nitrogen oxide

Any chemical compound that contains only nitrogen and oxygen. Nitrogen monoxide and nitrogen dioxide contribute to air pollution. See also nitrous oxide.

Nitrogen monoxide NO, or nitric oxide, is a colourless gas released when metallic copper reacts with concentrated nitric acid. It is also produced when nitrogen and oxygen combine at high temperature. On contact with air it is oxidized to nitrogen dioxide.

Nitrogen monoxide was discovered during the 1980s to act as a chemical messenger in small quantities within the human body, despite being toxic at higher concentrations and its rapid reaction with oxygen. The medical condition of septic shock is linked to overproduction by the body of nitrogen monoxide. Nitrogen monoxide has an unpaired electron, which can be removed to produce the nitrosyl ion, NO+.

Nitrogen dioxide nitrogen(IV) oxide, NO2, is a brown, acidic, pungent gas that is harmful if inhaled. It contributes to the formation of acid rain, as it dissolves in water to form a mixture of nitrous acid and nitric acid (this is an example of disproportionation, where one kind of reactant is both oxidized and reduced at the same time. It is the most common of the nitrogen oxides and is obtained by heating most nitrate salts (for example lead(II) nitrate, Pb(NO3)2). If liquefied, it gives a colourless solution (N2O4). It has been used in rocket fuels.