FIGURE 6.9(a) Brown NO2(g) and colorless N2O4(g) at equilibrium in a syringe. (b) The volume is suddenly decreased, giving a greater concentration of both N2O4 and NO2 (indicated by the darker brown color). (c) A few seconds after sudden volume decrease, the color becomes a much lighter brown as the equilibrium shifts from brown NO2(g) to colorless N2O4(g). This is predicted by Le Châtelier’s principle, since in the equilibrium 2NO2(g) N2O4(g) the product side has the smaller number of molecules..