ELECTROLYTIC DISSOCIATION. 391 



H + OH = HOH. 



The whole reaction is equivalent to the formation of water out of 

 both its ions, H and OH, and evidently independent of the nature of 

 the strong acid and of the strong base. The heat of any reaction of 

 this kind must, therefore, always be the same for equivalent quantities 

 of any strong acids and bases. In reality it is found to be 13,600 cal. 

 in all cases. This thermal equality was the most prominent feature 

 that thermochemistry had discovered. 



It was now asked in what respect the active state of the electrolytes 

 differs from the inactive one. On this question I gave an answer in 

 1887. At that time van't Hoff had formulated his wide-reaching law 

 that the molecules in a state of great dilution obey the laws that are 

 valid for the gaseous state, if we only replace the gas-pressure by the 

 osmotic pressure in liquids. As van't Hoff showed, the osmotic pressure 

 of a dissolved body could much more easily be determined by help of a 

 measurement of the freezing point of the solution than directly. Now 

 both the direct measurements made by De Vries, as also the freezing 

 points of electrolytic solutions, showed a much higher osmotic pressure 

 than might be expected from the chemical formula. As, for instance, 

 the solution of 1 gram-molecule of ethylic alcohol — C,H 5 OH = 46 

 grams — in one liter gives the freezing-point — 1.85° C, calculated 

 by van't Hoff the solution of 1 gram-molecule of sodium chloride — 

 NaCI = 58.5 grams — in one liter gives the freezing-point — 3.26 = 

 — 1.75 X 1.85° C. This peculiarity may be explained in the same 

 manner as the ' abnormal ' density of gaseous sal-ammoniac, viz., by 

 assuming a partial dissociation — to 75 per cent. — of the molecules of 

 sodium chloride. For then the solution contains 0.25 gram-molecules 

 of NaCl, 0.75 gram-molecules of CI and 0.75 gram-molecules of Na; 

 in all, 1.75 gram-molecules. Now we have seen before how we may 

 calculate the number of active molecules in the same solution of sodium 

 chloride, and we find by Kohlrausch's measurements precisely the num- 

 ber 0.75. From this I was led to suppose that the active molecules of 

 the salts are divided into their ions. These are wholly free and behave 

 just as other molecules in the solutions. In the same manner I cal- 

 culated the degree of dissociation of all the electrolytes that were deter- 

 mined at that time — they were about eighty — and I found in general a 

 very good agreement between the two methods of calculation. In a 

 few instances the agreement was not so good; I therefore made new 

 determinations for these bodies and some others. The new determina- 

 tions were all in good conformity with the theoretical prevision. 



The next figure (Fig. 6) shows the freezing-points of some solution 

 of salts, and of non-conductors. As abscissa is used the molecular 

 concentration of the bodies, as ordinates the molecular depression of 



