nitric oxide or nitrogen monoxide, a colorless gas formed by the combustion of nitrogen and oxygen as given by the reaction: energy + N₂ + O₂ → 2NO; m.p. -163.6°C;; b.p. -151.8°C;. Nitric oxide readily combines with oxygen or air to form nitrogen dioxide (NO₂), which can again be separated by ultraviolet light to produce nitric oxide and highly reactive oxygen atoms. These oxygen atoms combine with hydrocarbons producing noxious compounds that irritate the membranes of living organisms and destroy vegetation. Large amounts of nitric oxide are created by internal-combustion engines and manufacturing processes. Its quantity is greatly reduced by passing the oxide gas through a catalyst, thereby converting it back to its constituent nitrogen and oxygen gases.

In the environment, nitric oxide is a precursor of smog and acid rain. Nitric oxide in minute amounts serves as a source of energy in certain bacteria. In the body, it serves as a chemical messenger with a wide range of functions. It acts as a neurotransmitter and is necessary for penile erection. It affects blood pressure and is produced by macrophages in the immune system to help defend against infection and cancer. Despite its usefulness, nitric oxide can have a toxic effect on body cells and has been implicated in Huntington's disease and Alzheimer's disease.

Colourless, toxic gas (NO), formed from nitrogen and oxygen by the action of electric sparks or high temperatures or, more conveniently, by the action of dilute nitric acid on copper or mercury. First prepared circa 1620 by Jan B. Helmont, it was first studied in 1772 by Joseph Priestley, who called it “nitrous air.” An industrial procedure for the manufacture of hydroxylamine is based on the reaction of nitric oxide with hydrogen in the presence of a catalyst. The formation of nitric oxide from nitric acid and mercury is applied in a volumetric method of analysis for nitric acid or its salts. The gas is synthesized via enzyme-catalyzed reactions in humans and other animals, where it serves as a signaling molecule. Among its numerous biological roles, it causes dilation of blood vessels and as such is an important regulator of blood pressure. Nitric oxide is one of the components of air pollution generated by internal-combustion engines.

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