 We have two tubes containing bromine vapor. One tube is at atmospheric pressure. The other tube has had some of the air removed and is at low pressure. As the tubes go into the liquid nitrogen, the bromine vapor condenses. The bromine in the low pressure tube condenses more rapidly than the bromine in the atmospheric pressure tube. The low pressure tube has a smaller number of gas molecules. So the bromine molecules are able to travel a further distance between collisions. This allows them to reach the cold surface of the liquid nitrogen faster than the bromine molecules in the atmospheric pressure tube. In the atmospheric pressure tube, the bromine collides with large numbers of molecules. These collisions deflect the bromine away from the cold surface, so it takes a longer time for the bromine to diffuse down and condense.