Bromine gas diffusion. Time lapse movie of bromine vapour (orange) filling a gas jar as it evaporates from bromine liquid (dark brown, bottom of jar). Gas molecules move randomly, so when two gases are in contact they will mix through the process of diffusion. The diffusion rate depends mainly on the temperature and the atomic masses of the gases that are mixing. The gas colour allows the upwards diffusion of the bromine vapour from the liquid to be observed.