Litmus

Substance to test chemical acidity

Litmus is a water-soluble mixture of different dyes extracted from lichens. It is often absorbed into filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity. In an acidic medium, blue litmus paper turns red, while in a basic or alkaline medium, red litmus paper turns blue.

From Wikipedia, under CC BY-SA. More on occurri.