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A litmus test is used in chemistry to determine if a solution is acidic or basic using litmus or litmus paper.

Litmus comes from certain species of lichens. Litmus will turn blue when exposed to a base and red when exposed to an acid. Litmus paper is paper with litmus in or on it.

A litmus test is a very basic chemical test in which you dip a piece of litmus paper in an unknown solution and look at what color the paper changes to—either red or blue. If the paper doesn’t change color, then the solution is either chemically neutral (not an acid or a base) or it matches whatever color the litmus currently is.

Litmus testing is so common and well known that the phrase litmus test is also used to mean that something is a strong indication of a future success or failure, as in Record producers viewed the young singer’s career as a litmus test for a new generation of teenage musical stars.

The first records of the figurative sense of litmus test come from around 1955. It refers to the chemical litmus test, which originated in the 1300s, after scientists discovered that litmus changes color in reaction to acids and bases.

Chemical litmus tests are a common, simple experiment taught in elementary science classes when explaining acids and bases. Your teacher will ask you to dip litmus paper in an unknown liquid and then watch whether the paper changes color to determine if the liquid is an acid, a base, or neutral.

Outside of chemistry, the phrase litmus test is commonly used to describe an event or situation that is being closely monitored to be used when planning for the future. For example, national political parties often view local elections as litmus tests for which candidates or policies will be popular with a large number of voters.

One reason litmus tests are useful is that the two most common gases in air, nitrogen and oxygen, are chemically neutral. Otherwise, exposing litmus tests to air would make the papers change color.

This image shows a collection of litmus tests in which litmus paper has been turned red or blue by several different solutions.

Litmus Paper and the Litmus Test

— Anne Helmenstine (@azareal)

Almost all students will perform a litmus test experiment in chemistry class. But the figurative use of litmus test is also very popular, especially when discussing politics.

The litmus test as to whether you paid attention in school is if you know what a litmus test is

— gongtown (@gongs)

On my second day in Guwahati, I see Union Home Minister making a grand entry in the middle of frenzied supporters of Assam. The polls are a litmus test on the Saffron might in northeast whether it can hold onto the 2016 wave.

— Aarti Tikoo Singh (@AartiTikoo)

• litmus
• litmus paper
• acid
• base
• chemistry
• solution
• experiment

True or False?

When you perform a litmus test, you dip a piece of litmus paper in a solution to determine whether the liquid is an acid or a base.