Nitric acid is an inorganic mineral acid. Liquid and with a pungent odour, nitric acid can be mixed with water in any ratio. It reacts violently with ethanol and is colourless in its pure state. The chemical is one of the strongest acids and can even dissolve precious metals such as silver. Especially when exposed to light, concentrated nitric acid decomposes easily and often has a yellowish or reddish hue due to the nitrogen dioxide (NO2) dissolved in it. A solution of nitric acid and nitrogen dioxide is called "fuming nitric acid":
When it comes into contact with air or exceeds its boiling point, nitric acid decomposes, producing the reddish-brown, highly toxic nitrogen dioxide (NO2). This nitrogen dioxide gas resembles orange-brown smoke and gives "fuming nitric acid" its name. As described above, nitrogen dioxide is formed when nitric acid decomposes or reacts. It has a strong oxidising effect and thus promotes fire - so it can ignite some easily combustible materials such as wood or straw.
The synonym "separating water" for nitric acid stems from the fact that silver can be dissolved out of gold with a 50 percent solution of the chemical, i.e. the metals gold and silver can be separated from each other. Concentrated nitric acid reacts with other acids. One of the best-known compounds is that with hydrochloric acid (HCl), producing the so-called "aqua regis" (Latin), also known as royal acid, which can even dissolve precious metals such as gold and platinum. The salts of nitric acid are formed from the reaction of nitric acid with metals.
The salts (and esters) of nitric acid are called nitrates. The acid owes its name to them, derived from the common names of some of its alkali and alkaline earth salts, which end with the name saltpetre - for example sodium nitrate (Chile saltpetre), potassium nitrate (potassium saltpetre), ammonium nitrate (ammonium saltpetre), calcium nitrate (calcium saltpetre or mason's saltpetre) or barium nitrate (barytes saltpetre). Since 1908, nitric acid has been produced on an industrial scale by catalytic oxidation of ammonia using the so-called Ostwald process.
Nitric acid is used in different concentrations. A 65- to 69-percent solution is considered concentrated nitric acid, which is mostly used in laboratories. Industry usually uses nitric acid in a concentration of 68 percent. A 100 per cent solution of the chemical is rarely used. Nitric acid may be freely traded in a concentration of up to three percent.
