The acid test is the decisive test that shows what a thing, person or idea is really worth. The acid test of a raincoat is a three-hour downpour. It's often followed by whether: the acid test is whether a stranger can follow the map. It works in conversation and in essays.
When would you say the acid test?
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the acid test of + quality; the acid test for + person / thing; pass the acid test
- The acid test of the waterproof bag came during a three-hour downpour.
- The first crowded service will be the acid test for the new café team.
- The chair passed the acid test: it remained comfortable throughout a long rehearsal.
What changed
First versionThe litmus paper of a good map is whether a stranger can follow it.
RevisionThe acid test of a good map is whether a stranger can follow it.
In chemistry, litmus paper shows whether something is an acid, so it seems like a natural metaphor for a decisive check. But English doesn't use the paper itself this way. You need the whole idiom, the acid test (or the litmus test): the acid test of a good map. The literal image was replaced.
A test is any check, even a trial run: a quick test. The acid test is the decisive one, after which everything is clear. The litmus test is almost a synonym, used more about views and principles: Their response is a litmus test of their values. Litmus paper is only the real paper from a chemistry lab.
Your turn
The acid test ___ a clear instruction is whether a new user understands it.
Show the answer
of — What is being tested is introduced with of: the acid test of a clear instruction. On may come from a test on a topic, but it doesn't work here.
Name the decisive test for an everyday object, like an umbrella, a backpack or headphones, and say what it should show.
The acid test of a lunch box is whether it keeps soup inside when the bag tips over.
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