Period 6 element also dissolves in alkaline solutions of cyanide, which have been used in mining. Gold dissolves in mercury, forming amalgam alloys. Gold is insoluble in nitric acid, which dissolves silver and base metals, a property that has long been used to confirm the presence of gold in items, giving rise to the term "the acid test." Gold has been a valuable and highly sought-after precious metal for coinage, jewelry, and other arts since long before the beginning of recorded history. Gold standards have been a common basis for monetary policies throughout human history, later being supplanted by fiat currency starting in
Acid–base reaction reacts with an alkali salt (a metal hydroxide), the product is a metal salt and water. Acid–alkali reactions are also neutralization reactions. In general, acid–alkali reactions can be simplified to by omitting spectator ions. Acids are in general pure substances that contain hydrogen cations () or cause them to be produced in solutions. Hydrochloric acid () and sulfuric acid () are common examples. In water, these break apart into ions: The alkali breaks apart in water, yielding dissolved hydroxide ions: Acid–base reaction An acid–base reaction is a chemical reaction that occurs between an acid and a base, which can be