Acid Test


Meaning: A sure test, giving an incontestable result.

Origin
In the California Gold Rush, in the second half of the 19th century, prospectors and dealers needed to be able to tell gold from base metal. Gold doesn't react to most acids as other metals do, but does dissolve in Aqua Regia (a mixture of hydrochloric acid and nitric acid). To confirm that a find was gold it was given 'the acid test'. Various other tests can be made by the use of acid and these are all called 'acid tests'.
The earliest citation is from The Burlington Hawk-eye, Iowa, which sounds precisely the right sort of publication for such a phrase. It is dated, mid-Gold Rush - September 1877, in a piece entitled Counterfeit Coins - the perfection of a wicked art:
"This coin is of right weight, rings out clear and sharp, and will stand the acid test."

 

In Context:

The last week of school is definitely an acid test for teachers and students alike!

Think About It:  When one passes the acid test, is there a reward?