Thumbnail: The $$N_2O_{(g)} \rightleftharpoons 2NO_{2(g)}$$ system at Different Temperatures. Nitrogen dioxide ($$NO_2$$) gas converts to the colorless gas dinitrogen tetroxide ($$N_2O_4$$) at low temperatures, and converts back to $$NO_2$$ at higher temperatures. The bottles in this photograph contain equal amounts of gas at different temperatures. Figure used with permission from Wikipedia (CC BY-SA 3.0).