Which acid is listed as capable of dissolving gold when used alone?

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

Answer: (C)

Gold tends to resist most acids because you need a strong oxidizer that can both push gold into a higher oxidation state and keep it in solution as a stable complex. Selenic acid is one of those powerful oxidizing acids that can do this on its own, so gold can be dissolved by it without needing a second reagent.

Nitric acid alone can’t dissolve gold because gold isn’t readily oxidized by nitric acid; it stays inert in that environment. Hydrochloric acid by itself isn’t an oxidizer either, so it can’t convert gold into a soluble form. Aqua regia dissolves gold, but it’s a mixture of nitric and hydrochloric acids working together—the nitric acid oxidizes, and the chloride provides a stabilizing complex for the dissolved gold—so it isn’t an acid used alone.