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

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

Playing in Friday's Chatrier day session against exciting Brazilian teenager Joao Fonseca, with temperatures set to reach 34C, will be a litmus test.

From BBC • May 28, 2026

They also provide a valuable litmus test for contemporary romance.

From Salon • Mar. 19, 2026

Olympic qualification doesn’t involve a political litmus test—only citizenship, sportsmanship and skill.

From The Wall Street Journal • Feb. 17, 2026

Still, many wondered if the Lakers’ litmus test would come from facing the defending champion Oklahoma City Thunder on Monday night.

From Los Angeles Times • Feb. 10, 2026

When they are strongly impregnated with the gas, they redden infusion of litmus, and exhibit some other of the characteristics of acids; and, even in a weak state, they blacken silver and lead.

From Memoranda on Tours and Touraine Including remarks on the climate with a sketch of the Botany And Geology of the Province also on the Wines and Mineral Waters of France by Holdsworth, J. H.