Gold element silver (as electrum) and also naturally alloyed with copper and palladium. Less commonly, it occurs in minerals as gold compounds, often with tellurium (gold tellurides). Gold is resistant to most acids, though it does dissolve in aqua regia, a mixture of nitric acid and hydrochloric acid, which forms a soluble tetrachloroaurate anion. Gold is insoluble in nitric acid, which dissolves silver and base metals, a property that has long been used to refine gold and to confirm the presence of gold in metallic objects, giving rise to the term "acid test". Gold also dissolves in alkaline solutions of cyanide,
Brønsted–Lowry acid–base theory to become its conjugate base, A. The base, B, can accept a proton to become its conjugate acid, HB. Most acid-base reactions are fast so that the components of the reaction are usually in dynamic equilibrium with each other. Consider the following acid–base reaction: Acetic acid, CHCOOH, is an acid because it donates a proton to water (HO) and becomes its conjugate base, the acetate ion (CHCOO). HO is a base because it accepts a proton from CHCOOH and becomes its conjugate acid, the hydronium ion, (HO). The reverse of an acid-base reaction is also an acid-base reaction, between the