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The chemical compound nitrogen dioxide is an orange/brown gas and smells rather disgustingly (biting), though characteristical. It is poisonous.
Nitric oxide (NO) spontaneously produces the dioxide when exposed to air:
2 NO + O2 --> NO2
It is also generated in various reactions by nitric acid, e.g. when metals, such as Copper or Silver, are exposed to nitric acid.
It reacts, even at normal temperature & pressure, with water, where it disproportionates to nitric and nitrous acid. That reaction is used in the Ostwald process.
Vapor pressure, mmHg: 400 @ 80 oC
Vapor density (air=1): 1.58
Specific gravity/density: 1.4494 at 20°C
Solubility in water: Reacts
Odor threshold: 2.0 mg/m3
Heat of vaporization: 9.110 kcal/mole