Aqua regia (Latin: royal water) or aqua regis is a highly corrosive mixture of acids, fuming yellow or red solution, also called nitro-hydrochloric acid. The mixture is formed by freshly mixing nearly pure nitric acid and maximum-concentration (38%) hydrochloric acid, usually in a volume ratio of 1:3 respectively. It was named so because it can dissolve the so-called royal metals, or noble metals, gold and platinum. However, ruthenium, tantalum, iridium, osmium, titanium, rhodium and a few other metals are capable of withstanding chemical attack from it.
Aqua regia first appeared in the work of the medieval European alchemist Pseudo-Geber.
When Germany invaded Denmark in World War II, the Hungarian chemist George de Hevesy dissolved the gold Nobel Prizes of the German physicists Max von Laue (1914) and James Franck (1925) in aqua regia to prevent the Nazis from confiscating them. The German government had prohibited Germans from accepting or keeping any Nobel Prize after the jailed peace activist Carl von Ossietzky had received the Nobel Peace Prize in 1935. De Hevesy placed the resulting solution on a shelf in his laboratory at the Niels Bohr Institute. It was subsequently ignored by the Nazis who thought the jar—one of perhaps hundreds on the shelving—contained common chemicals. After the war, de Hevesy returned to find the solution undisturbed and precipitated the gold out of the acid. The gold was returned to the Royal Swedish Academy of Sciences and the Nobel Foundation who recast the medals and again presented them to Laue and Franck.