The Romance of Modern Chemistry

FACTS ABOUT SOLUTIONS molecules of the dissolved substance in a given volume. So that the chemist can compare the number of molecules present in solutions of salt and sugar by finding out the temperatures at which these solutions freeze or boil. Working on the same principle, one could get a very fair idea of the amount of solid dissolved in sea water by comparing its freezing-point with those of a number of common salt solutions of known strengths. The sea water must contain about as much dissolved solid as that particular salt solution which has the same freezing- point The topic of the freezing and boiling of solutions is in reality closely related to the interesting question how the water can be separated from the dissolved substance—how, for example, pure fresh water can be obtained from sea water. The problem is not quite so simple as was thought by the examination candidate who suggested that in order to procure fresh from salt water it was only necessary to set aside and skim off the salt after standing. This method certainly works in the case of milk, but then milk is not simply a solution ; the particles of fat which separate from milk on standing are not dis- solved—they are only suspended, and gradually come to the top because they are lighter than water. One way, however, of getting pure water from a solution of salt is to freeze it; when that takes place the solution does not freeze as a whole; the solid crystals which separate consist of ice alone—that is, they are pure water. In virtue of this .fact, an iceberg formed in sea water would, if melted, be found to yield approximately fresh water. Any salt which it still contained must have been merely imprisoned or entangled among the freezing particles of water. 308