58 lONISATION 



conceded that, as stated above, all ions are hydrated. Therefore 



potassium and chlorine must be hydrated to almost the same 



extent. Bousfield has shown that 9 water molecules are attached 



to both ions of potassium chloride when completely dissociated, 



64-6 65-5 

 Now, as the speed of K to CI is as — — : — -, i.e. as 16 : 19, almost 



as 4 : 5, it may be considered that K has 4 and CI 5 water molecules 

 per ion. 



In the group of alkali metals tabulated above it will be seen 

 that the lightest metal, lithium, furnishes the most sluggish ion 

 of the three, and conversely, the most mobile ion is that of the 

 heaviest metal, potassium, sodium being intermediate both in 

 atomic weight and in speed. This is supposed to mean that 

 lithium is more heavily hydrated than sodium, and sodium more 

 than potassium. The number of molecules of water combined 

 with their chlorides when completely dissociated is respectively, 

 21, 13 and 9. If the 5 molecules of water which form an envelope 

 for the chlor-ion be subtracted from the total, lithium is found 

 to be hydrated to the extent of 16 and sodium to 8 molecules. 



Effect of Temperature. 



Increase in temperature according to the kinetic theory and 

 laws of energy will increase the speed of ions, provided, of course, 

 that dissociation is complete. Partially dissociated salts are more 

 completely ionised by increase in temperature. For equal incre- 

 ment of temperature, different ions increase in speed according 

 to their degree of hydration. The more highly hydrated the ion, 

 the greater is its temperature coefficient. This is explicable on 

 the hypothesis that a rise of temperature will favour the disruption 

 of hydrate-complexes and decrease the size of the ion, and so 

 reduce the frictional resistance to its passage through the fluid. 



When dealing with surface tension (p. 48), the Helmholtzian 

 double layer or surface electrical charge was mentioned. This 

 may now be attributed to the different ionic speeds. Whichever 

 of the two ions has the greater mobility will get into the surface 

 layer and, of course, will carry its charge with it. This will cause 

 the mobilisation on the immediately opposite " side " of the 

 surface of oppositely charged ions. 



There exists an enormous electrostatic attraction between ions 

 of opposite sign. The introduction of other electrolytes into a 

 solution may therefore alter not only the rate of migration of the 

 original ions but the nature of the surface charge. The addition 

 of HCl to a solution of KCl would increase the diffusion potential 

 that would be produced at the boundary between solutions of KCl 



