Freshly prepared aqua regia is colorless, but it turns yellow, orange or red within seconds from the formation of nitrosyl chloride and nitrogen dioxide.

These reactions result in the volatile products nitrosyl chloride and chlorine gas: as evidenced by the fuming nature and characteristic yellow color of aqua regia.

Nitrosyl chloride (NOCl) can further decompose into nitric oxide (NO) and elemental chlorine (Cl2): This dissociation is equilibrium-limited.

Because nitric oxide readily reacts with atmospheric oxygen, the gases produced also contain nitrogen dioxide, NO2 (red fume): Aqua regia is primarily used to produce chloroauric acid, the electrolyte in the Wohlwill process for refining the highest purity (99.999%) gold.

This method is preferred among most over the more traditional chromic acid bath for cleaning NMR tubes, because no traces of paramagnetic chromium can remain to spoil spectra.

If full dissolution of the platinum is desired, repeated extractions of the residual solids with concentrated hydrochloric acid must be performed: and The chloroplatinous acid can be oxidized to chloroplatinic acid by saturating the solution with molecular chlorine (Cl2) while heating: Dissolving platinum solids in aqua regia was the mode of discovery for the densest metals, iridium and osmium, both of which are found in platinum ores and are not dissolved by aqua regia, instead collecting as insoluble metallic powder (elemental Ir, Os) on the base of the vessel.

As a practical matter, when platinum group metals are purified through dissolution in aqua regia, gold (commonly associated with PGMs) is precipitated by treatment with iron(II) chloride.

[8] The third of Basil Valentine's keys (c. 1600) shows a dragon in the foreground and a fox eating a rooster in the background.

[11] When Germany invaded Denmark in World War II, Hungarian chemist George de Hevesy dissolved the gold Nobel Prizes of German physicists Max von Laue (1914) and James Franck (1925) in aqua regia to prevent the Nazis from confiscating them.

De Hevesy placed the resulting solution on a shelf in his laboratory at the Niels Bohr Institute.