litmus

English

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Litmus test (chemistry)

From Middle English litmose, lytmose, litemose, from Old Norse litmosi (“moss used for dyeing”), from lita (“to dye, stain”) +‎ mosi (“moss”), the former from litr (“colour, dye, blee”), from Proto-Germanic *wlitiz, *wlituz (“appearance, blee”), from Proto-Indo-European *wel- (“to see”). Cognate with Old English wlite (“appearance, form, brightness, countenance”). More at moss.

litmus (countable and uncountable, plural litmuses)

(uncountable) A dyestuff extracted from certain lichens, that changes color when exposed to pH levels greater than or less than certain critical levels.
A simple test of acidity in a liquid using litmus, usually in the form of litmus paper.
A simple test of any attribute; a litmus test.

dyestuff from lichen