4o0 Dr. F. Tinker on Osmotic Pressure. 



1. The Peocesses Operative during Solution. 



It is obvious that when two fluids are mixed together 

 without appreciable total volume change, each component 

 of the mixture separates the molecules o£ the other com- 

 ponent from one another. An immediate consequence of 

 this fact is that the partial pressures and concentrations of 

 the two components are less in the mixture than in the pure 

 substances themselves *, and if either component is capable 

 of " evaporating" from the mixture into another phase, its 

 partial pressure in that phase is also reduced. It is thus not 

 necessary to assume, as some have done |, that the lowering 

 of the vapour pressure of the solvent by the process of 

 solution is due to solvation, or even to the blocking by the 

 solute molecules of solvent molecules which would otherwise 

 evaporate J. It is the reduction in the pressure and con- 

 centration of the solvent inside the solution which causes 

 the lowering of the vapour pressure, whilst solvation and 

 other solution effects which also occur are the cause of 

 abnormalities in the reduced vapour pressure rather than the 

 primary cause of the reduction itself. 



The present section is a development of the theory of 

 fluid mixtures from the fundamentals of the kinetic theory. 

 The treatment has been made as broad and general as possible 

 so that it can be extended to solutions of any strength. The 

 only assumptions made are the following : — 



(i.) The pressure of any component inside a fluid mixture 

 is inversely proportional to the free space available 

 to each molecule of that component, i. e. it is 

 inversely proportional to the 



total free space in a given vol. \ 



o 



no. of mols. of the component in the given vol./ ' 



(ii.) The exact relationship between the partial pressure of 

 the component and the free space available to its 

 molecules is given by the equation, 



,. , / total free space in given vol. \ -^^ 



partial pressure r. , ± -^ -. . }=RT. 



\no. or mols. ot compt. in given vol. / 



* The simplest and most obvious case is that of mixing two gases 

 without total volume change. Thus, if one volume of gas A at atmos. 

 press, is mixed with 1 vol. gas B also at atmos. press, to give 2 vols, of 

 mixture also at atmos. press., the partial pressures of A and B are only 

 half the pressures inside pure A and B. 



t Poyntinff, Phil. Mag. (5) xlii. p. 298 (1896) ; Callendar, Proc. Roy. 

 Soc. A. xc. (1908) ; Dolezalek, Zeit. Phys. Chem. bxxiii. p. 40 (1913), 

 and other papers. 



% Lowry, Phil. Mag. (6) xiii. p. 552 (1897). 



