CHEMICAL THEORY OF SOLUTIONS. PAKT I. 79 



(c) The Equilibrium between Liquid and Solid Phases, 



(1) Solubility Curves. 



Let us first take the case in wliicli the solid phase is a 

 normal component ; and as the two normal components must 

 show quite similar relations it is enough to consider only one 

 of them. According to equation (17) the molar fraction of ©i 

 in a solution, Avliich is in equilibrium with the solid ©j, must 

 be constant at a constant temperature. The solubility curves of 

 Si must therefore have the same form as the equifractional curve 

 shown in Fig. 18. This has been verified by H. Hirobe^^ in the 

 solubility curves of naphthalene in the system phenol-naphthalene- 

 chlorobenzene. Equation (61) has been found to represent the 

 solubility curves with tolerable approximation, the deviation in 

 the value of x not exceeding 0.015. 



When the solid phase is the associated component the solu- 

 bility curves must be straight lines parallel to the axis of y, as the 

 equifractional curves of G^^ as well as those of Ca are such. Hirobe 

 has found that the solubility curves of phenol in the system at 

 various temperatures almost exactly fulfill this requirement. 



We have seen in § 3 {a) of the preceding chapter that the 

 solubility curves of a component in an ideal solution are straight 

 lines parallel to one of the axes. But we are not justified in 

 concluding from such a course of a solubility curve that the 

 solution is an ideal one, because this is but a consequence of a 

 symmetrical relation between the components. When two com- 

 ponents are symmetrically related to a third the solubility curve 

 of the latter must necessarily be such a straight line. 



1) See Art. 12 ut" tbib Volume. 



