Acid – IGCSE Chemistry Definition and Reactions
IGCSE Chemistry definition of acid: a substance producing H+(aq) in water. Covers properties, reactions with metals, bases and carbonates, and common exam mistakes.
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.
Acids appear in nearly every IGCSE Chemistry paper. The definition alone carries a free mark on Paper 4, and the four core reactions of acids — with metals, bases, metal oxides, and carbonates — are tested across Papers 1–4. Candidates who memorise the general equations but forget the ionic explanation of what makes a substance acidic lose the Supplement marks that separate a B from an A.
The 0620 definition
An acid is a substance that produces hydrogen ions, H+(aq), when dissolved in water. The (aq) is important: it means the ions are in aqueous solution, surrounded by water molecules.
Common examples: hydrochloric acid HCl, sulfuric acid H₂SO₄, nitric acid HNO₃. All three are strong acids — they ionise completely.
Weak acids (Supplement): ethanoic acid CH₃COOH and carbonic acid H₂CO₃ ionise only partially. A 0.1 mol/dm³ solution of a weak acid has a higher pH than the same concentration of a strong acid.
Properties of acids
| Property | Detail |
|---|---|
| pH | Below 7 |
| Litmus | Turns blue litmus red |
| Universal indicator | Red/orange/yellow depending on strength |
| Electrical conductivity | Conducts (free H+ and anions move) |
| Taste (lab safety: never taste) | Sour |
The four core reactions
1. Acid + metal → salt + hydrogen
Only metals above hydrogen in the reactivity series react. The test for hydrogen: a burning splint gives a squeaky pop.
Example: Mg + 2HCl → MgCl₂ + H₂
2. Acid + base → salt + water (neutralisation)
A base is a metal oxide or metal hydroxide that reacts with an acid.
Example: CuO + H₂SO₄ → CuSO₄ + H₂O
3. Acid + alkali → salt + water
An alkali is a soluble base — it produces OH− in water.
Example: NaOH + HCl → NaCl + H₂O
The ionic equation for every neutralisation: H+(aq) + OH−(aq) → H₂O(l)
4. Acid + carbonate → salt + water + carbon dioxide
The test for CO₂: it turns limewater milky.
Example: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Naming the salt
The metal comes from the base/metal/carbonate. The ending comes from the acid:
| Acid | Salt ending | Example salt |
|---|---|---|
| Hydrochloric acid, HCl | chloride | Sodium chloride |
| Sulfuric acid, H₂SO₄ | sulfate | Copper(II) sulfate |
| Nitric acid, HNO₃ | nitrate | Zinc nitrate |
Worked exam question
Magnesium ribbon is added to dilute hydrochloric acid. (a) Write the balanced equation. (1) (b) State two observations. (2) (c) Name the salt formed. (1)
(a) Mg + 2HCl → MgCl₂ + H₂ [1]
(b) Effervescence / bubbles of gas [1]; magnesium dissolves / gets smaller [1]. Accept: solution warms.
(c) Magnesium chloride [1]
Common exam mistakes
- Writing “hydrochloric” as the salt name instead of “chloride”. The salt is never called hydrochloric anything.
- Forgetting to balance the equation — the 2 in front of HCl is a separate mark.
- Confusing strong/weak with concentrated/dilute. Concentration is how much acid is dissolved; strength is how fully it ionises. You can have dilute strong acid and concentrated weak acid.
- Writing H₂ as an observation instead of describing what you would see (bubbles, effervescence).
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