Diffusion is the passive movement of particles from an area of high concentration to an area of low concentration.
Learn this phrase! Examiners like to ask for definitions, and then insist that you are word perfect, or they start taking marks off. If you learn this, you'll be fine.
Translated, it means that particles which float about randomly (e.g. liquids and gases) will naturally rearrange themselves until they are equally spread out.
If I have a room full of blindfolded people, with all the blondes in one corner and the brunettes in another and tell them to walk about randomly, after a short while they will all be thoroughly mixed up. That's pretty much how diffusion works.
NB: Diffusion can happen through a permeable/partially permeable membrane. This makes no difference, except that with a partially permeable membrane, only some things move through it. This is how substances move into and out of cells.
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