Indicator – IGCSE Chemistry Definition
IGCSE Chemistry definition of indicator: a substance changing colour to show acidic or alkaline conditions. Covers litmus, methyl orange, phenolphthalein for 0620.
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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.
Indicators are tested in both theory and practical contexts across all IGCSE Chemistry papers. You must know the colour changes of three specific indicators and understand when each is appropriate to use — particularly in titrations.
The 0620 definition
An indicator is a substance that changes colour depending on whether the solution is acidic or alkaline.
Key indicators and colour changes
| Indicator | In acid | At neutral / endpoint | In alkali |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Methyl orange | Red | Orange (endpoint) | Yellow |
| Phenolphthalein | Colourless | Colourless | Pink |
Litmus
The simplest indicator. Available as paper (red litmus paper and blue litmus paper) or as a solution. Used for quick acid/alkali identification, not for titrations.
- Acids turn blue litmus red
- Alkalis turn red litmus blue
Methyl orange
Used in titrations involving strong acids. The sharp colour change from red to yellow (or the reverse) makes the endpoint easy to see. The endpoint colour is orange.
Phenolphthalein
Used in titrations involving strong alkalis. The solution is colourless in acid and turns pink when alkali is in excess. The endpoint is the first permanent pale pink colour.
Universal indicator vs single indicators
Universal indicator is a mixture of several indicators that produces a continuous range of colours from red (pH 1) to violet (pH 14). It tells you the approximate pH but is unsuitable for titrations because there is no single sharp colour change at the endpoint.
Worked exam question
A student carries out a titration of hydrochloric acid with sodium hydroxide. (a) Suggest a suitable indicator. [1] (b) State the colour change at the endpoint. [1] (c) Explain why universal indicator is not suitable. [1]
Mark scheme
(a) Methyl orange OR phenolphthalein [1]
(b) Methyl orange: red to orange/yellow [1] OR Phenolphthalein: colourless to pink [1] (must match the indicator named in part (a))
(c) Universal indicator gives a gradual colour change / range of colours, so it is difficult to identify the exact endpoint [1]
Common exam mistakes
- Giving the wrong direction of colour change — always state the colour change from the starting condition (e.g. “red to yellow” not just “yellow”).
- Saying litmus is suitable for titrations — it is not, because its colour change is not sharp enough.
- Confusing the colours of phenolphthalein — it is colourless in acid, not white or clear.
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