Litmus paper is used to indicate the acidity or alkalinity of a solution by changing color. It is made from a natural dye extracted from lichens, and when dipped into a solution, it can change from red to blue, or from blue to red, depending on the pH level of the solution.

In acidic solutions, litmus paper turns red, while in basic solutions, it turns blue. Litmus paper acts as an acid-base indicator. Indicators are substances that change color over a certain range of pH values. Litmus paper has a pH range of 4.5 to 8.3, which means that it will change color from red to blue as the pH of a solution increases from 4.5 to 8.3.

Litmus paper is useful for quick and easy identification of the acidity or alkalinity of a solution, but it is not an accurate way to measure the exact pH of a solution. For more precise measurements, pH meters or other specific indicators are needed.