Some physico-chemical themes / by Alfred W. Stewart with five plates and thirty-seven diagrams in the text.

  • Alfred Walter Stewart
Date:
1922
Catalogue details

Licence: In copyright

Credit: Some physico-chemical themes / by Alfred W. Stewart with five plates and thirty-seven diagrams in the text. Source: Wellcome Collection.

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    that this behaviour is characteristic of all double salts and salt hydrates. There is a certain temperature—to which van’t Hoff gave the name transition temperature—which marks the upper or lower existence-limit for a double salt or a salt hydrate. Each salt or hydrate has its own peculiar transition temperature. It must not be assumed that conglomerates always exist below and double salts above the transition tem¬ perature; for this is not the case. For example, the double salt leonite, MgS04 . K2S04.4H20, exists only between 61*5° and 18°. Above 61'5° and below 18° crystallisation leads to the formation of magnesium and potassium sulphates in con¬ glomerate form. Again, at temperatures below 92°, the double salt, CuCl2.2KC1.2H20, crystallises from solution; but above 92° only the separate copper and potassium chlorides appear. On the other hand, when a solution containing magnesium and sodium sulphates is evaporated below 22° the crystals formed are a conglomerate of the two sulphates; above 22°, however, the double salt astrakanite, MgS04. Na2S04.4H20, is pro¬ duced on evaporation of the solution. The dominating factor in the problem is, of course, the heat of change which is required to convert the simple salts into the double salt, or vice versa, at the transition temperature. Ac¬ cording to Le Chatelier’s theorem, at the higher temperature, that change will take place which is accompanied by the ab¬ sorption of heat. If, therefore, the production of a double salt from its components is an endothermic reaction at the transi¬ tion temperature, the double salt will be stable above the tran¬ sition point; whereas, if the formation of the double salt be an exothermic process, then the double salt will be stable at low temperatures and its components will exist uncombined above the temperature of transition. It may be noted that the characteristic of the transition temperature is the change of physical state which occurs as a system passes through the transition point; and this brings out the correspondence between the transition temperature of a double salt and the point of liquefaction of a gas or the melting-point of a solid. In some cases the physical resem¬ blances between the two processes are very close indeed. When sodium sulphate decahydrate, for example, is heated to its transition temperature, so much water is liberated that the