NO2-N2O4 equilibrium. Animation showing the effect of pressure on the equilibrium between nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4). The syringe is filled with a mixture of the two gases. NO2 molecules are represented as red, while N2O4 molecules are represented as white. Drawing out the plunger increases the volume of the gas and reduces its pressure. This leads to a colour change as the equilibrium shifts in favour of the red NO2 molecules. As the plunger is depressed, the volume decreases and the pressure increases. The equilibrium shifts as more NO2 molcules are converted to white N2O4 molecules. The increasing concentration of N2O4 causes the colour of the gas in the syringe to become paler, and eventually white.