|MadSci Network: Physics|
Dear Andrew, I suppose that the idea of your son is to make the salt in a solution settle to the bottom of a tube by centrifugal force. Well, IN PRINCIPLE this is possible, and the principle is exploited in density-gradient centrifugation, using cesium chloride as the salt. But even in this case, you just get slight differences in salt concentration between the top and the bottom of the centrifuge tube at high spinning rates. The reason for this inefficient "desalination" is the tendency of the dissolved salt ions to redistribute themselves in the space where their concentration is reduced. This entropy-driven tendency is pitched against the centrifugal force. Entropy thus plays a much more important role if we are dealing with a true solution (like that of sodium chloride in water), as opposed to a suspension or a colloid. There are much cheaper and more energy-efficient methods of desalination, e.g. by ion exchange or vacuum distillation. Best Regards Werner Sieber
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