Ni′tric Acid (), is one of the most important acids. It was formerly, and sometimes is still, called aqua fortis. It is prepared by distilling a mixture of saltpeter, usually the cheaper Chile variety (sodium nitrate), with sulphuric acid. The strongest nitric acid is about half again as heavy as water, and is an intensely corrosive liquid, which is colorless when pure, but is usually colored yellow by the presence of lower oxides of nitrogen. It fumes in the air, colors the skin yellow, producing painful burns, and usually dissolves or oxidizes all the commoner metals except gold and platinum, with the violent expulsion of choking, red fumes. Aluminum is dissolved but slowly by it, and in some instances the strong acid must be diluted with water before it will act upon a metal. Dilute nitric acid is generally less active the more water it contains. Strong nitric acid, mixed with sulphuric acid, acts upon cotton and glycerine to form the explosives, guncotton and nitroglycerin. Nitric acid is extensively used in chemical operations, particularly to dissolve metals and to oxidize substances. When the metals or their oxides or carbonates dissolve in this acid, salts called nitrates are produced. Mixed with hydrochloric acid, nitric acid forms aqua regia (royal water), which is capable of dissolving gold and other substances that are not attacked by a single acid. Nitric acid is also used to some extent in medicine.
