Nitric acid oxidizes copper, nitrogen dioxide equilibrium

Concentrated nitric acid reacts with copper to form NO gas which immediately reacts with oxygen in the air to form brown nitrogen dioxide gas. Collecting this gas sets up an equilibrium between nitrogen dioxide gas and colorless dinitrogen tetroxide gas. Applying pressure stresses the the system and according to Le Chatelier's principle, equilibrium shifts towards the side with less moles which in this case is the colorless dinitrogen tetroxide and the mixture turns a slightly lighter brown color.