An acid test in the literal sense is most commonly the use of a strong acid to distinguish gold from base metals.
If someone has passed the “acid test,” it usually means that he has proven his worth or value through experience or trial.
When gold was in wide circulation, jewelers and assayers needed a method of testing golden objects and nuggets that were brought to them for cash.
Because nitric acid dissolves base metals but not gold, a drop was applied to the suspect object, and if the metal didn’t dissolve, it had passed the acid test and was confirmed to be gold.
