An acid test is any qualitative chemical or metallurgical assay

Metallurgical assay

A metallurgical assay is a compositional analysis of an ore, metal, or alloy.Some assay methods are suitable for raw materials; others are more appropriate for finished goods. Raw precious metals are assayed by an assay office...

 which uses acid

Acid

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

; most commonly, and historically, the use of a strong acid to distinguish gold

Gold

Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

 from base metal

Base metal

In chemistry, the term base metal is used informally to refer to a metal that oxidizes or corrodes relatively easily, and reacts variably with diluted hydrochloric acid to form hydrogen. Examples include iron, nickel, lead and zinc...

s. Figuratively, acid test is any definitive test for some attribute, e.g. of a person's character, or of the performance of some product.

Chemistry

The traditional acid test for gold consists of placing a small drop of a strong acid, such as nitric acid

Nitric acid

Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...

, onto the metal's surface. Most metals fizz or bubble, whilst precious metals are unaffected. The acid test is thus decisive, immediate, cheap and simple.

Figurative meanings

The figurative meaning of the expression, where it is applied to tests of character, or definitive tests to other materials, became popular during and after the California Gold Rush

California Gold Rush

The California Gold Rush began on January 24, 1848, when gold was found by James W. Marshall at Sutter's Mill in Coloma, California. The first to hear confirmed information of the gold rush were the people in Oregon, the Sandwich Islands , and Latin America, who were the first to start flocking to...

, but was current before then, as shown by this quote from the Wisconsin paper The Columbia Reporter, November 1845:

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 examples of the figurative use of the phrase are the web sites Acid1

Acid1

Acid1, originally called the Box Acid Test, is a test page for web browsers. It was developed in October 1998 and was important in establishing baseline interoperability between early web browsers, especially for the Cascading Style Sheets 1.0 specification...

, Acid2

Acid2

Acid2 is a test page published and promoted by the Web Standards Project to expose web page rendering flaws in web browsers and other applications that render HTML. Named after the acid test for gold, it was developed in the spirit of Acid1, a relatively narrow test of compliance with the Cascading...

 and Acid3

Acid3

Acid3 test is a web test page from the Web Standards Project that checks a web browser's compliance with elements of various web standards, particularly the Document Object Model and JavaScript....

, which are designed to test web browser

Web browser

A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...

s for compliance with current web standards. Another example is the quick ratio

Quick ratio

In finance, the Acid-test or quick ratio or liquid ratio measures the ability of a company to use its near cash or quick assets to extinguish or retire its current liabilities immediately. Quick assets include those current assets that presumably can be quickly converted to cash at close to their...

 method, nicknamed "acid test", used by financial analysts to assess a business' liquidity.

The use of the term "acid test" for experiences with the psychedelic drug

Psychedelic drug

A psychedelic substance is a psychoactive drug whose primary action is to alter cognition and perception. Psychedelics are part of a wider class of psychoactive drugs known as hallucinogens, a class that also includes related substances such as dissociatives and deliriants...

 LSD

LSD

Lysergic acid diethylamide, abbreviated LSD or LSD-25, also known as lysergide and colloquially as acid, is a semisynthetic psychedelic drug of the ergoline family, well known for its psychological effects which can include altered thinking processes, closed and open eye visuals, synaesthesia, an...

 was popularised by the Merry Pranksters

Merry Pranksters

The Merry Pranksters were a group of people who formed around American author Ken Kesey in 1964 and sometimes lived communally at his homes in California and Oregon...

, and derives from the drug's common name, "acid". Many users experience a dissolution between themselves and the "outside world", and such experiences are considered to be a test of one's character.