Acid test

An acid test is a qualitative chemical or metallurgical assay utilizing acid. Historically, it often involved the use of a robust acid to distinguish gold from base metals. Figuratively, the term represents any definitive test for attributes, such as gauging a person's character or evaluating a product's performance.

Chemistry
Testing for gold with acid capitalizes on gold's status as a noble metal, resistant to corrosion, oxidation, or acid. The procedure includes rubbing the gold-colored item on black stone, leaving a visible mark. The mark undergoes scrutiny by applying nitric acid, which dissolves the mark of any item not gold, stainless steel, zinc, tungsten, aluminum, platinum, or palladium. If the mark persists, further testing involves aqua regia (nitric acid and hydrochloric acid). If the mark dissolves, the item proves to be genuine gold. More precise assessment of fineness or purity is achieved using varying strengths of aqua regia and comparative testing against known fineness.

Geology
In geological applications, detecting the presence of calcite or other forms of calcium carbonate in alkaline soils or during lithological analysis involves using dilute hydrochloric acid and observing effervescence.

Figurative meanings
The figurative usage of the expression gained popularity during and after the California Gold Rush, but had currency even before then. An example from the Wisconsin paper The Columbia Reporter, November 1845, illustrates this: "Twenty-four years of service demonstrates his ability to stand the acid test, as Gibson’s Soap Polish has done for over thirty years."

Other instances of the figurative use of the phrase include websites like Acid1, Acid2, and Acid3, designed to test web browsers for compliance with current web standards. Financial analysts also employ the "acid test" nickname for the quick ratio method, which assesses the liquidity of a business.

The term "acid test" took on a different meaning in the realm of experiences with the psychedelic drug LSD, popularized by the Merry Pranksters. This association stems from the drug's colloquial name, "acid" (which chemically it indeed was).