Alkali – IGCSE Chemistry Definition
IGCSE Chemistry definition of alkali: a soluble base producing OH−(aq) in water. Covers examples, reactions, pH, and common exam mistakes 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.
Alkalis are one of the most frequently tested concepts in IGCSE Chemistry because they link acids, bases, neutralisation, pH, and salt preparation into a single story. Examiners regularly ask candidates to distinguish an alkali from a base — a distinction worth at least one mark on Paper 2 and Paper 4.
The 0620 definition
An alkali is a soluble base that produces hydroxide ions, OH−(aq), when dissolved in water.
Key examples: sodium hydroxide NaOH, potassium hydroxide KOH, calcium hydroxide Ca(OH)₂ (sparingly soluble, but enough dissolves to give a pH above 7).
Ammonia solution NH₃(aq) is also an alkali — it reacts with water to form NH₄OH, releasing OH− ions.
Properties of alkalis
| Property | Detail |
|---|---|
| pH | Above 7 |
| Litmus | Turns red litmus blue |
| Universal indicator | Blue/purple |
| Feel | Soapy (do not test by touch) |
| With ammonium salts | Produces ammonia gas on warming |
Alkalis in neutralisation
The reaction between an alkali and an acid is:
Alkali + acid → salt + water
Ionic equation for every neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l)
This ionic equation is a Supplement requirement and regularly appears on Paper 4. The key point is that the salt ions are spectator ions — only H⁺ and OH⁻ react.
Example: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Alkalis and salt preparation
Alkalis are used in titration to prepare soluble salts. Because both the acid and alkali are solutions, you cannot simply add excess and filter — you must use a burette and indicator to find the exact volume needed.
Worked exam question
A student adds dilute hydrochloric acid to potassium hydroxide solution. (a) Name the type of reaction. [1] (b) Write the balanced equation. [1] (c) State the ionic equation. [1]
Mark scheme
(a) Neutralisation [1]
(b) KOH + HCl → KCl + H₂O [1]
(c) H⁺(aq) + OH⁻(aq) → H₂O(l) [1]
Common exam mistakes
- Saying “all bases are alkalis”. Only soluble bases are alkalis. CuO is a base but not an alkali.
- Writing the ionic equation without state symbols — the (aq) and (l) are required for the mark.
- Confusing alkali metals (Group I elements) with alkalis (the hydroxide solutions). Sodium is an alkali metal; sodium hydroxide is an alkali.
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