Litmus – IGCSE Chemistry Definition and Key Facts

Litmus is the simplest indicator on the Cambridge 0620 syllabus. While universal indicator and pH measurements are more informative, litmus questions still appear regularly — particularly in practical contexts and as the starting point for acid-alkali identification.

The 0620 definition

Litmus is an indicator made from lichens. It is available as litmus paper (red or blue) or as litmus solution. It turns red in acidic solutions and blue in alkaline solutions.

Colour changes

Test solution	Blue litmus paper	Red litmus paper	Litmus solution
Acid (pH < 7)	Turns red	Stays red	Turns red
Neutral (pH 7)	Stays blue	Stays red	Stays purple
Alkali (pH > 7)	Stays blue	Turns blue	Turns blue

The key exam point: to confirm a solution is acidic, you use blue litmus paper (it changes from blue to red). To confirm it is alkaline, you use red litmus paper (it changes from red to blue). Using the wrong colour litmus means no colour change is observed, which tells you nothing useful.

How litmus is used in practice

Testing gases

Litmus paper is used to test for certain gases:

Gas	Test	Result
Ammonia (NH₃)	Hold damp red litmus over the gas	Turns blue (ammonia is alkaline)
Hydrogen chloride (HCl gas)	Hold damp blue litmus in the gas	Turns red (HCl is acidic)
Chlorine (Cl₂)	Hold damp litmus in the gas	Turns red then bleaches white

The chlorine test is distinctive: litmus first turns red (chlorine in water forms an acid) and then is bleached colourless. This two-step change is a diagnostic test for chlorine gas.

Testing solutions

A drop of litmus solution added to a test tube of unknown solution quickly shows whether it is acidic or alkaline. This is faster than using pH paper but less precise.

Litmus vs universal indicator

Feature	Litmus	Universal indicator
Colours produced	Red or blue (or purple)	Full spectrum from red to purple
Information given	Acid or alkali only	Approximate pH value
Precision	Low — cannot distinguish strong from weak acid	Moderate — gives pH to nearest unit
Use case	Quick classification	Estimating pH

For Paper 4 questions asking “estimate the pH,” universal indicator or a pH meter is needed — litmus alone cannot give a pH value.

Litmus in titrations

Litmus is generally not suitable as a titration indicator. Its colour change at the endpoint (purple) is not sharp enough to identify the exact neutralisation point. Indicators like methyl orange or phenolphthalein, which change colour over a narrow pH range, are preferred for titrations.

Worked exam question

A student tests an unknown solution using blue litmus paper and red litmus paper. The blue litmus paper turns red. The red litmus paper stays red. (a) What can the student conclude? [1] (b) Suggest a more informative test. [1]

(a) The solution is acidic / has pH below 7 [1]

(b) Use universal indicator to find the pH value / use a pH meter [1]

Common exam mistakes

1. Saying “litmus turns green in neutral” — litmus in a neutral solution is purple, not green. Green is the neutral colour of universal indicator.
2. Using litmus to “measure pH” — litmus cannot measure pH; it can only classify a solution as acidic or alkaline.
3. Forgetting to specify which colour of litmus paper is used. To show a solution is acidic, you need blue litmus (which changes to red). Red litmus in an acid stays red — no useful change.
4. Not mentioning that the litmus paper should be damp when testing gases — dry litmus does not respond to dry gases.