Blue litmus paper remains blue when contacted by an alkaline solution, while red litmus paper turns blue. Litmus paper may also be used to determine the acidity and neutrality of a solution.
Litmus is a dye composed of several different lichens in a powder form that easily dissolves into water. It is typically more chemically reactive when applied to filter paper. When used to determine the acidity of a solution, red litmus paper remains red, while blue litmus paper turns red. If the solution is neutral, both colors of litmus paper retain their original color. Litmus may be dissolved directly into a solution to determine its alkalinity or acidity.