Diffusion, also called molecular diffusion, is the process by which molecules of a given substance move from an area of relatively high concentration to an area of lower concentration. When the molecules have diffused so that they are in a uniform concentration, this state is called equilibrium. This phenomenon plays a key role in many disciplines of biology, physics, and chemistry. It is closely related to the way in which cells take up nutrients, thus life could not exist without it.

Regardless of the state of matter that a group of molecules finds itself in, all the molecules are moving to some degree. They not only move, but they do so randomly, in no particular pattern. This random motion is what allows diffusion to occur. We can easily observe diffusion in action by placing a drop of colored dye in a glass of water. At first, the dye is more or less concentrated in one area. Soon, however, due to random molecular motion, it spreads throughout the water until it reaches equilibrium, giving the water a uniform color.

Liquids and gases have a high tendency toward diffusion, which increases along with temperature. Most solids that can be dissolved in liquids also dissolve faster, the higher the temperature of the liquid solvent. Diffusion is one form of what scientists call passive transport, meaning movement that occurs without any additional energy being required. It is closely related to osmosis, the process by which molecules are transported across the walls and membranes of individual plant and animal cells. Diffusion also plays a role in the processes involved in the life of a cell, particularly the transportation of molecules like amino acids from one place to another within a cell.

Osmosis is basically diffusion that occurs across a selectively permeable membrane, or one through which certain molecules can pass, but others cannot. If, on one side of a membrane, there is a solution of a high concentration, while on the other side is the same solution in a lower concentration, the solvent will tend to move to the area of lower concentration until the two solutions reach equilibrium. This can only occur if the molecules of the solute -- the dissolved substance -- are larger than those of the solvent. The smaller solvent molecules can pass through the membrane, while the larger ones are too large to do so, and must remain on the other side.