PROPERTIES OF MATTER 35 



should aid in wetting certain kinds of cell surfaces. Wetting is neces- 

 sary in turn for the transfer of solutes through these surfaces. 



Certain of these materials are concentrated in the surface, producing 

 a film of high viscosity. When gases are bubbled through such solu- 

 tions a froth or foam is formed and is stabilized by the high viscosity 

 of the film. Proteins, for example, exhibit this behavior, leading to 

 foams like meringue. Addition of any material that markedly lowers 

 the viscosity of the film causes the foam to break. Likewise, addition 

 of something that replaces the solute originally concentrated in the 

 surface breaks the foam when the new film layer has a low viscosity. 



Osmosis and Osmotic Pressure 



When two gases are brought into contact, they diffuse by way of 

 molecular motion and rapidly come to equilibrium in a uniform 

 mixture. During the diffusion process, local differences in the 

 concentrations of the two gases produce a force responsible for uni- 

 formity of mixing. A similar phenomenon is observed when solutions 

 of different concentration are brought together. Owing to the greater 

 friction between molecules of solute and solvent in the liquid state, 

 diffusion is less rapid. Eventually, however, the system comes to 

 equilibrium and the solute is uniformly distributed throughout. 



When a membrane through which solvent but not solute can diffuse 

 (called semipermeable) is placed between solution and pure solvent, 

 solvent diffuses into and dilutes the solution. This flow is termed 

 osmosis, defined as the net transfer of solvent from a dilute solution 

 through a semipermeable membrane to a more concentrated solution. 

 Osmosis continues until a pressure develops or is applied of sufficient 

 magnitude to exactly oppose the force of diffusion arising from the 

 difference in concentration. This compensating pressure is called the 

 osmotic pressure and depends upon the concentration of the solute 

 restrained by the membrane. When the osmotic pressure is applied to 

 the system, osmosis ceases and osmotic equilibrium is attained. How- 

 ever, as in other equilibria, the system is not completely static. Rather, 

 diffusion of solvent continues, but the rates through the membrane 

 are the same in the opposing directions. Thus there is no net transfer 

 of solvent. 



Osmotic pressures may be deliberately applied, as with piston 

 pumps, columns of liquid, etc. Or they may develop in a closed 

 system like a cell by a mechanical resistance to the increase in volume 

 resulting from diffusion of solvent. Thus a cell with its solutes unable 

 to diffuse through the cell wall swells with solvent until the strains of 

 stretching exert the osmotic pressure and osmosis ceases. In the event 



