Litmus is a water-soluble dye extracted from certain lichens and absorbed on to filter paper. The resulting piece of paper becomes a pH indicator (one of the oldest), used to test materials for acidity. Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic conditions, the colour change occurring over the pH range 4.5-8.3 (at 25°C). The active ingredient of Litmus is called Erythrolitmin. Low pH value substances are called acids, and high ones are called alkalis.

pH indicators | Paper

リトマス | Лакмус