SODIUM CHLORIDE -BERTHOLLET'S LAWS 441 



rate of action (for example, in the case of CaCl 2 + Na 2 CO 3 , 75 per' 

 cent, of CaCO 3 was precipitated in five minutes, 85 per cent, in thirty 

 minutes, and 94 per cent, in two days) determined by the temperature, 

 relative mass, and amount of water (a large mass of water decreases the 

 rate), but that the limit of decomposition was also dependent on these 

 influences. However, even in researches of this kind the conditions 

 of reaction are complicated by the non- uniformity of the media, inas- 

 much as a portion of the substance is obtained, or remains in the form of 

 a precipitate, so that the system is heterogeneous. The investigation of 

 double saline decompositions offers many difficulties which cannot be 

 considered as yet entirely overcome. Although many efforts have long 

 since been made, the majority of the researches were carried on in 

 aqueous solutions, and as water is itself a saline compound and 

 able to combine with salts and enter into double decomposition with 

 them, such reactions taking place in solutions in reality present very 

 complex cases. 27 In this sense the" reaction between alcohols and acids 



* 7 As an example two methods may be mentioned, Thomsen's and Ostwald's. Thomsen. 

 (1869) applied a thermo-chemical method to exceedingly dilute solutions without taking" 

 the water into further consideration. He took solutions of caustic soda containing 

 100H 3 per NaHO, and sulphuric acid containing PLS0 4 + lOOHoO. In order that these 

 solutions may be mixed in such quantities that atomic proportions of acid and alkali would 

 act, for forty grams of caustic soda (which answers to its equivalent) there. should 

 be employed 49 grams of sulphuric acid, and then + 15,689 heat units would be evolved. 

 If the normal sodium sulphate so formed be mixed with n equivalents of sulphuric 



acid, a certain amount of heat is absorbed, namely a quantity equal to L--- .- heat 



units. An equivalent of caustic soda, in combining with an equivalent of nitric acid, 

 evolves + 13,617 units of heat, and the augmentation of the amount of nitric acid entails 

 an absorption of heat for each equivalent equal to 27 units ; so also in combining with 

 hydrochloric acids + 13,740 heat units are absorbed, and for each equivalent of hydro-.. 

 chloric acid beyond this amount there are absorbed 82 heat units.. Thomsen mixed 

 each one of three neutral salts, sodium sulphate, sodium chloride and sodium nitrate, 

 with an acid which is not contained in it ; for instance, he mixed a solution of sodium 

 sulphate with a solution of nitric acid and determined the number of heat units then 

 absorbed. An absorption of heat ensued because a normal salt was taken in the 

 first instance, and the mixture of all the .above normal salts with acid produces au 

 absorption of heat. The amount of heat absorbed enabled him to obtain an insight into 

 the process taking place in this mixture, for sulphuric acid added to sodium sulphate 

 absorbs a considerable quantity of heat, whilst hydrochloric and nitric acids absorb a 

 very small amount of heat in this case. By mixing an equivalent of sodium sulphate. 

 with various numbers of equivalents of nitric acid, Thomsen observed that the amount of 

 heat absorbed increased more and more as the amount of nitric acid was increased ; thus 

 when HNOs was taken per ^Na i SO 4 , 1,752 heat units were absorbed per equivalent of 

 soda contained in the sodium sulphate. When twice as much nitric acid was taken, 2,026. 

 heat units, and when three times as much, 2,050 heat units were absorbed. Had the 

 double decomposition been complete in the case where one equivalent of nitric acid was, 

 tuken per equivalent of Na 2 SO 4 , then- according to calculation from similar data there 

 should have been absorbed -2,989 units of heat, while in reality only - 1,752 units were. 

 absorbed. Hence Thomsen concluded that a displacement of only about two-thirds of the 

 sulphuric acid had taken place that is, the ratio k : k' for the reaction iNa. 2 SO*-.HN0 3 . 



