Aqua regia (lit. "royal water"), aqua regis (lit. "king's water"), or nitro-hydrochloric acid is a highly-corrosive mixture of acids, a fuming yellow or red solution. The mixture is formed by freshly mixing concentrated nitric acid and hydrochloric acid, usually in a volume ratio of 1:3. It was named so because it can dissolve the so-called royal or noble metals, gold and platinum. However, titanium, iridium, ruthenium, tantalum, osmium, rhodium and a few other metals are capable of withstanding its corrosive properties.

Aqua regia is primarily used to produce chloroauric acid, the electrolyte in the Wohlwill process. This process is used for refining highest quality (99.999%) gold.

Aqua regia is also used in etching and in specific analytic procedures. It is also used in some laboratories to clean glassware of organic compounds and metal particles. This method is preferred over the "traditional" chromic acid bath for cleaning NMR tubes, because no traces of paramagnetic chromium can remain to later spoil acquired spectra. While chromic acid baths are discouraged because of the high toxicity of chromium and the potential for explosions, aqua regia is itself very corrosive and has been implicated in several explosions due to mishandling.

Coordinates: 41°53′31″N 12°29′12″E / 41.891967°N 12.486595°E / 41.891967; 12.486595

The Regia was a structure in Ancient Rome in the Roman Forum that originally served as the residence or one of the main headquarters of kings of Rome and later as the office of the Pontifex Maximus, the high priest of Roman religion. It occupied a triangular patch of terrain between the Temple of Vesta, the Temple of Divus Julius and Temple of Antoninus and Faustina. Only the foundations of Republican/Imperial Regia remain. Like the Curia it was destroyed and rebuilt several times, as far back as the Roman monarchy. Studies have found multiple layers of similar buildings with more regular features, prompting the theory that this "Republican Regia" was to have a different use.^{[further explanation needed]}

According to ancient tradition it was built by the second king of Rome, Numa Pompilius as a royal palace. Indeed, the Latin term regia can be translated as royal residence. It is said that he also built the Temple of Vesta and the House of the Vestal Virgins as well as the Domus Publica. This created a central area for political and religious life in the city and Kingdom. When Caesar became Pontifex Maximus he exercised his duties from the Regia.