Why Is It Called a “Litmus Test”?

In a 2024 New York Times piece exploring the controversy over married couples in Japan using separate surnames, the Times headline read: “A Litmus Test in Japan: Should Spouses Have Different Surnames?” Likewise, a story about the Los Angeles Kings dropping a hockey game to the Las Vegas Knights was described as a “litmus test” for the California team.

In the media and elsewhere, various parties seem to constantly be faced with a litmus test about their political views, athletic prowess, or overall capabilities. But what exactly is litmus, and why is it constantly being invoked as a metaphor for testing one’s worth?

1. The Origin of Litmus Test
2. Litmus Test vs. Acid Test

The Origin of Litmus Test

While litmus test is typically used as an idiom, there’s actually a literal meaning to it. Litmus is a kind of coloring or dye derived from lichen, the hybrid fungi-algae growth commonly found on rocks. It’s used in chemistry to differentiate an acid from a base. If one dips litmus paper in a solution and it turns red, it’s acidic, but if it turns blue, it's alkaline. The litmus test is binary, or a kind of pass-fail measure.

The Oxford English Dictionary cites litmus as being derived from the Dutch leecmos and dates use of litmus in the literal sense to the 1500s. The figurative term first shows up in print in the late 1800s. (“The alleged investigation will not stand the litmus paper test of public opinion,” asserted an 1896 Pittsburgh Press article.)

To put something to a litmus test is to imply the situation relies solely on one single determining factor. Something is, or something is not—the litmus test determines which. The Kings failed their litmus test of getting past the Stanley Cup champions the Knights; voters in Japan will or will not vote for a prime minister candidate based on the litmus test of how they feel about separate surnames.

One could also go on a date and see how their company treats waitstaff. If they’re kind, that could mean passing a litmus test of compatibility. A literal litmus test essentially sets a goal that has an objective value: The solution is or is not acidic. A figurative litmus test sets a subjective threshold.

If you use litmus test to refer to worthiness or quality, you might be thinking of another, similar idiom: the acid test.

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Litmus Test vs. Acid Test

Like litmus test, acid test is another term borrowed from chemistry, where nitric acid is used to evaluate the presence of gold. The idiom acid test is to challenge the value or quality of something. You may find an acid test incorporates several factors whereas a litmus test is nominally used for a single determining factor. Voters, for example, may weigh several issues during a presidential debate, meaning the candidates will be undergoing an acid test. If they’re asked one pointed question about a single incendiary topic, that may be the litmus test of the debate.

The two idioms otherwise hew fairly close together, with each used to assess the overall quality or worth of an opinion, person, or event. Because the literal term is so easily applied—the litmus test is merely watching a cheap piece of paper change color, or not—a figurative litmus test is also typically something relatively easy to arrange or witness. A 20-page quiz on history is not exactly a litmus test of one’s knowledge. If anything, it’s more of an acid test—or just a plain test.

Sitting down to eat at a new restaurant, on the other hand, might prompt one to choose a single dish as a litmus test. If a new eatery makes a hard-to-prepare meal, a patron may use that as evidence of its overall competency. But another customer may want to eat several meals or courses and assess a business’s overall service, making it an acid test.

If you label something a litmus test, it’s probably something easily applied, evaluated, and experienced that will help others form an opinion. Asking someone if they know the etymology of the litmus test is, in its way, a method of measuring their grasp of linguistics—a litmus test of a litmus test.

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