Copper penny with concentrated nitric acid

The reaction of a copper penny (minted pre-1982) and concentrated nitric acid (15 M) is shown. Red-brown nitrogen dioxide is generated and some of the copper dissolves to form a blue solution of copper(II) nitrate.

Oxidation and reduction of copper

A piece of copper metal is heated in a flame and oxidized to black copper oxide by air. The oxidized copper is reduced when placed in a hydrogen atmosphere. When removed from the hydrogen atmosphere, the metal is again oxidized.