Litmus Paper: How It Works

“Litmus test” has become an everyday phrase for any simple test that gives a clear answer. The original litmus test deserves the reputation. A strip of dyed paper, a drop of solution, and within a second you know whether you’re dealing with an acid or an alkali. No calculations, no calibration, no electricity.

Here’s what litmus actually is, why it changes colour, and what it can and can’t tell you.

What litmus is made of

Litmus is a natural dye extracted from certain species of lichen, the crusty organisms (partnerships between a fungus and an alga) that grow on rocks and trees. Lichens from the genus Roccella, found on rocky coasts, were traditionally used, along with a few others.

The lichens are ground up and fermented, historically in the presence of ammonia and potash, which converts colourless compounds in the lichen into a complex mixture of blue-purple dyes. The mixture contains around ten to fifteen different compounds. One of the main coloured components is called azolitmin, and many of the dyes share a phenoxazone-type ring structure that’s responsible for the colour.

The Netherlands became famous for litmus production from the 16th century onwards, and the word itself probably comes from Old Norse roots meaning “dye moss”.

Why it changes colour

Like all acid–base indicators, the dyes in litmus are weak acids. We can write the equilibrium in general form:

HIn ⇌ H⁺ + In⁻ (red) (blue)

• In acidic solution, the high concentration of H⁺ pushes the equilibrium to the left, so the red acid form dominates.
• In alkaline solution, hydroxide ions remove H⁺, pulling the equilibrium to the right, so the blue conjugate base dominates.

Gaining or losing a proton changes the arrangement of alternating double bonds in the dye molecule. That shifts the wavelengths of light it absorbs, so we see a different colour. See acid–base indicators for more on this.

The change happens gradually over a fairly wide range, roughly pH 4.5 to 8.3. Below about 4.5, litmus is fully red; above about 8.3, fully blue; in between, it’s purple.

Red, blue and neutral litmus paper

Litmus paper is filter paper soaked in litmus solution and dried. It comes in three types:

Type	Colour	Use	Changes to
Blue litmus paper	blue	tests for acids	red in acid
Red litmus paper	red	tests for alkalis	blue in alkali
Neutral litmus paper	purple	tests for both	red in acid, blue in alkali

Blue litmus paper is made with litmus that has been treated so that it’s in its blue form; red paper has had a little acid added to put it in the red form.

The key logic: a paper can only show a change towards the other colour. Blue paper in acid turns red. Blue paper in alkali stays blue, which tells you nothing new. That’s why you usually test with both.

Result	Blue paper	Red paper	Conclusion
Acid	turns red	stays red	acidic
Alkali	stays blue	turns blue	alkaline
Neutral	stays blue	stays red	neutral (roughly pH 5–8)

A memory aid: “Blue to Red means acid” (BRA), and red to blue means alkali.

How to use it correctly

1. Place a small piece of litmus paper on a clean white tile.
2. Dip a clean glass rod into the test solution and touch it to the paper. (Dipping paper straight into a sample can contaminate it.)
3. Look at the colour immediately.
4. Test with both red and blue paper unless you only need to know one thing.
5. Rinse the rod between samples.

Testing gases

To test a gas, dampen the litmus paper with distilled water first and hold it in the gas. Gases need to dissolve in water to form ions:

• Ammonia turns damp red litmus blue. This is the standard test for ammonia gas.
• Hydrogen chloride turns damp blue litmus red.
• Carbon dioxide may turn damp blue litmus slightly red, because it dissolves to form weak carbonic acid, but the change is faint.
• Chlorine turns damp blue litmus red, then bleaches it white. This two-step change is the standard test for chlorine: the red comes from acids formed when chlorine reacts with water, and the bleaching from hypochlorous acid.

What litmus can’t do

It can’t measure pH. Litmus tells you acid, alkali or roughly neutral. It can’t distinguish pH 1 from pH 4, or pH 9 from pH 13. For that, use universal indicator or a pH meter.

It isn’t good for titrations. Its colour changes gradually over nearly four pH units, and through an in-between purple that’s hard to judge. Indicators with sharper changes, such as phenolphthalein or methyl orange, are used instead. See how to choose an indicator.

It isn’t reliable with coloured or bleaching solutions. Strongly coloured liquids mask the result, and oxidising agents such as bleach can destroy the dye.

Near neutral, it’s ambiguous. A weakly acidic solution at pH 6 and a weakly alkaline one at pH 8 can both give similar purple-ish shades.

Litmus vs other indicator papers

	Litmus paper	Universal indicator paper	pH meter
Tells you	acid or alkali	pH to about ±1	pH to ±0.01
Cost	very low	low	higher
Shelf life	long if dry	long if sealed	needs calibration and care
Best for	quick yes/no tests, gas tests	estimating pH	accurate measurements

A short history

Litmus is one of the oldest chemical indicators. Lichen dyes were used for colouring cloth in medieval Europe, and the Spanish alchemist Arnaldus de Villa Nova is sometimes said to have noted colour changes in lichen dyes around 1300, although that attribution is uncertain. What is well documented is that Robert Boyle, in his 1664 book Experiments and Considerations Touching Colours, described using plant dyes, including those from lichens, to distinguish acids from alkalis systematically. His work helped turn a dyer’s curiosity into a scientific tool.

Paper strips impregnated with litmus became common laboratory supplies in the 19th century and have hardly changed since.

Common misconceptions

• “Litmus stays purple in water because water is neutral.” Actually, red and blue papers keep their own colours in pure water; only neutral (purple) litmus paper is purple.
• “If blue litmus doesn’t change, the solution must be alkaline.” It could equally be neutral. Test with red litmus too.
• “Litmus turns green in alkali.” That’s universal indicator. Litmus goes blue.
• “Litmus can tell you how strong an acid is.” It can’t. A weak and a strong acid both turn blue litmus red.

Quick quiz

1. Which paper would you use to test whether a solution is alkaline?
2. Damp red litmus paper turns blue near a test tube. Which common gas might be present?
3. Blue litmus paper turns red and then white. Which gas is present?
4. Why must litmus paper be damp when testing gases?

Answers: (1) Red litmus paper; it turns blue in alkali. (2) Ammonia. (3) Chlorine. (4) Gases must dissolve in water to form the H⁺ or OH⁻ ions that change the indicator’s colour.

Key takeaways

• Litmus is a mixture of natural dyes from lichens that behaves as a weak acid indicator.
• It’s red in acid (below about pH 4.5), blue in alkali (above about pH 8.3), and purple in between.
• Blue paper tests for acids; red paper tests for alkalis; test with both for a full answer.
• Damp litmus is used in the standard tests for ammonia and chlorine.
• It classifies but doesn’t measure; use universal indicator or a pH meter for actual pH values, and see the pH scale explained.