26 PHYSICAL CHEMISTRY 



posed to be in the form of hydrates. Sulphuric acid forms the 

 following hydrates (see Fig. 5) : sH 2 S0 4 • H 2 0, 3H 2 S0 4 ■ H 2 0, 

 H 2 S0 4 -H 2 0, H 2 S0 4 -2H 2 0, H 2 S0 4 ■ 3HA H 2 S0 4 • 4H 2 

 and H 2 S0 4 • I2H 2 0, at percentages 96, 94, 85, 79, 65, 58, and 30 

 of sulphuric acid (C. E. Davis, 1915). 



It should be emphasized that strong electrolytes exist chiefly 

 as ions in the dilute solutions with which we have to deal in bio- 

 chemistry. Take, for example, the neutralization of HC1 with 

 KOH. Since HC1, KOH and KC1 take no part in the reaction 

 and are practically never present, the reaction becomes 

 H" -+- OH' = H 2 0. CI' and K' are present in the same quan- 

 tities before and after the reaction takes place. That the re- 

 action is simply the formation o-f water from its ions is illustrated 

 by the fact that no matter what strong base or acid is substituted 

 for the above (at 20 ), 13700 calories of heat are produced for 

 1 mol of acid and alkali used. 



Ionization must be considered in studying the solubility of 

 electrolytes. If we pass hydrochloric acid gas into a saturated 

 solution of NaCl, the salt will partially crystallize out. In a 

 saturated solution of NaCl, the product of the concentrations of 

 Na" and CI' is a constant, and when we add more CI' by the 

 addition of HC1, salt crystallizes out until the product assumes 

 its original value. (This principle is used in the recrystallization 

 of NaCl by the introduction of HC1 gas into the saturated solu- 

 tion.) 



Some electrolytes dissociate into three or more ions. They disso- 

 ciate by steps, thus K 2 S0 4 dissociates into K' and KSO/ and 

 the latter dissociates into K' and S0 4 ". In certain cases the 

 first step may be complete and the second be incomplete under 

 ordinary conditions. The dissociation of H 2 CO s and H 3 P0 4 

 will be considered in a later chapter. 



In case one of the ions of an electrolyte is weak and the other 

 strong hydrolysis may occur. K 2 C0 3 dissociates into K* and 

 KCO/. K' combines with OH' of the water and forms KOH 

 while the other ion unites with H' of water to form KHC0 3 . 

 Since the KOH dissociates more OH ions than the bicarbonate 

 produces H ions the reaction is alkaline. The further steps in 

 this hydrolysis will be considered later. Hydrolysis is the union 

 with the ions of water already present. When these are ex- 



