Litmus
Summary: Litmus is a common indicator used to test whether a solution is acidic or alkaline. It turns red in acidic solutions and blue in alkaline solutions. Litmus paper is the most widely used form. Tags: igcse chemistry definition acids-bases indicators Created: 2026-07-14 Last Updated: 2026-07-23
Litmus is a naturally-derived indicator — a substance that changes colour depending on the pH of the solution it is added to. It is extracted from lichens and is absorbed onto filter paper to make litmus paper.
Colour Changes
| Solution Type | Colour of Litmus |
|---|---|
| Acidic (pH < 7) | Red |
| Neutral (pH = 7) | Purple (no change) |
| Alkaline (pH > 7) | Blue |
Forms of Litmus
- Blue litmus paper: Turns red in acid. Used to test for acids.
- Red litmus paper: Turns blue in alkali. Used to test for alkalis.
- Litmus solution: Purple liquid that turns red in acid or blue in alkali.
How It’s Used
- Dip a piece of litmus paper into the solution
- Observe the colour change
- Blue litmus turning red = acidic solution
- Red litmus turning blue = alkaline solution
Key Facts
- Litmus only tells you whether a solution is acidic or alkaline — it does NOT give a pH value
- For a precise pH, use universal indicator or a pH meter
- Litmus is referenced in the IGCSE syllabus alongside thymolphthalein and methyl orange
- Blue litmus → red means acid; Red litmus → blue means alkali
See Also
- Methyl Orange
- Thymolphthalein
- Universal Indicator
- Indicators and pH
- Acids and Bases
- IGCSE-Chem-Index
This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.