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Processed with VS1Distinguish a test that sorts or signals from one that proves whether something works or is good enough.
VOA source audio
Source: VOA Learning English · Words and Their Stories
View original article & transcriptThe program compares two science-based expressions. A “litmus test” is a sign or criterion used to sort something into a category or judge acceptability. An “acid test” is a demanding proof of quality, truth, value, or effectiveness. The story notes that the proposed historical origin of “acid test” is uncertain.
Cultural note: These are figurative expressions in everyday discussion, not claims that someone used a chemistry test. “Litmus test” often signals a criterion that decides where something belongs; “acid test” asks whether it holds up under a real challenge. Treat the proposed etymology in the source as uncertain, not established fact.
Keep exploringA company says its new service is easy to use. A short screening question can sort likely users, while real customer experience can show whether the claim holds up. Which is the litmus test, and which is the acid test?
Phrase guide · 5 expressions
Original explanations and examples, created for learners. Save a card for later or hear the phrase spoken by your browser.
Identify a deciding sign or condition that places something in a category.
Meaning
A criterion or sign used to judge whether something is acceptable or belongs in a particular category.
Tone & use
Often used in public debate, hiring, and decision-making. It may imply one decisive criterion, so say what the criterion is.
Example
“For the volunteer role, reliability is a litmus test because the team depends on people arriving on time.”
Describe a demanding check that shows whether a claim works.
Meaning
A demanding check that proves the quality, value, truth, or effectiveness of something.
Tone & use
Use when actual results or a difficult situation reveal whether a claim holds up. It is figurative in ordinary conversation.
Example
“The first week with real customers will be the acid test for the new support process.”
Meaning
Remain convincing when examined carefully.
Tone & use
Useful for evaluating claims, evidence, plans, or explanations; more explicit than “acid test.”
Example
“The explanation sounded plausible, but it did not hold up under scrutiny of the original records.”
Meaning
Meet the requirements of an evaluation or prove adequate in a particular situation.
Tone & use
A broad everyday phrase. Name the test or standard when the context is not obvious.
Example
“The backup system passed the test when the main server briefly went offline.”
Use neutral words when a figurative test is not the clearest choice.
Meaning
A sign or piece of information that points to a condition or likely result.
Tone & use
More literal and often more neutral than “litmus test.” An indicator can be useful without being decisive proof.
Example
“A rising number of repeat visitors is one indicator that readers find the guide useful.”
Knowledge check
Choose the expression that best fits each situation.
Try it · Choose the best expression
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