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Related to litmus: litmus paper
1. A chemical test used to determine acidity or alkalinity in a solution. The students performed a litmus test in class to learn whether the chemical solution was an acid or a base.
2. A test used to determine someone's true intentions or beliefs. I used his reaction to my favorite movie as a litmus test to determine if he was worth dating.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
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.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
a litmus testJOURNALISM
COMMON If something is a litmus test of the quality or success of a particular thing, it is an effective way of proving it or measuring it. My personal litmus test when I have to decide whether to keep or discard something is whether or not I look at and enjoy it every day. The success of wind power represents a litmus test for renewable energy. Note: Litmus paper is used to test the acidity of substances. It turns red in acid conditions and blue in alkaline conditions.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012