Bromine diffusion. Image 2 of 2. Bromine vapour (orange) diffused upwards to fill two gas jars. Bromine liquid is weakly bonded, and so it will readily evaporate at room temperature. Because gas molecules can move independently of each other and do so randomly, a gas spreads out from its source in a process called diffusion. The diffusion rate depends mainly on the temperature (affecting the speed of the gas molecules) and the medium through which the diffusion takes place. In this case, the medium is air. The gas colour allows the diffusion to be observed. For the vapour partially diffused up the glass jars, see image A150/309.