Nitrogen dioxide

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 metalls, 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.