Diffusion, in physical chemistry, the movement of molecules from a region of high concentration to a region of low concentration, so that the molecules become equalay distributed throughout the system. Diffusion is most noticeable in gases and liquids, but also takes place to some extent in solids. If a bottle containing hydrogen is placed mouth downward over one filled with chlorine, which is 35 times as heavy, the two gases will mix until uniform in composition throughout. A little heavy sulfuric acid at the bottom of a vessel of water will slowly distribute itself all through the water. Blocks of lead and gold in contact will diffuse slightly at their point of contact. Diffusion can also occur between gases and liquids, gases and solids, and liquids and solids.

In water solutions that are separated by a semipermeable membrane, such as the cell membranes that surround plant and animal cells, water will diffuse through the membrane. This process, especially important in living organisms, is called osmosis.