nitric acid, chemical compound, HNO3, colorless, highly corrosive, poisonous liquid that gives off choking red or yellow fumes in moist air. It is miscible with water in all proportions. It forms an azeotrope (constant-boiling mixture) that has the composition 68% nitric acid and 32% water and that boils at 120.5°C. The nitric acid of commerce is typically a solution of 52% to 68% nitric acid in water. Solutions containing over 86% nitric acid are commonly called fuming nitric acid. White fuming nitric acid (WFNA) is similar to the anhydrous variety, and red fuming nitric acid (RFNA) has a reddish brown color from dissolved nitrogen oxides. When treated with hydrogen fluoride, both varieties form inhibited fuming nitric acid, which has increased corrosion resistance in metal tanks, e.g., when used as an oxidizer in liquid fuel rockets.
Nitric acid is a strong oxidizing agent. It ionizes readily in solution, forming a good conductor of electricity. It reacts with metals, oxides, and hydroxides, forming nitrate salts. Chief uses of nitric acid are in the preparation of fertilizers, e.g., ammonium nitrate, and explosives, e.g., nitroglycerin and trinitrotoluene (TNT). It is also used in the manufacture of chemicals, e.g., in making dyes, and in metallurgy, ore flotation, etching steel, photoengraving, and reprocessing of spent nuclear fuel. It is produced chiefly by oxidation of ammonia (the Ostwald process). Small amounts are produced by the treatment of sodium nitrate with sulfuric acid. Nitric acid was known to the alchemists as aqua fortis; the name is used in commerce for impure grades of it. Aqua regia is a mixture of nitric and hydrochloric acids. Niric acid is a component of acid rain.