The Romance of Modern Chemistry

FACTS ABOUT SOLUTIONS The separation of pure water from a salt solution, however, can be carried out not only by freezing, but by boiling. The latter process, indeed, is more easily effected, and gives a more perfect separation; it is, therefore, used on a large scale for the production of fresh water from sea water. The whole operation of heating the sea water and condensing the steam which comes off is known as distillation, and the plant necessary for carrying this out is part of the regular equipment of an ocean liner. It will be obvious to the reader that one result of boiling a solution, and thus getting rid of some of the water, will be to increase the strength of the solution, provided, of course, that the dissolved substance itself has no tendency to volatilise. Now, as already stated, the greater the concentration, the higher is the boiling- point If, therefore, a thermometer is put in a solution, say, of sugar, which is boiling in an open vessel, the readings of the thermometer will get higher and higher. Any one who has made fondant for confectionery pur- poses will have observed this. In this operation sugar and water are mixed in certain proportions in a pan and heated; the temperature rises rapidly to the point at which the mixture begins to boil, and then slowly thereafter as the water is boiled off and the sugar solution concentrates. The subsequent behaviour of the sugar solution when cold varies with the extent to which this concentration has been allowed to proceed ; it depends on the temperature at which the boiling has been stopped. In the endeavour to bring home to the reader the curious properties exhibited by solutions, the writer has taken sugar or salt as the dissolved substance. These compounds have the advantage of being perfectly well 309