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Phrase(s): litmus test
1. Lit. a test used to determine the acidity or alkalinity of chemical substances. (Acid turns litmus paper red and alkaline compounds turn it blue.) • I used a litmus test to show that the compound was slightly acid. 2. Fig. a question or experiment that seeks to determine the state of one important factor. • His performance on the long exam served as a litmus test to determine whether he would go to college. • The amount of white cells in my blood became the litmus test for diagnosing my disease.
McGrawhill's American Idioms And Phrasal Verbs