Blue litmus paper turns red in acidic solutions.
Litmus is a mixture of water-soluble dyes extracted from lichens and used as an acid–base indicator. In an acidic environment, the dye’s molecular structure changes so that the paper appears red; in an alkaline environment, red litmus generally turns blue.
The word litmus comes from the Old Norse term “litmosi,” meaning colored moss. Litmus testing has roots in medieval natural-dye use, and the material became a practical way to distinguish acids from bases before modern electronic pH meters.
Litmus is a quick qualitative test, not a precise pH measurement. Its color transition occurs across a range near pH 4.5–8.3, and neutral litmus is commonly purple. A related trap is chlorine: it can bleach blue litmus white, which is a chemical bleaching reaction rather than the normal acid-indicator color change.