The Romance of Modern Chemistry

CHAPTER XXVIII FROM SOLUTIONS TO CRYSTALS IN the foregoing chapter reference was made to the curious ways in which common substances affect the properties of water, and to the methods of getting pure water from a solution. It was there sug- gested that by cooling a salt solution until it began to freeze a separation of water from the dissolved sub- stance could be affected, since it is pure ice which crystallises first. Strictly speaking, this method would not work with a strong solution, for cooling in this case might result in the separation of the salt itself in the crystalline form before the freezing-point was reached. This phenomenon of salt crystallisation depends on the fact that substances as a rule are more soluble in hot than in cold water. Thus, for example, a satu- rated solution of saltpetre (potassium nitrate)—that is, a solution which cannot dissolve any more nitrate—• contains 24 per cent, of the salt at 68° Fahrenheit, 48 per cent, at 130°, and 71 per cent, at 212°. Hence if a saturated solution of saltpetre were prepared at 130° Fahrenheit, and was then cooled down, ultimately to 68°, it would give up as crystals all the salt which it contained over and above 24 per cent. In such a case the saltpetre is said to have “crystallised out” from the solution. 314