Weak Acid – IGCSE Chemistry Definition
IGCSE Chemistry definition of weak acid: an acid that partially ionises in water. Covers examples, reversible equilibrium, and exam tips for Cambridge 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Weak acids are a Supplement topic that pairs directly with strong acids. The examiner expects candidates to use the correct arrow notation and to explain the pH difference at equal concentrations — both are common Paper 4 targets.
The 0620 definition
A weak acid is an acid that partially ionises in water (aqueous solution).
Only a small fraction of the acid molecules break apart into ions. The rest remain as whole molecules in solution. An equilibrium is established between the molecules and ions.
CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq) — reversible, partial ionisation
H₂CO₃(aq) ⇌ 2H⁺(aq) + CO₃²⁻(aq)
Notice the equilibrium arrow (⇌) — this is essential for the mark on Paper 4.
Key examples
| Weak acid | Formula | Where found |
|---|---|---|
| Ethanoic acid | CH₃COOH | Vinegar |
| Citric acid | C₆H₈O₇ | Citrus fruits |
| Carbonic acid | H₂CO₃ | Fizzy drinks (CO₂ + H₂O) |
Consequences of partial ionisation
Because fewer H⁺ ions are produced:
- Higher pH than a strong acid at the same concentration
- Lower electrical conductivity — fewer ions to carry current
- Same reactions as strong acids (with metals, bases, carbonates) but often slower because the H⁺ concentration is lower
This last point is critical: a weak acid still reacts with magnesium to give hydrogen gas. It just produces bubbles more slowly than the same concentration of a strong acid.
The reversible arrow matters
Writing → for a weak acid is a common error that costs the mark. The equilibrium arrow ⇌ shows that:
- The forward reaction (ionisation) and backward reaction (recombination) occur simultaneously
- At any given moment, most molecules are undissociated
Worked exam question
Ethanoic acid (0.1 mol/dm³) has a pH of 3. Hydrochloric acid (0.1 mol/dm³) has a pH of 1. (a) Explain this difference. [3] (b) State one similarity in the reactions of these two acids. [1]
Mark scheme
(a) Ethanoic acid is a weak acid / partially ionises [1]; hydrochloric acid is a strong acid / completely ionises [1]; so HCl produces more H⁺ ions at the same concentration, giving a lower pH [1]
(b) Both react with metals / bases / carbonates to form salts / hydrogen / water / CO₂ [1] (any valid shared reaction)
Common exam mistakes
- Confusing weak with dilute. A weak acid can be concentrated; a dilute acid can be strong. They are independent.
- Using a forward arrow (→) instead of the equilibrium arrow (⇌) for the ionisation of a weak acid.
- Saying weak acids do not react with metals — they do, just more slowly.
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