HYDRATION OF IONS 51 



of ions apart from hydration is proportional to their mass, e.g. 

 potassium and chlorine have approximately similar atomic 

 weights and similar speeds. Any variation from this proportion 

 is usually attributed to the different hydration of the ions. It is 

 generally 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 



r*t rt fifj 



dissociated. Now, as the speed of K to Cl is as : , 



39 35 -5 



i.e. as 16 : 19, almost as 4 : 5, it may be considered that K has 4 

 and Cl 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 

 increment of temperature, different ions increase in speed accord- 

 ing to their degree of hydration. The more highly hydrated the 

 ion, the greater is its temperature co-efficient. This is explicable 

 on the hypothesis that a rise of temperature will favour the dis- 

 ruption 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. 47), 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 an equal and opposite charge. As has been said above, 



