Explain the difference between a strong acid and a weak acid.

Edexcel GCSE Chemistry (1CH0) — 3.1 Acids · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Hydrochloric acid and ethanoic acid are both acids, but they behave differently when dissolved in water.

Model answer (4 marks)

A strong acid is one that is completely ionised in aqueous solution, meaning all of its molecules dissociate to give H⁺ ions. A weak acid is only partially ionised, so only a fraction of its molecules give H⁺ ions.

Because of this, a strong acid has a higher concentration of H⁺ ions than a weak acid when both are at the same overall concentration.

Consequently, a strong acid has a lower pH than a weak acid of the same concentration. For example, 0.1 M HCl (strong) has a pH ≈1, whereas 0.1 M CH₃COOH (weak) has a pH ≈2.3.

Examiner tips

  • Use the word ‘completely’ for strong acids and ‘partially’ for weak acids. Include the pH comparison and give one example of each acid type. Show the link between ionisation and H⁺ concentration.

Common mistakes

  • Confusing ionisation with concentration. Forgetting to mention the pH difference or the example acids. Using ‘strong’ and ‘weak’ without explaining the ionisation behaviour.

Mark scheme (4 marks)

  1. A strong acid is completely ionised in aqueous solution
  2. A weak acid is only partially ionised in aqueous solution
  3. Therefore a strong acid has a higher concentration of H+ ions than a weak acid (at the same overall concentration)
  4. A strong acid has a lower pH than a weak acid of the same concentration, with a correct example of each type

Key terms in this question

strong acid · weak acid

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