HETEROGENEOUS EQUILIBRIUM 



241 



increasing temperature, but the rate of increase also increases. 

 The p-t curve is accordingly concave upward, and the slope 

 continues to increase. As the critical point of water is ap- 

 proached, the difference between the properties of liquid and 

 vapor diminishes rapidly, and vanishes at the critical tem- 

 perature. Hence the equation for the p-f curve becomes 

 indeterminate, and the vapor pressure curve ends. 



Bm 



/oo 



\ 



f 

 I 



/oo 200 300 Bm 



400 



Fig. 2. The binary system, H2O-KNO3. Diagrams A, B, and C are 

 the projections of the curve representing the three-phase equilibrium, 

 vapor + saturated solution + solid KNO3, in the solid p-t-x model 

 on the pressure-composition I (p-x), pressure-temperature (p-t), and 

 temperature-composition (i-x) planes, respectively. 



IV. Application of Equation [97] to Systems of Two 



Components 



4. Application of the Phase Rule to a System in Which No 

 Compounds Are Formed. H2O-KNO3. We will now consider 

 the case of a simple binary system, choosing the system, water- 

 KNO3, as an illustration. The relationship between pressure, 

 temperature, and composition is shown in Fig. 2, A, B, and C, 



