Diffusion commonly refers to the spontaneous movement of a substance (gas, liquid, or solid) into its surrounding area. The molecules, or particles, that make up the substance distribute over time from an area of higher concentration to an area of lower concentration in order to create, at equilibrium, a uniform distribution of particles throughout the system. Diffusion is a natural process that requires no added energy to occur. It increases the entropy of the system and hence is an energetically favorable and irreversible process.

An example of diffusion is the release of a drop of ink into a beaker of water. The ink will be visibly distinguishable from the water for some amount of time, but it will diffuse eventually to all areas of the beaker. The collision of the ink molecules with the water molecules keeps diffusion from happening quickly. This is an example of a liquid...