Litmus paper is a simple test commonly used to determine whether a solution is acid or base. Acidity is important to life, and general health and wellness. Most biological processes have a very narrow pH range in which they can take place. A change in blood or urine acidity can be critical in determining and diagnosing the presence of disease.
The first recorded use of litmus dyes was by the Spanish alchemist Arnaldus de Villa Nova around 1300 AD. Litmus paper made with these dyes was probably invented in the early 1800s by French chemist J.L. Gay-Lussac.
Litmus paper is a white paper that has been soaked in a dye solution, which will change color in an acid or a base. There are two types of litmus paper. Blue litmus paper turns pink in the presence of an acid. Red litmus paper turns blue in the presence of a base.
Litmus paper is commonly used in high school science classes to check for acidic or basic conditions in a gas or liquid. It is also useful for checking the acidity of soil, and in biological applications such as checking the acidity of urine.
Litmus paper only defines acid or base, but cannot tell the actual pH of the solution. An approximate pH can be found using a universal indicator or pH paper. To find the exact pH, a pH meter is needed.
The expression "giving the litmus test" in modern language can be a reference to using litmus paper as a definitive test for determining whether a substance is acid or base, but the term has also come to describe the act of determining the truth of a situation.