Litmus paper

A paper, normally purple in colour, which turns red in an acid solution and blue in an alkaline solution.

Litmus is a water-soluble mixture of different dyes extracted from certain lichens which is available as a solution or can be absorbed on to a porous paper. The resulting solution or piece of paper becomes a pH indicator which is used to determine whether a solution is acid or alkaline.

Litmus paper turns red under acidic conditions at a pH of 4.5 or below and turns blue under alkaline conditions at a pH of greater than 8.3. Weak acids and alkalis with a pH of between 4.5 and 8.3, appear to be neutral.