Chapter V 



— 47 



Mechanism of Osmosis 



do not yet know enough about the properties of Hquids under all condi- 

 tions of pressure and concentration to formulate a perfect equation. 



When a solute is dissolved in a 

 Hquid solvent its molecules disperse 

 throughout the volume of solvent. 

 The molecules of the solvent are 

 now dispersed through somewhat 

 more space than they were origi- 

 nally. And the solute molecules, 

 depending upon the concentration, 

 are dispersed through from some- 

 what more volume (for concen- 

 trated solutions) to very much more 

 volume ( for dilute solutions). The 

 total volume of solute plus solvent, 

 however, is usually somewhat re- 

 duced. Table 17 shows the mag- 

 nitude of this reduction for sucrose 

 solutions (cf. also Bousfield and 

 LowRY, 1910). Because of the dispersion of the molecules and of the 

 intermolecular forces of attraction between the molecules, the vapor pres- 

 sure of both solute and solvent is reduced. The relation of these for an 

 ideal solution is given bv the diagram from Raoult's law shown in Figure 

 13. 



Table 17. — Volume of zveight-normal solutions of cane sugar at 0° C. {Sp. gr. cane 

 sugar at 0" C. = 1.59231 ; from Morse, 1914) :— 



Fig. 12. — Diagram showing the relations 

 between attractive and repulsive forces between 

 molecules. (From Hildebrand, 1924). 



This diagram emphasizes that although the vapor pressure above the 

 solution may be higher (in the case of a more volatile solute) or lower 

 (in case of a less volatile solute) than that above the original solvent, the 

 individual vapor pressures of the two components are both lowered. 



In liquids, diffusion pressure is an expression of the balance between 

 repulsive forces and attractive forces characteristic of the liquid state and 

 is responsible for its low compressibiHty. In solids the forces of mutual 

 attractions are so great and of repulsion so little that diffusion in the sense 

 of molecular migration as it occurs in liquids is slow and almost escapes 

 detection. The fact that the term diffusion pressure may be applied to the 

 molecules of a solid solute depends upon the fact expressed by Professor 

 Kendall (1937) that sohds in solution are in the liquid state. That is, 

 when solution takes place, the forces that serve to maintain the molecules 

 of a solid in that state are overcome and the molecules in solution act under 

 forces like those in the liquid. 



