The Romance of Modern Chemistry

FACTS ABOUT SOLUTIONS This may be shown by a very simple experiment, in which some concentrated sugar solution is put at the bottom of a tall glass jar, the upper part being then carefully filled with water. If left to themselves, the sugar molecules gradually penetrate the water which occupies the upper part of the jar, until there are as many of them at the top as at the bottom. This inter- esting phenomenon of diffusion is not peculiar to sugar in water; it is characteristic of all dissolved substances. The rate of diffusion, however, differs markedly from one case to another; for example, sodium chloride (common salt) diffuses three to four times as rapidly as cane sugar. The idea of diffusion is not new to the reader, for at an earlier stage we have adopted the view that the molecules of a gas are in constant motion, by virtue of which they also are ready to diffuse, to expand, and occupy fully any space which is put at their disposal. The magnitude of this diffusive and expansive force—the pressure, in other words—can be ascertained by interposing some surface in the path of the expanding gas, and thus stopping its further diffusion. Similarly, in the case of dissolved cane sugar, we may ascertain the magnitude of its diffusive force by inter- posing between the sugar solution and the water into which it naturally diffuses some diaphragm which shall allow only the water to pass, and which, like a sieve, shall stop the diffusion of the sugar molecules. Such diaphragms have been discovered, and are known as “ semi-permeable membranes,” the name having reference to the fact that the membrane is permeable for water, but not for the dissolved substance. The interposition of such a diaphragm between a strong sugar solution and water prevents the sugar molecules doing what they would 804