High-speed footage of the reaction between copper metal and concentrated nitric acid. The reaction produces a green solution of copper nitrate, and clouds of toxic brown nitrogen dioxide gas. The gas is heavier than air, so flows over the brim of the beaker and along the surface. Water is also produced. The equation for this reaction is: Cu + 4HNO3 --> Cu(NO3)2 + 2NO2 + 2H2O