Editing Molecular diffusion (section)

{{short description|Thermal motion of liquid or gas particles at temperatures above absolute zero}}
[[File:DiffusionMicroMacro.gif|thumb|260px|Diffusion from a microscopic and macroscopic point of view. Initially, there are [[solute]] molecules on the left side of a barrier (purple line) and none on the right. The barrier is removed, and the solute diffuses to fill the whole container. <u>Top:</u> A single molecule moves around randomly. <u>Middle:</u> With more molecules, there is a clear trend where the solute fills the container more and more uniformly. <u>Bottom:</u> With an enormous number of solute molecules, all randomness is gone: The solute appears to move smoothly and systematically from high-concentration areas to low-concentration areas, following Fick's laws.]]

'''Molecular diffusion''' is the motion of [[atom]]s, [[molecule]]s, or other [[particle]]s of a [[gas]] or [[liquid]] at [[temperature]]s above [[absolute zero]]. The rate of this movement is a function of temperature, [[viscosity]] of the fluid, size and density (or their product, mass) of the particles. This type of [[diffusion]] explains the net [[flux]] of molecules from a region of higher concentration to one of lower concentration. 

Once the concentrations are equal the molecules continue to move, but since there is no concentration gradient the process of molecular diffusion has ceased and is instead governed by the process of [[self-diffusion]], originating from the random motion of the molecules. The result of diffusion is a gradual mixing of material such that the distribution of molecules is uniform. Since the molecules are still in motion, but an equilibrium has been established, the result of molecular diffusion is called a "dynamic equilibrium". In a [[Phase (matter)|phase]] with uniform temperature, absent external net forces acting on the particles, the diffusion process will eventually result in complete mixing.

Consider two systems; S<sub>1</sub> and S<sub>2</sub> at the same [[temperature]] and capable of exchanging [[Molecule|particles]]. If there is a change in the [[potential energy]] of a system; for example μ<sub>1</sub>>μ<sub>2</sub> (μ is [[Chemical potential]]) an [[energy]] flow will occur from S<sub>1</sub> to S<sub>2</sub>, because nature always prefers low energy and maximum [[entropy]].

Molecular diffusion is typically described mathematically using [[Fick's laws of diffusion]].