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 conjugate acid of the base in the first reaction and the conjugate base of the acid. In the above example, acetate is the base of the reverse reaction and hydronium ion is the acid. The power of the Brønsted–Lowry theory is that, in contrast to Arrhenius theory, it does not require an acid to dissociate. The essence of Brønsted–Lowry theory is that an acid only exists as such in relation to a base, and "vice versa". Water is amphoteric as it can act as an acid or as a base. In the image shown at the right one molecule of