Diffusion can take place in gases, liquids, solids, in all states of formation.

Diffusion associated with the interpenetration of molecules of substances occurs in those substances in which free movement of molecules throughout the volume of the body is possible, i.e. in liquids and gases. In solids, there is no free movement of molecules, because they are linked by intermolecular attraction in the crystal lattice. On the other hand, in solids, the so-called. thermal diffusion, for example with thermal conductivity. In this case, the molecules do not mix with each other, but only the transfer of kinetic energy occurs, therefore diffusion is called thermal.