Diffusion: substances in gasses and liquids always spread out from areas of high concentration to areas of low concentration, until the substance is evenly spread throughout its container. The entropy of the system increases.
Before: concentration is high After: concentration is low
entropy is low (ordered) entropy is high (disordered)
random motion of each particle
results in random distribution
Diffusion always causes substances to become less concentrated, never more concentrated, this is because each particle in the substance moves randomly, outward movement in all directions is random, but inward movement to a central point is not at all random. Inward movement would violate the 2nd Law of Thermodynamics, because the entropy of the system would spontaneously decrease.
Diffusion happens much faster in hot substances than in cold ones. This provides powerful evidence that the hotter a substance is, the faster its atoms/molecules move.
view a simulations of diffusion: link1 link2
link3 (think about a blood hound detecting molecules when you view this one)
In class we discussed how your sense of smell is actually a means of detecting molecular shape.