12. Spreading of Substances

So far, the discussion of the chemical potential has concentrated primarily on chemical reactions and phase transitions. But another property of substances is also of great importance: their tendency to spread out or disperse in space. The phenomenon of diffusion will be explained in this context. The subject area of this chapter also includes the discussion of the effect of a small amount of solute on certain properties of a solution. The properties we have in mind are the lowering of vapor pressure of the solvent, the elevation of its boiling point, the lowering of its freezing point, and last but not least the origin of osmotic pressure. These phenomena are found everywhere, in households and in nature but also in engineering. In everyday life, a prime example for freezing-point depression is the melting effect of road salt. Or have you ever asked yourself why juice is drawn out of sugared strawberries but cherries swell up and burst after a long rain? Then have a look at Sect. 12.4 dealing with osmosis. For a quantitative discussion of all these phenomena, we first have to learn about indirect mass action and the corresponding colligative lowering of chemical potential.