The Romance of Modern Chemistry

 FROM SOLUTIONS TO CRYSTALS detaching any of these water molecules. They are, as a matter of fact, chemically combined with the salt to form a composite molecule, and they will not be drawn by methods which suffice to dry up ordinary moisture. Water which is held by a salt in this way is described as “ water of crystallisation.” Although blotting-paper fails to effect the separation of a salt and its water of crystallisation, the bond of union is really not very strong, and it may be said that the love between the two grows cold as the temperature rises. So that by merely warming a salt which contains water of crystallisation, the water is driven off as vapour, and finally the salt alone—the “ anhydrous ” salt, as it is called—is left. Water molecules, however, are not all alike in the tenacity with which they cling to the salt molecule. Some can be detached only by the application of a higher temperature than is required for others. Of this graded affection blue vitriol—or copper sulphate, to give it its chemical name—furnishes an interesting example. The ordinary crystals of this substance are blue in colour, and contain 36 per cent, of their weight of water; each molecule of the salt carries with it five molecules of water of crystallisation. If the crystals are exposed for some time to the temperature of 212° Fahrenheit, say in a steam oven, four out of the five molecules go off, and the residue is pale blue. The last molecule is more faithful, but a rise of temperature to 400° compels even this one to take its departure, and a white powder is left as the anhydrous salt. In some cases the molecules of water in a crystallised salt begin to evaporate of their own accord even at the ordinary temperature. A domestic example of this 819