Strong Acid – IGCSE Chemistry Definition
IGCSE Chemistry definition of strong acid: an acid that completely ionises in water. Covers examples, comparison with weak acids, 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.
The strong/weak acid distinction is a Supplement topic that appears on Paper 4 virtually every year. Examiners specifically target the misconception that “strong” means “concentrated” — understanding the difference between these terms is worth guaranteed marks.
The 0620 definition
A strong acid is an acid that completely ionises in water (aqueous solution).
This means every molecule of the acid breaks apart into ions. There are no undissociated acid molecules remaining.
HCl(aq) → H⁺(aq) + Cl⁻(aq) — 100% ionised, forward arrow only
H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq) — 100% ionised
HNO₃(aq) → H⁺(aq) + NO₃⁻(aq) — 100% ionised
Notice the single forward arrow (→) — this indicates the ionisation goes to completion.
Strong vs weak: the key comparison
| Feature | Strong acid | Weak acid |
|---|---|---|
| Ionisation | Complete (100%) | Partial |
| Arrow in equation | → (forward only) | ⇌ (reversible / equilibrium) |
| pH (same concentration) | Lower | Higher |
| H⁺ concentration | Higher | Lower |
| Conductivity (same conc.) | Higher | Lower |
| Examples | HCl, H₂SO₄, HNO₃ | CH₃COOH, H₂CO₃ |
Why pH differs at the same concentration
If you make 0.1 mol/dm³ solutions of HCl and CH₃COOH:
- HCl ionises completely → [H⁺] = 0.1 mol/dm³ → pH ≈ 1
- CH₃COOH partially ionises → [H⁺] < 0.1 mol/dm³ → pH ≈ 3
Same concentration, different pH — because strength (degree of ionisation) differs.
Worked exam question
Hydrochloric acid is a strong acid. Ethanoic acid is a weak acid. Both are at the same concentration. (a) Define the term strong acid. [1] (b) Explain why the pH of hydrochloric acid is lower than that of ethanoic acid at the same concentration. [2]
Mark scheme
(a) An acid that completely / fully ionises in water / aqueous solution [1]
(b) HCl completely ionises [1]; so it produces a higher concentration of H⁺ ions (than ethanoic acid at the same concentration), giving a lower pH [1]
Common exam mistakes
- Confusing strong with concentrated. Strong = fully ionised. Concentrated = lots of solute per dm³. These are independent properties.
- Using a reversible arrow for a strong acid. HCl → not ⇌. The single arrow shows complete ionisation.
- Saying strong acids are “more reactive” — strength describes ionisation, not reactivity. Both strong and weak acids react with metals and carbonates.
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