Litmus paper is paper used to test whether a solution is acidic or basic.
Light Blue litmus paper turns red under acidic conditions, and red litmus paper turns blue under basic or alkaline conditions, with the color change occurring over the pH range 4.5–8.3 at 25 °C (77 °F). The neutral litmus paper is purple. Litmus can also be prepared as an aqueous solution that functions similarly. Under acidic conditions, the solution is red, and under alkaline conditions, the solution is blue.
We can use Wet litmus paper to test for water-soluble gases that affect acidity or basicity; the gas dissolves in the water. The resulting solution colors the litmus paper. For instance, alkaline ammonia gas turns red litmus paper blue.
Chemical reactions other than acid-base can also cause a color change to litmus paper. For instance, chlorine gas turns blue litmus paperwhite - the litmus dye is bleached because of hypochlorite ions. This reaction is irreversible, so the litmus is not acting as an indicator in this situation.