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,
Formic acid carboxylic acids. Because of its high acidity, solutions in alcohols form esters spontaneously. Formic acid shares some of the reducing properties of aldehydes, reducing solutions of gold, silver, and platinum to the metals. Heat and especially acids cause formic acid to decompose to carbon monoxide (CO) and water (dehydration). Treatment of formic acid with sulfuric acid is a convenient laboratory source of CO. In the presence of platinum, it decomposes with a release of hydrogen and carbon dioxide. Soluble ruthenium catalysts are also effective. Carbon monoxide free hydrogen has been generated in a very wide pressure range (1–600 bar). Formic