(Top) The student holds a sealed syringe full of a mixture of nitrogen dioxide (NO2, a brown gas) and dinitrogen tetroxide (N2O4, a colourless liquid). These chemicals exist in a dissociative equilibrium, with two molecules of NO2 combining to form a molecule of N2O4, and vice versa. Changes in temperature and pressure change the equilibrium. (Bottom) When the student has applied pressure, it forces the equilibrium to the N2O4 side (as that has fewer molecules and less pressure), this reduces the amount of NO2 and reduces the brown colour.