litmus
/ˈlɪtməs
/noun [mass noun]
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.
Origin
Middle English: from Old Norse lit-mosi, from litr 'dye' + mosi 'moss'.