Litmus, a dye used by chemists to learn whether a solution is an acid or a base. The color of the dye can change from blue to red and from red to blue. Blue litmus turns red in acid solutions, and red litmus turns blue in basic solutions.

Litmus is obtained from organisms called lichens. The dye is prepared by crushing the lichens and fermenting them in a mixture of ammonia and potash. The fermented matter, which is blue, is mixed with chalk to form a paste, which is dried and powdered. Red litmus is made by adding acid. Litmus is usually used in the form of litmus paper—absorbent paper that has been treated with litmus.