diffusion

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Net spontaneous and random movement of molecules or particles in a fluid (gas or liquid) from a region in which they are at a high concentration to a region of lower concentration, until a uniform concentration is achieved throughout. The difference in concentration between two such regions is called the concentration gradient. No mechanical mixing or stirring is involved. For instance, a drop of ink added to water will diffuse down the concentration gradient until evenly mixed.

Diffusion occurs because particles in a liquid or gas are free to move according to the kinetic theory of matter. The molecules move randomly, but there is more chance that they will move out of the ink drop than into it, so the molecules diffuse until their colour becomes evenly distributed throughout. Diffusion occurs more rapidly across a higher concentration gradient and at higher temperatures.

Diffusion is quite different from the movement of molecules when a fluid is flowing. In this case movement is not random; all molecules are moving together and in the same direction.

In biological systems, diffusion plays an essential role in the transport, over short distances (for example across cell membranes), of molecules such as nutrients, respiratory gases (carbon dioxide and oxygen), and neurotransmitters. It provides the means by which small molecules pass into and out of individual cells and micro-organisms, such as an amoeba, that possess no circulatory system. Plant and animal organs whose function depends on diffusion – such as the lung – have a large surface area. Diffusion of water across a semi-permeable membrane is termed osmosis. Some important processes which involve diffusion are: the uptake of the products of digestion from the gut, gas exchange in the lungs, and gas exchange in the leaves of plants.

One application of diffusion is the separation of isotopes, particularly those of uranium. When uranium hexafluoride diffuses through a porous plate, the ratio of the 235 and 238 isotopes is changed slightly. With sufficient number of passes, the separation is nearly complete. There are large plants in the USA and UK for obtaining enriched fuel for fast nuclear reactors and the fissile uranium-235, originally required for the first atom bombs. Another application is the diffusion pump, used extensively in vacuum work, in which the gas to be evacuated diffuses into a chamber from which it is carried away by the vapour of a suitable medium, usually oil or mercury.

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