litmus

American  

[lit-muhs] / ˈlɪt məs /

noun

1. a blue coloring matter obtained from certain lichens, especially Roccella tinctoria. In alkaline solution litmus turns blue, in acid solution, red: widely used as a chemical indicator.

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litmus British  

/ ˈlɪtməs /

noun

1. a soluble powder obtained from certain lichens. It turns red under acid conditions and blue under basic conditions and is used as an indicator

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litmus Scientific  

/ lĭt′məs /

1. A colored powder, obtained from certain lichens, that changes to red in an acid solution and to blue in an alkaline solution. Litmus is a mixture of various closely related heterocyclic organic compounds.

2. ◆ Litmus is typically added to paper to make litmus paper, which can be used to determine whether a solution is basic or acidic by dipping a strip of the paper into the solution and seeing how the paper changes color.

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litmus Cultural  

1. In chemistry, a kind of paper used to tell whether a solution is an acid or a base. Acids turn blue litmus paper red; bases turn red litmus paper blue. Other testing paper or sophisticated instruments can be used to measure the pH of a solution more precisely.

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Discover More

The term litmus is often used to refer to a general and simple test: “Your vote on this issue is a litmus test of your political philosophy.”

Etymology

Origin of litmus

1495–1505; earlier lytmos < Old Norse litmosi dye-moss, equivalent to lit- color, dye + mosi moss

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Explanation

Litmus is a special dye that's used to determine how acidic a substance is. Scientists use litmus to test liquids or gases to see if they're an acid or a base. Litmus is the mix of dyes that's used to make litmus paper, a scientific tool that makes it easy to see if a material has an acidic pH. Made from lichen, litmus can be absorbed onto paper which turns blue to indicate an alkaline substance and red if it's acidic. Litmus comes from the Middle Dutch word lijkmoes, which combines lac, "red resinous substance" and moes, "pulp."

Example Sentences

See Examples For:

litmus

These questions can be a litmus test that readers can apply to check for meaningful dialogue around AI safety.

From The Wall Street Journal ● Oct. 22, 2026

Chesney’s mental training ultimately serves as a litmus test in discovering who is cut from a different cloth.

From Los Angeles Times ● Aug. 7, 2026

"This is a litmus test. A barometer of feeling nationally and locally."

From BBC ● May 8, 2026

In retrospect, its uncompromising litmus tests and chilly demeanor were real weaknesses, creating a pervading fear that any false step could expel one from good graces, and that a return trip was always, always treacherous.

From Slate ● Apr. 13, 2026

For instance, a weak solution of litmus in water is blue when a thin layer is examined, and red when it is a thicker or more concentrated layer.

From Colour Measurement and Mixture by W. de W. Abney

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