HETEROGENEOUS EQUILIBRIUM 247 



about 266°, and a pressure of about 7.9 atmospheres. Our 

 equation is 



0-953 , 

 dt , ,. , 0.953. ,. ' 



^'^ ~ '^ + o:or7 ^'' ~ '^ 



and the numerator is zero because the negative entropy differ- 

 ence (solid-liquid), multiplied by the ratio a: V(l — 2:0 is equal to 

 the positive entropy difference (vapor-liquid). On further 

 increase in temperature x continues to increase, the negative 

 second term becomes larger than the positive first term, and 

 the numerator becomes negative. The denominator is still 

 positive, so the p-t curve has a negative slope; pressure de- 

 creases with increasing temperature. On further increase in 

 temperature, the numerator continues to become more strongly 

 negative, until at the melting point of pure KNO3 it is the 

 entropy difference (solid-Uquid) for KNOj. 



8. The Maximum Temperature of the Equilibrium, KNO3 + 

 Solution + Vapor. The changes which have been taking place 

 in the denominator will now be considered. The specific 

 volume of the vapor phase at all points is much larger than that 

 of any other phase, its smallest value at the maximum pressure 

 being about 100 cc. per gram. As the pressure decreases from 

 this point, the specific volume of the vapor increases; the effect 

 of this is merely to alter the rate of decrease of pressure which 

 takes place from this point. But as the liquid phase approaches 

 KNO3 in composition, the amount of water becoming very 

 small, the second term in the denominator becomes of im- 

 portance. The specific volume difference between fused and 

 solid KNO3 is but a few tenths of a cubic centimeter; when the 

 water content is only 0.1 per cent, the negative volume differ- 

 ence (solid-liquid) is multiplied by the ratio 999/1, and at 0.01 

 per cent water, by 10,000. As the water content decreases, 

 the coefficient of the second term in the denominator, (v — vO> 

 increases rapidly, the denominator approaches zero, and the slope 

 of the p-t curve, dp/dt, becomes infinite. At this one point 

 the curve is vertical; on further increase in temperature the 



