Comparing Strong and Weak Acids
IGCSE Chemistry strong vs weak acids: complete and partial ionisation, pH differences at the same concentration, conductivity, and reaction rates for Cambridge 0620 Supplement.
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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 distinction between strong and weak acids is a Supplement topic on the Cambridge 0620 syllabus. It is one of the most commonly confused areas — candidates frequently mix up strong/weak with concentrated/dilute. This page clarifies the difference.
Key definitions
Strong acid
A strong acid is one that completely ionises (dissociates) in water. Every molecule of the acid produces H⁺ ions.
HCl → H⁺ + Cl⁻ (100% ionised — forward arrow only)
HNO₃ → H⁺ + NO₃⁻
H₂SO₄ → 2H⁺ + SO₄²⁻
Weak acid
A weak acid partially ionises in water. Only a small fraction of the molecules produce H⁺ ions. An equilibrium is established.
CH₃COOH ⇌ CH₃COO⁻ + H⁺ (only ~1% ionised — reversible arrow)
H₂CO₃ ⇌ 2H⁺ + CO₃²⁻
C₆H₈O₇ ⇌ ions (citric acid)
Notice the arrow: strong acids use a single forward arrow (→) because the reaction goes to completion. Weak acids use a reversible arrow (⇌) because an equilibrium exists.
Comparing strong and weak acids at the same concentration
If both acids are at 0.1 mol/dm³:
| Property | Strong acid (e.g. HCl) | Weak acid (e.g. CH₃COOH) |
|---|---|---|
| Degree of ionisation | 100% (complete) | ~1% (partial) |
| Concentration of H⁺ ions | High (0.1 mol/dm³) | Low (~0.001 mol/dm³) |
| pH | Low (~1) | Higher (~3) |
| Universal indicator colour | Red | Orange/yellow |
| Electrical conductivity | High (many ions) | Lower (fewer ions) |
| Rate of reaction with Mg | Fast, vigorous fizzing | Slower, gentler fizzing |
| Volume of H₂ produced with excess Mg | Same | Same |
The last point is crucial: at the same concentration and volume, both acids contain the same total amount of acid molecules. The weak acid reacts more slowly because fewer H⁺ ions are available at any moment, but as H⁺ ions are used up, the equilibrium shifts to produce more. Eventually, the same total amount of hydrogen gas is produced.
The concentration vs strength distinction
| Strength | Concentration | |
|---|---|---|
| What it measures | Degree of ionisation | Amount of acid per unit volume |
| Determined by | The nature of the acid itself | How much acid is dissolved |
| Can be changed? | No — a strong acid is always strong | Yes — by adding water or more acid |
| Units | No units (complete or partial) | mol/dm³ |
Four possible combinations
| Combination | Example | pH |
|---|---|---|
| Dilute strong acid | 0.001 mol/dm³ HCl | ~3 |
| Concentrated strong acid | 10 mol/dm³ HCl | Very low |
| Dilute weak acid | 0.001 mol/dm³ CH₃COOH | ~4-5 |
| Concentrated weak acid | 10 mol/dm³ CH₃COOH | ~2-3 |
A concentrated weak acid can have a lower pH than a dilute strong acid — because even though it is only partially ionised, the sheer number of molecules means a significant number still ionise.
Identifying strong vs weak acids experimentally
Method 1: Measure pH at the same concentration
Use a pH meter or universal indicator. At the same concentration, the strong acid has the lower pH.
Method 2: Compare rates of reaction
Add equal-sized pieces of magnesium ribbon to equal volumes of equal concentration acid. The strong acid reacts faster (more vigorous bubbling) because it has a higher concentration of H⁺ ions available.
Method 3: Measure electrical conductivity
At the same concentration, the strong acid conducts better because it has more ions in solution.
Common strong and weak acids
| Strong acids | Weak acids |
|---|---|
| Hydrochloric acid (HCl) | Ethanoic acid (CH₃COOH) — vinegar |
| Sulfuric acid (H₂SO₄) | Carbonic acid (H₂CO₃) — fizzy drinks |
| Nitric acid (HNO₃) | Citric acid (C₆H₈O₇) — citrus fruits |
Worked exam question
Two acids, A (strong) and B (weak), both have a concentration of 0.10 mol/dm³. (a) Explain why acid A has a lower pH than acid B. [2] (b) Equal volumes of each acid are added to separate beakers containing excess magnesium. Compare the rate and total volume of hydrogen produced. [3]
(a) Acid A is a strong acid and completely ionises, producing a higher concentration of H⁺ ions [1]. Acid B only partially ionises, so it has fewer H⁺ ions in solution and a higher pH [1].
(b) Acid A reacts faster / more vigorously because it has a higher concentration of H⁺ ions [1]. As acid B reacts, its equilibrium shifts to produce more H⁺ ions [1]. The total volume of hydrogen produced is the same because both acids contain the same total amount of acid (same concentration and volume) [1].
Common exam mistakes
- Confusing strong/weak with concentrated/dilute — these are independent properties. A strong acid can be dilute; a weak acid can be concentrated.
- Saying a weak acid produces less gas than a strong acid of the same concentration — with excess metal, the total gas volume is the same; only the rate is different.
- Using a single forward arrow for a weak acid equation — weak acids establish an equilibrium, so use a reversible arrow (⇌).
- Saying “strong acids are more dangerous” — a very concentrated weak acid can be more corrosive than a very dilute strong acid. Danger depends on both strength and concentration.
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