A dye obtained from certain lichens that is red under acid conditions and blue under alkaline conditions.‘In the presence of a basic solution, litmus will lose hydronium ions and turn blue.’
- ‘During the 1660s, Robert Boyle discovered that certain plant extracts, such as litmus, can be used to distinguish acids from bases.’
Middle English from Old Norse lit-mosi, from litr ‘dye’ + mosi ‘moss’.
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