The Complexity of the Solid State

Abstract

IT is a well-known fact that if we distil off a part of a quantity of pure water, no physical or chemical property is changed by the process; for example, the vapour pressure of the remaining liquid, the vapour tension of the distillate, and the vapour tension of the original quantity of liquid are identical. Precisely the same result is obtained if we distil a quantity of the crystalline aggregation of water, namely, ice. But if we take a mixture of water and alcohol, the result of a partial distillation is quite different; for now the vapour tension of the remaining portion is lower and that of the distillate higher than the vapour tension of the original mixture. This phenomenon is caused by the fact that alcohol is more volatile than water, so that the vapour which is being condensed is richer in alcohol than the evaporating liquid. Consequently the evaporating liquid becomes poorer and the distillate richer in alcohol than the original liquid. Distillation effects, as we see here, a partial separation, and that explains the observed changes in vapour tension. Now there are also crystalline mixtures, which differ from the liquid mixture of the same substances therein only in that the state of aggregation is a crystalline one. Every crystal in this case contains the substances, being mixed, and is named a mixed crystal. If we submit now such a mixed crystal to distillation, the effect is exactly the same as in the case of the mixture of alcohol and water.