Strong and Weak Acids (Higher)

This lesson provides a detailed treatment of strong and weak acids for Edexcel GCSE Chemistry (1CH0) Higher tier. You need to understand the difference between strong and weak acids in terms of ionisation, distinguish this from concentration, and explain how it affects pH and rate of reaction.

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Ionisation of Acids in Water

When an acid dissolves in water, its molecules dissociate (split apart) to produce hydrogen ions (H⁺) and negative ions. The extent to which this happens determines whether the acid is classified as strong or weak.

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Strong Acids

A strong acid is one that completely ionises (fully dissociates) in aqueous solution. Every single molecule of the acid breaks apart to form ions.

Examples of Strong Acids

Strong 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)

Key Features