Diffusion is the transport of a chemical by random motion due to a state of disequilibrium.
For example, diffusion causes the movement of a chemical within a phase
(e.g., water) from a location of relatively high concentration to a place of lower
concentration until the chemical is homogeneously distributed throughout the
phase. Likewise, diffusive transport will drive a chemical between media (e.g., water
and air) until their equilibrium concentrations are reached and thus, the chemical
potentials or fugacities are equal in each phase.