256 MOREY ART. G 



16. Application of Equation [97] to a System in Which Com- 

 pounds Are Formed. HiO-CaCk. We have considered the 

 appUcation of equation (8) to the simplest type of system, that 

 in which there is but one phase of variable composition, and no 

 compounds are formed. It will be of interest to see what 

 additional complications are introduced by the formation of 

 compounds, and as illustration the system, H20-CaCl2, will be 

 chosen. Projections of the solid pressure-temperature-com- 

 position model are shown in Fig. 3.* 



The invariant point, ice + CaCla-GHaO + solution + vapor, 

 is at — 55°, and the pressure is but a fraction of a milhmeter. 

 The compound, CaCl2-6H20, contains 50.66 per cent CaCl2, 

 and the cryohydrate solution, 29.8 per cent. The equation of 

 the pressure-temperature curve of the solutions saturated with 

 CaCl2-6H20is 



As in the preceding case the volume change of the water vapor 

 is the dominating factor at low temperatures, causing the curve 

 to be concave upward (Fig. 3). As the temperature is raised 

 the fractional coefficient of the second term becomes of increas- 

 ing importance, as before, and again a point of inflection of the 

 p-t curve is reached at 18°; the solution at this temperature 

 contains 42 per cent CaCl2, so the coefficient of the second term 

 is now 0.42/(0.5066-0.42), or about 4.2. The curvature falls 

 off rapidly with increase in the CaCl2 content, and becomes zero 

 at 28° and 48.5 per cent CaCl2. Since at this point 



X^ rj" — ry' 



0.5066 - x^ n' - V^ 



the ratio of the entropy of vaporization to the entropy of solu- 

 tion is 0.485/(0.506 - 0.885), or about 23 to 1. With further 



* H. W. Bakhuis Roozeboom, Z. physik. Chem., 4, 31 (1889). 



