litmus
n. uncountablen. a substance that changes color when it touches an acid or a base. It turns red in acids and blue in bases, so it is used to test if something is acidic or alkaline.
n. a water-soluble mixture of dyes extracted from lichens, used as a pH indicator. It changes color depending on the acidity or alkalinity of the solution it is placed in.
The teacher used litmus paper to test the lemon juice.
In the chemistry lab, students dipped blue litmus paper into the solution to confirm it was acidic.
Before the advent of digital sensors, litmus served as the primary tool for determining the chemical nature of a substance, providing a simple visual cue for basic or acidic environments.
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.