CHAPTER V 



OSMOTIC PHENOMENA 



I. OSMOTIC PRESSURE OF THE SOLUTE 



a) Non-electrolytes. If a parchment-paper bag be filled 

 with aqueous sugar solution and, after the opening has 

 been sealed, the bag be submerged in water, the walls will 

 soon be distended by an internal pressure. If the original 

 solution is strong enough, the walls will be stretched to their 

 limit of extensibility, and at last ruptured. If, during the 

 distension of the bag, the water around it be tested, it will 

 be found to be nearly or quite free from sugar ; after the 

 bag is ruptured, however, we find the sugar diffusing rapidly 

 to the limits of the water. Therefore parchment paper 

 hinders greatly the diffusion of the sugar, i. e., it is only 

 slightly permeable to dissolved sugar molecules. This fact 

 forms the basis for an explanation of the phenomenon of 

 distension and rupture just mentioned. In tending to dif- 

 fuse indefinitely, the dissolved molecules (there is no disso- 

 ciation in the case of sugar and other non-electrolytes) bom- 

 bard the walls of any chamber in which they may be 

 inclosed. 1 The fact that they possess this property of in- 

 definite diffusion only when within the limits of the solvent 

 makes it necessary that such a chamber be surrounded by the 

 pure solvent, and that the solvent permeate its walls. The 

 pressure thus produced upon the walls of the bag is really 

 the diffusion tension of the solute. If diffusion could take 

 place without obstruction, this pressure would not be made 

 apparent, but would exist none the less. The water itself 

 exerts but little pressure upon the walls of the bag, since 



1 J. H. VAN'T HOFF, " Die Rolle des osmotischen Druckes in der Analogic zwischen 

 LOsungenund Gasen," Zeitschr. f. physik. Chem,, Vol. I (1887), pp. 481-508. 



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