Video: Copper penny with nitric acid
When concentrated nitric acid is dropped onto a copper penny, the nitric acid is reduced to red-brown nitrogen dioxide gas while the copper metal is oxidized to copper(II) ion. The nitrato-copper(II) complex is green while dilution with water gives the hexaaquacopper(II) ion.Cu(s) ® Cu2+(aq) + 2 e-
HNO3(aq) + H+(aq) + e- ® NO2(g) + H2O(l)