76 ELECTRICAL CONDUCTIVITIES, ETC. 



TEMPERATURE COEFFICIENTS OF CONDUCTIVITY AND THE SOLVATE THEORY 



OF SOLUTION. 



The temperature coefficients of conductivity are expressed both in conductivity 

 units and in per cent. Certain relations between the coefficients in conductivity 

 units and the solvate theory of solution have already been pointed out for a few 

 substances.* We can now see how general these relations are. We have seen that 

 the chief factor conditioning the increase in conductivity with rise in temperature 

 is the increase in the velocities with which the ions move. If we assume that the 

 force which drives the ions is constant, the velocity would be conditional chiefly by 

 the viscosity of the medium through which the ion moves, and by the mass and size 

 of the ion. The force that drives the ion would be greater at the more elevated 

 temperatures, and the viscosity of the medium through which the ion moves would 

 be less. Both of these factors would increase the ionic velocities and, consequently, 

 the conductivity with rise in temperature. 



There is, however, another factor which must be taken into account. That many 

 ions in aqueous solution are hydrated seems now to be generally accepted. We 

 have shown that these hydrates are relatively unstable; the higher the temperature 

 the less complex the hydrate existing in solution. One example will make this point 

 clear. In a normal solution of aluminium chloride, every molecule of the salt, or the 

 ions resulting from it, is combined with about 30 molecules of water at the freezing 

 point of the solution. Practically all of the water can be removed from such a 

 solution by boiling it, except six molecules to one of aluminium chloride, this being 

 the number brought out of solution as water of crystallization. The smaller the 

 number of molecules of water combined with the ion the less the mass of the ion, 

 and the less its resistance when moving through the solvent. Consequently, the 

 ion will move faster the higher the temperature. 



When we refer to the mass of the ion decreasing with rise in temperature, we do 

 not refer to the charged atom or group of atoms which we usually term the ion, but 

 to this charged nucleus plus a larger or smaller number of molecules of water which 

 are attached to it, and which it must drag along with it in its motion through the 

 remainder of the solvent. 



The above conclusion can be tested by the results of experiment. If this factor 

 of diminishing complexity of the hydrate of the ion with rise in temperature plays 

 any prominent role in determining the large temperature coefficient of conductivity, 

 then we should expect to find those ions with the largest hydrating power, having 

 the largest temperature coefficients of conductivity. This condition can be tested 

 by the results, as can be seen from the tables on page 77. 



The hydrating power of a salt (or the ions resulting from it) is roughly proportional 

 to the number of molecules of water with which the salt crystallizes. This is the 

 same as to say that the salt which has the greatest power to bring water with it out 

 of solution as water of crystallization would be the salt which, in solution, would 

 combine with the largest amount of water. Water of crystallization is, then, a good 

 general criterion of the degree of hydration in aqueous solution. 



'Amer. Chcm. Journ., 35, 445 (1906). 



