Diffusion
The spontaneous spreading of one substance through another as a result of random molecular motion.
Formula / equation
J = −D (dc/dx)
Unit
m²/s
In depth
Diffusion always proceeds down a concentration gradient and continues until the mixture is uniform, because that is the state of maximum entropy. It is rapid in gases, far slower in liquids and almost imperceptible in solids, and it speeds up with temperature.
Examples in the real world
A perfume detectable across a room; a drop of ink spreading through still water over several minutes.