Although this view put forward by Heyn has not been conclusively proved, it must be said that there is much evidence in its favour. Further investigation is, however, required before a final decision as to the interpretation of the curves can be reached.
Determination of the Composition of Compounds, without Analysis.—Since the equilibrium between a solid and a liquid phase depends not only on the composition of the liquid (solution) but also on that of the solid, it is necessary
to determine the composition of the latter. In some cases this is easily effected by separating the solid from the liquid phase and analyzing it. In other cases, however, this method is inapplicable, or is accompanied by difficulties, due either to the fact that the solid phase undergoes decomposition (e.g. when it contains a volatile constituent), or to the difficulty of completely separating the mother liquor; as, for example, in the case of alloys. In all such cases, therefore, recourse must be had to other methods.
In the first place, synthetic methods may be employed.[[313]] In this case we start with a solution of the two components, to which a third substance is added, which, however, does not enter into the solid phase.[[314]] We will assume that the initial solution contains x gm. of A and y gm. of B to 1 gm. of C. After the solution has been cooled down to such a temperature that solid substance separates out, a portion of the liquid phase is removed with a pipette and analyzed. If, now, the composition of the solution is such that there are x′ gm. of A and y′ gm. of B to 1 gm. of C., then the composition of the solid phase is x - x′ gm. of A and y - y′ gm. of B. When x = x′, the solid phase is pure B; when y = y′, the solid phase is pure A.
We have assumed here that there is only one solid phase present, containing A and B. To make sure that the solid phase is not a solid solution in which A and B are present in the same ratio as in the liquid solution, a second determination of the composition must be made, with different initial and end concentrations. If the solid phase is a solid solution, the composition will now be found different from that found previously.
The composition of the solid phase can, however, be determined in another manner, viz. by studying the fusion curve and the curve of cooling. From the form of the fusion curve alone, it is possible to decide whether the two components
form a compound or not; and if the compounds which may be formed have a definite melting point, the position of the latter gives at once the composition of the compounds (cf. p. [231]).
This method, however, cannot be applied when the compounds undergo decomposition before the melting point is reached. In such cases, however, the form of the cooling curve enables one to decide the composition of the solid phase.[[315]] If a solution is allowed to cool slowly, and the temperature noted at definite times, the graphic representation of the rate of cooling will give a continuous curve; e.g. ab in Fig. 76. So soon, however, as a solid phase begins to be formed, the rate of cooling alters abruptly, and the cooling curve then exhibits a break, or change in direction (point b). When the eutectic point is reached, the temperature remains constant, until all the liquid has solidified. This is represented by the line cd. When complete solidification has occurred, the fall of temperature again becomes uniform (de).
