Base – IGCSE Chemistry Definition
IGCSE Chemistry definition of base: a substance neutralising an acid to form salt and water. Covers types, examples, reactions, and common exam errors for 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 base–alkali distinction is one of the most commonly examined definitions in IGCSE Chemistry. Paper 2 and Paper 4 regularly include a question requiring candidates to define “base” and then explain why not all bases are alkalis. Getting this definition precise is easy marks.
The 0620 definition
A base is a substance that reacts with an acid to produce a salt and water only.
Bases include metal oxides (e.g. CuO, MgO), metal hydroxides (e.g. NaOH, Cu(OH)₂), and ammonia (NH₃).
An alkali is the special subset of bases that dissolve in water to produce OH⁻(aq) ions.
Types of bases
| Type | Example | Soluble? | Also an alkali? |
|---|---|---|---|
| Metal oxide | CuO | No | No |
| Metal oxide | Na₂O | Yes | Yes |
| Metal hydroxide | NaOH | Yes | Yes |
| Metal hydroxide | Cu(OH)₂ | No | No |
| Ammonia | NH₃(aq) | Yes | Yes |
Key reactions
Base + acid → salt + water
CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)
This is the standard method for preparing a soluble salt from an insoluble base: add excess base to warm acid, then filter off the unreacted solid and crystallise the filtrate.
Alkali + acid → salt + water
NaOH(aq) + HNO₃(aq) → NaNO₃(aq) + H₂O(l)
Because both reactants are solutions, you cannot add excess. A titration with an indicator finds the exact volumes.
Base + ammonium salt → salt + water + ammonia
Ca(OH)₂ + 2NH₄Cl → CaCl₂ + 2H₂O + 2NH₃
This is the test for ammonium ions: warm with a base and test for ammonia gas (turns damp red litmus blue).
Worked exam question
Copper(II) oxide is a base. (a) Define the term base. [1] (b) Describe how you would use copper(II) oxide and dilute sulfuric acid to prepare pure, dry crystals of copper(II) sulfate. [4]
Mark scheme
(a) A substance that reacts with an acid to form a salt and water (only) [1]
(b) Add excess CuO to warm dilute H₂SO₄ [1]; stir until no more dissolves / black solid remains [1]; filter to remove excess CuO [1]; heat the filtrate gently / evaporate to the point of crystallisation, then cool and filter to collect crystals / dry between filter paper [1]
Common exam mistakes
- Defining a base as “a substance with pH above 7” — that describes an alkali in solution, not a base. Insoluble bases like CuO do not have a pH because they do not dissolve.
- Forgetting that ammonia is a base. NH₃ + HCl → NH₄Cl — no water is formed, but ammonia still neutralises the acid by accepting H⁺.
- Writing “base + acid → salt + water + gas” — bases do not produce gas. Only carbonates give CO₂ with acid.
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