An acid test is any qualitative chemical or metallurgical assay which uses acid; most commonly, and historically, the use of a strong acid to distinguish gold from base metals.
Testing for gold with acid focuses on the fact that gold is a noble metal which is resistant to change by corrosion, oxidation, or acid. The acid test for gold is to rub the gold-colored item on black stone, which will leave an easily visible mark. The mark is tested by applying nitric acid, which dissolves the mark of any item that is not gold. If the mark remains, it is tested by applying aqua regia (nitric acid and hydrochloric acid). If the mark is removed, then this test dissolves the gold, proving the item to be genuine gold. More accurate testing of the item for its fineness or purity can be done through the use of differing strengths of aqua regia and comparative testing of gold items of known fineness.
This article uses material from the Wikipedia article “Acid Test Gold”, which is released under the Creative Commons Attribution-Share-Alike License 3.0.