Diffusion is the redistribution of substances by the random motion of their molecules. Substances diffuse from regions where their concentration is high to regions where it is lower. The greater the difference in concentration, the faster the diffusion takes place.
The action of diffusion is in apparent contradiction to the laws of gravity. When a gas is allowed to enter the bottom of a vessel that already contains a lighter gas, the heavy gas diffuses upwards and the lighter gas diffuses downwards until the two are uniformly mixed. The light gas molecules diffuse more rapidly than the heavier ones under the same conditions.
Diffusion also takes place in liquids that can mix with each other. When separate layers of two such liquids are brought into contact, they diffuse into each other even when the denser liquid is in the lower layer. Solids diffuse into liquids in which they are soluble. Some solids also diffuse into other solids with which they are in contact. An example of this is the diffusion of gold into lead.
In living organisms, diffusion is a very important natural process. In the human body, for example, oxygen diffuses from the lungs into the bloodstream, where its concentration is lower, and is carried to the body's tissues. At the same time, carbon dioxide, which is more concentrated in the blood, diffuses into the lungs and is exhaled.
The term "diffusion" is also used to describe the process by which light is scattered when it passes through fog or frosted glass, or when it is reflected from a rough surface.