In chemistry, a litmus test is used to find out if a solution is an acid or a base. A special kind of paper is dipped in the liquid being tested during a litmus test.
A scientific litmus test can tell you the pH of a gas or liquid — in other words, it shows exactly how acidic the substance is. A litmus test uses litmus paper to test the substance; it turns red if it's acidic and blue if it's alkaline, or basic. The term litmus test is also used to talk about a simple, decisive test of something: "The vote was a litmus test of the voters' feelings about the economy."