Which acid is described as the only single acid capable of dissolving gold?

• A. Nitric acid
• B. Hydrochloric acid
• C. Selenic acid
• D. Aqua regia

Answer: (C)

Gold is remarkably resistant to most acids; to dissolve it you need more than just a single acid. The practical dissolution mechanism relies on an oxidizing environment plus a source of chloride that can form stable gold complexes. Nitric acid or hydrochloric acid alone don’t both oxidize gold and stabilize the dissolved gold as a complex, so gold tends not to dissolve in either of them by itself. A mixture called aqua regia does the job: the oxidizer (nitric acid) raises gold to a higher oxidation state, while the chloride ions from hydrochloric acid create stable complex ions like [AuCl4]−, pulling gold into solution and preventing it from reprecipitating. Because of this need for both oxidation and complexation, there isn’t a standard “single” acid that dissolves gold in ordinary chemistry. Selenic acid is not the typical solution used for dissolving gold, so the widely taught method is aqua regia, not a lone acid.