The Romance of Modern Chemistry

HIGH TEMPERATURES this is one of the most ancient and valuable laboratory operations. When, for instance, a salt solution—in other words, a mixture of salt and water—is boiled and the steam condensed, it is found to be pure water, perfectly free from salt. This operation of boiling and then condensing the vapour—“ distillation,” as it is called—obviously makes it possible to separate salt and water, simply because the water is easily vaporised, in contrast to the salt. The same principle may be applied in numberless other cases. Metals, for instance, which are comparatively volatile, such as mercury and zinc, may be separated by distillation from others, such as copper and iron, which are mixed with them and which are much less easily vaporised. A rise of temperature, however, not only makes it possible to melt and then vaporise many solid substances, but it has also the general effect of weakening the bonds which hold together the atoms in a molecule. On heating a chemical compound the chances are that when a certain temperature is reached it begins to break up into simpler compounds, or even into the constituent atoms. Ihis change is known as decomposition or dissociation, Ihe former term is applied to the case in which the atoms or simpler molecules, having been once separated by heat, show no signs of coming together again on cooling; they have done with each other for good and all. But in many cases the interesting observation has been made that the separation caused by heating the compound molecule is spontaneously reversed on cooling, and the compound is re-formed, provided, of course, that the atoms or simpler molecules have been allowed to remain side by side. The effect of heating such a compound is described as “ dissociation,” and this is followed on cooling by an “ association ” of the separated atoms or molecules 194