Litmus paper red:
changes colour from red to blue when conditions change from acidic to alkaline.

Recommended application:
General test for acidic or alkaline reaction: It serves in neutralisation reactions to check whether the change from acid to alkaline or vice versa has taken place. Litmus paper is the bestknown indicator paper.

Chemistry:
Litmus is not a single, chemically defined substance but a natural product containing a mixture of substances. It is extracted from lichen and probably represents the oldest known indicator dye.