Strong and Weak Acids — AQA GCSE Chemistry (Higher Tier)
The strength of an acid describes how completely it ionises in water. This is different from its concentration.
Strong acids
A strong acid is completely ionised in aqueous solution — almost all its molecules release H⁺ ions. Examples: hydrochloric, sulfuric and nitric acids.
Weak acids
A weak acid is only partially ionised — only a small proportion of its molecules release H⁺ ions, and an equilibrium is set up. Examples: ethanoic (acetic), citric and carbonic acids.
pH and hydrogen ion concentration
The pH is a measure of the H⁺ ion concentration:
- As the pH decreases by 1 unit, the H⁺ concentration increases by a factor of 10.
- So a strong acid has a lower pH than a weak acid of the same concentration.
Strength vs concentration
These are different ideas:
- Strength = the degree of ionisation (strong = fully ionised; weak = partly).
- Concentration = the amount of acid dissolved in a given volume (concentrated = lots dissolved; dilute = little).
A weak acid can be concentrated, and a strong acid can be dilute.
Exam tips
- Strong acid = fully ionised; weak acid = partially ionised.
- pH ↓ 1 → H⁺ concentration ↑ ×10.
- Do not confuse strength (ionisation) with concentration (amount dissolved).
- Learn examples of strong (HCl, H₂SO₄, HNO₃) and weak (ethanoic, citric, carbonic) acids.