Heat of Mixture of Substances. 551 



BY 



the vapour pressure is small may be written Pj 2-^— 03 and 



Pi, 2 '*~pz respectively, and we Lave *— = ^— when no new 



molecules are formed, the internal heats of evaporation are 

 equal to one another in such a case. This can only be 

 approximately true, however (we should have pointed out 

 before), since the intrinsic pressure per cm. 2 when we con- 

 sider its effect upon such a small surface as that presented by 

 the volume of occupation of a molecule, must depend some- 

 what on the area of this surface. The heats of evaporation 

 could be obtained from the vapour-pressure curves of the 

 mixture by means of Clapeyron's equation and compared 

 with one another, and information thus obtained as to whether 

 or no new molecules are formed in the mixture. 



Test lohether in a dilute solution of molecules 1 in a liquid 

 of molecules 2, a molecule 1 simply replaces and occupies 

 the same position as a molecule 2. 

 Suppose a mass of gas of molecules 1 which is at low 

 pressure dissolves in a liquid of molecules 2 forming: a dilute 

 solution. Suppose that a dissolved molecule 1 simply re- 

 places a molecule 2 and occupies its position. Now the heat 

 of evaporation of a molecule 2 of the pure liquid into a 

 vacuum we have seen is 



The heat of evaporation of a molecule ] is therefore the above 



expression multiplied by — * = /??2 , and substituting 



( 2 \/wi) 2 

 ote t and T<? 1 for certain of the quantities aie 2 and Tc 2 occurring 

 in A 2 . But since A 2 has the form 3 it does not vary much 

 with the magnitude of the variables it contains, and we may 

 therefore as before take it approximately equal to a constant. 

 If the values of A l and A 2 in the expressions for the internal 

 heat of evaporation of liquids 1 and 2 are available, this con- 



A 4- A 



slant is best taken equal to — l — — - . If these values are not 



available the constant may be obtained from some single 

 liquid, as explained before. At a temperature of 20° C. the 

 heat of evaporation of a grm.-mol. of molecules 1 would be 

 approximately equal to 



3742 ( y] 2 ) S V w i2 v 7 mo calories. 



