Litmus paper is a type of pH indicator paper used to determine the acidity or alkalinity of a substance. It is impregnated with a mixture of natural dyes extracted from lichens, primarily Roccella tinctoria. Litmus paper is available in two common forms: blue litmus paper and red litmus paper.

The working principle of litmus paper relies on the concept of acid-base indicators. These indicators are substances that change color in response to changes in the hydrogen ion concentration (pH) of a solution. Litmus paper contains indicators that exhibit different colors in acidic and alkaline conditions.

Here's how it works:

Blue Litmus Paper: Blue litmus paper is blue in its original form. When it comes into contact with an acidic solution, the blue litmus paper turns red. This color change occurs because the acidic solution donates hydrogen ions (H+) to the litmus paper, causing the blue dye to become protonated and change to a red form.

Red Litmus Paper: Red litmus paper is red in its original form. When it comes into contact with an alkaline (basic) solution, the red litmus paper turns blue. This color change happens because the alkaline solution accepts hydrogen ions from the litmus paper, causing the red dye to become deprotonated and change to a blue form.

It's important to note that litmus paper is a qualitative indicator, meaning it provides a visual indication of whether a substance is acidic or alkaline. However, it does not provide an exact measurement of the pH value. For precise pH measurements, more accurate instruments such as pH meters or pH test strips are used.

Litmus paper is widely used in various fields, including chemistry, biology, and medicine, for quick and simple pH testing. It is a cost-effective and convenient tool to determine the approximate pH of a solution and to classify it as acidic or alkaline based on the observed color change.