diffusion
The intermingling of substances by the natural movement of their particles. Diffusion is the net movement of molecules or atoms from a region of high concentration (or high chemical potential) to a region of low concentration (or low chemical potential). This is also referred to as the movement of a substance down a concentration gradient. A gradient is the change in the value of a quantity (e.g., concentration, pressure, temperature) with the change in another variable (usually distance). For example, a change in concentration over a distance is called a concentration gradient, a change in pressure over a distance is called a pressure gradient, and a change in temperature over a distance is a called a temperature gradient. Diffusion refers to the process by which molecules intermingle as a result of their kinetic energy of random motion. Consider two containers of gas A and B separated by a partition. The molecules of both gases are in constant motion and make numerous collisions with the partition. If the partition is removed as in the lower illustration, the gases will mix because of the random velocities of their molecules. In time a uniform mixture of A and B molecules will be produced in the container. The tendency toward diffusion is very strong even at room temperature because of the high molecular velocities associated with the thermal energy of the particles. Since the average kinetic energy of different types of molecules (different masses) which are at thermal equilibrium is the same, then their average velocities are different. Their average diffusion rate is expected to depend upon that average velocity, which gives a relative diffusion rate where the constant K depends upon geometric factors including the area across which the diffusion is occuring. The relative diffusion rate for two different molecular species is then given by.