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litmus lit·mus (lĭt'məs)
A water-soluble blue powder derived from lichens that changes to red with increasing acidity and to blue with increasing basicity.
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. ◇ 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.
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.
Note: 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.”