In the preceding chapter we discussed the behaviour of systems formed of two components, only one of which was volatile, in those cases where the two components separated from solution in the pure state. In the present chapter we shall consider those systems in which combination between the components can occur with the formation of definite compounds; such as are found in the case of crystalline salt hydrates. Since a not inconsiderable amount of study has been devoted to the systems formed by hydrated salts and water, systems which are of great chemical interest and importance, the behaviour of these will first call for discussion in some detail, and it will be found later that the relationships which exist in such systems appear also in a large number of other two-component systems.
The systems belonging to this group may be divided into two classes according as the compounds formed possess a definite melting point, i.e. form a liquid phase of the same composition, or do not do so. We shall consider the latter first.
1. The Compounds formed do not have a Definite Melting Point.
Concentration-Temperature Diagram.—In the case of salts which can form crystalline hydrates, the temperature-concentration diagram, representing the equilibria of the
different possible systems, must necessarily be somewhat more complicated than where no such combination of the components occurs. For, as has already been pointed out, each substance has its own solubility curve; and there will therefore be as many solubility curves as there are solid phases possible, the curve for each particular solid phase being continuous so long as it remains unchanged in contact with the solution. As an example of the relationships met with in such cases, we shall first of all consider the systems formed of sodium sulphate and water.
Sodium Sulphate and Water.—At the ordinary temperatures, sodium sulphate crystallises from water with ten molecules of water of crystallisation, forming Glauber's salt. On determining the solubility of this salt in water, it is found that the solubility increases as the temperature rises, the values of the solubility, represented graphically by the curve AC (Fig. 33), being given in the following table.[[209]] The numbers denote grams of sodium sulphate, calculated as anhydrous salt, dissolved by 100 grams of water.
Solubility of Na2SO4,10H2O.
| Temperature. | Solubility. |
| 0° | 5.02 |
| 10° | 9.00 |
| 15° | 13.20 |
| 18° | 16.80 |
| 20° | 19.40 |
| 25° | 28.00 |
| 30° | 40.00 |
| 33° | 50.76 |
| 34° | 55.00 |