A weak acid only partially ionises in water. Only a small fraction of its molecules release H⁺ ions; the majority remain as undissociated molecules. An equilibrium is established.
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