Litmus paper is a type of paper used to test the acidity or alkalinity of a solution. It is treated with natural dyes derived from lichens, which change color when exposed to different pH levels. A red color indicates an acidic solution (pH less than 7), while a blue color shows an alkaline solution (pH greater than 7). Litmus paper is commonly used in laboratories, schools, and various industries for quick and easy pH testing. It provides a simple way to determine the chemical properties of a substance without the need for complex equipment.