Diffusion is important in many life processes. Because of the random nature of the motion of molecules, the rate of diffusion of molecules out of any region in a substance is proportional to the concentration of molecules in that region, and the rate of diffusion into the region is proportional to the concentration of molecules in the surrounding regions. Thus, while molecules continuously flow both into and out of all regions, the net flow is from regions of higher concentration to regions of lower concentration. Generally, the greater the difference in concentration, the faster the diffusion. Since an increase in temperature represents an increase in the average molecular speed, diffusion occurs faster at higher temperatures.
Diffusion often masks gravitational effects.
It is true, however, that at equilibrium the two gases will not be uniformly mixed. These differences, which are due to gravity, are almost impossible to measure in the laboratory, although they interact with other factors in determining the distribution of gases in planetary atmosphere. Generally, gases diffuse much faster than liquids, and liquids much faster than solids. Diffusion may take place through a semipermeable membrane, which allows some, but not all, substances to pass. Diffusion of a solute across a membrane is called dialysis , especially when some solutes pass and others are retained.