246 MOREY ART. G 



mixtures in closed tubes and observed the temperature at which 

 the crystals disappeared. 



As the temperature is raised past the point of inj9ection of the 

 p-t curve, the KNO3 content of the liquid increases and the 

 coefficient of the second term in the numerator increases corre- 

 spondingly. At 115°, the boiling point of the saturated solu- 

 tion, the ratio a; V(l — a:') is about 2.5; at the point of inflection, 

 about 4. As this coefficient continues to increase, the numer- 

 ator decreases more and more rapidly, and the value of dp/dt 

 decreases; but, as it is still positive, the pressure continues to 

 increase with temperature. With a little further increase in 

 temperature, the ratio x^/{l — x^) becomes such that the entire 

 second term equals the first term, and the difference is zero; 

 the numerator is now zero, so dy/dt is zero, and the curve 

 has a horizontal tangent. Since at this point 



it follows that 



x^ _ _ yfj-Tj^ 



1 — x' v' — v 



The ratio of the entropy difference (vapor-liquid) to the entropy 

 difference (solid-liquid) is equal to the ratio of KNO3 to water in 

 the saturated solution; the saturated solution at this point 

 contains about 95.3 per cent KNO3, so this ratio is approxi- 

 mately 95.3/4.7, or 20. The entropy of the water vapor at this 

 temperature and pressure can be obtained from steam tables, 

 that of KNO3 from specific heat data, and the entropy of the 

 liquid can accordingly be calculated. It should be remembered 

 that we are here dealing with entropy differences, not absolute 

 entropy, and when we take off the entropy of the steam from a 

 steam table we must remember that the assumption is made 

 in the steam table that the entropy of liquid water at its freez- 

 ing point is zero. 



7. The Maximum Pressure of the Equilihrium, KNOz -\- 

 Solution + Vapor. The point of maximum pressure is found 

 at a KNO3 content of about 95.3 per cent, a temperature of 



