DIFFUSION OF SOLUTES 91 



4. Concentration of the Medium through Which Diffusion Occurs. — In 

 general the more concentrated the medium, i.e. the more molecules per unit 

 volume in the medium through which the diffusing molecules must pass, the 

 slower the rate of diffusion. Bromine gas, as shown earlier in this chapter, 

 diffuses more rapidly through a vacuum than through air. 



Diffusion of Solutes. — The molecules or ions of a solute possess sufficient 

 kinetic energ}' to move from place to place within the limits of a solution. 

 The simplest method of demonstrating the diffusion of a solute is to intro- 

 duce a crystal of copper sulfate, or some other compound which is colored 

 when in solution, into the bottom of a tall glass cylinder filled with water. 

 The cylinder should then be placed in an environment of equable temperature 

 where it will be free from disturbance. The diffusion of the molecules or 

 the ions which pass into solution in the water can be followed by the slow 

 change in color of the water. One of the most striking facts illustrated in 

 such experiments is the extremely slow rate of diffusion of solutes through 

 water. This is partly due to the fact that in such an experiment the steepness 

 of the dift'usion gradient decreases with time, but principally to the fact that 

 the densely packed molecules of the liquid enormously impede the diffusion 

 of the dissolved molecules or ions. 



The true rate of diffusion of solutes is probably even less than the rates 

 indicated in such experiments, since in all such set-ups convection currents 

 may develop in the water and aid in the distribution of the solute throughout 

 the body of the liquid. The diffusion of solutes is often demonstrated by 

 employing a gel rather than a liquid as the medium into which diffusion 

 occurs (Chap. V). Such a technique avoids errors introduced by the develop- 

 ment of convection currents. 



The direction of the diffusion of any solute occurs in accordance with 

 its own differences in diftiision pressure, regardless of the rate or direction 

 of diffusion of other solutes in the same system. The rate of diffusion of 

 solute particles is governed by principles essentially similar to those which 

 control the rate of diffusion of gases and is controlled by the following factors : 



I. Size and Mass of the Diffusing Particle. — Small molecules or ions dif- 

 fuse more rapidly than large ones. A hydrogen ion, for example, diffuses 

 many times more rapidly than a glucose molecule. Similarly, highly hydrated 

 ions diffuse more slowly than those which have fewer water molecules bound 

 to them, since the association of water of hydration with a molecule or ion 

 in eft"ect increases its size. The mass of the particle will also be a factor 

 influencing the speed of its diffusion. As betAveen two particles of the same 

 size, but different masses, the heavier particle will dift'use more slowly. 



