Litmus paper is absorbent paper treated with litmus, a mixture of dyes which change colour with PH.
One of many acid-base indicators, litmus reacts to concentrations of hydrogen ions (in other words, the pH), turning increasingly red under acidic conditions (above pH 7).
The indicator molecule can exist in two different-coloured states: one associated with hydrogen and the other not.
In an acidic solution, excess hydrogen ions lead the red, hydrogen-associated form to dominate. Conversely, in a basic (alkaline) solution the equilibrium is shifted and the blue, the hydrogen disassociated form dominates.
Answered by Alex Cheung