The process by which different substances mix as a result of the random motions of their component atoms, molecules and ions.

In gases, all the components are perfectly miscible with each other and mixing ultimately becomes nearly uniform, though slightly affected by gravity. The diffusion of a solute through a solvent to produce a solution of uniform concentration is slower, but otherwise very similar to the process of gaseous diffusion.

In solids, diffusion occurs very slowly at normal temperatures.