At all temperatures above absolute zero, the individual atoms that constitute any substance are always in movement. Indeed, the amount of this movement is what constitutes temperature itself.
When this movement becomes large enough, the bonds holding the atoms of a solid together break, and the substance becomes either liquid or gaseous. These gaseous or liquid atoms can move more freely, since they are no longer bound to their nearest neighbors.
If you could follow the movements of any one atom in a liquid or gas, it would appear to bounce at random between its collisions with other atoms or molecules. This random movement is called diffusion.
The "atoms" in the box at right are moving at random, similar to the way they would move if dissolved in water (but here the water molecules are left out for simplicity). Each particle moves in a random direction (and for a random distance) every time the view is changes, constrained only by the walls from which it "bounces". The paths for each of the two particles is shown as they move. To clear all previous activity and restart the simulation, click the mouse when the cursor is in the box
The movement of diffusion can also be thought of in terms of the classic analogy of a drunk meandering randomly near a streetlamp. At any time the drunk may be found here or there on the sidewalk, but on average, he/she will keep coming back to the same lamp.
Even large particles (ones that we can see under the microscope) move around in solution as a result of the bouncing of individual atoms. This visible bouncing is called Brownian Motion, which was described mathematically by Albert Einstein in 1926.
Diffusion can lead to a net movement of atoms or molecules when large numbers are present, and when there is an imbalance in the number of particles between one part of a container and another (creating a concentration difference or gradient). To see the result of such large scale diffusion, move to the next chapter.