Strong and Weak Acids [H]

This lesson covers strong and weak acids, as specified in the AQA GCSE Chemistry specification (4.4.2 — Higher Tier only). You need to understand the difference between strong and weak acids in terms of ionisation, relate acid strength to pH and hydrogen ion concentration, and distinguish between acid strength and acid concentration. This is a Higher Tier topic, marked [H] on the specification, and will only appear on Higher Tier exam papers.

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Ionisation of Acids

When an acid dissolves in water, its molecules ionise (split apart) to release hydrogen ions (H+). The degree to which this happens determines whether the acid is strong or weak.

Strong Acids

A strong acid is one that is completely ionised (fully dissociated) in aqueous solution. Every molecule of the acid splits up to release H+ ions.

Examples of strong acids:

Strong Acid	Formula	Ionisation in water
Hydrochloric acid	HCl	HCl → H+ + Cl-
Sulfuric acid	H2SO4	H2SO4 → 2H+ + SO4 2-
Nitric acid	HNO3	HNO3 → H+ + NO3-