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,
Acidophiles in acid mine drainage economically and from a conservation perspective. Control of these acidophiles and their harnessing for industrial biotechnology shows their effect need not be entirely negative. The use of acidophilic organisms in mining is a new technique for extracting trace metals through bioleaching, and offers solutions for the phenomenon of acid mine drainage in mining spoils. Upon exposure to oxygen (O) and water (HO), metal sulfides undergo oxidation to produce metal-rich acidic effluent. If the pH is low enough to overcome the natural buffering capacity of the surrounding rocks (‘calcium carbonate equivalent’ or ‘acid neutralising capacity’), the surrounding area may become acidic,