A process by which particles become evenly distributed throughout a medium. The random movement of particles (see Brownian Motion) drives the diffusion of small molecules and particles in liquid. The diagram below shows two different types of molecules in two separate compartments. Once the divider is removed, the both types of molecules diffuse through the whole container.
In the Wikimedia Commons video below, food coloring is dropped into two glasses of water. Can you guess which glass is hot and which one is cold? Click here for the answer.