Diffusion is the movement of molecules from a region of high concentration to a region of low concentration down a concentration gradient until evenly distributed. Concentration gradient: the difference in concentration between two regions.
Why does diffusion occur?
1. There is a great deal of empty space between molecules of all substances. (The spaces are more in gases, less in liquids, and least in solids). 2. As all the molecules are always in a constant random motion they collide all the time.