SODIUM CHLORIDE BERTHOLLET'S LAWS 447 



of salts ; the displacement, by its means, of basic hydrates insoluble in 

 water ; the separation of volatile nitric acid by the aid of non-volatile 

 sulphuric acid, as well as the decomposition of common salt by means 

 of sulphuric acid, when gaseous hydrochloric acid is formed may be 

 taken as examples of reactions which proceed to the end, inasmuch as 

 one of the resultant substances is entirely removed from the sphere of 

 action, but they in no way indicate the measure of affinity. 30 



As a proof that double decompositions like the above are actually 

 accomplished in the sense of Berthollet's doctrine, the fact may be cited 

 that common salt may be entirely decomposed by nitric acid, and nitre 

 may be completely decomposed by hydrochloric acid, just as they are 

 decomposed by sulphuric acid ; but this only takes place when, in the 

 first instance, an excess of nitric acid is taken, and in the second instance, 

 an excess of hydrochloric acid, for a given quantity of the sodium salt, 

 and when the resultant acid passes off. If sodium chloride be put intd 

 a porcelain evaporating basin, nitric acid added to it, and the mix- 

 ture heated, then both hydrochloric and nitric acids are expelled by 

 the heat. Thus the nitric acid partially acts on the sodium chloride, 

 but on heating, as both acids are volatile, they are both converted into 



150 Common salt not only enters into double decomposition with acids but also with 

 every salt. However, as clearly follows from Berthollet's doctrine, this form of decom- 

 position will only in a few cases render it possible for new metallic chlorides to be ob- 

 tained, because the decomposition will not be carried on to the end unless the metallic 

 chloride formed separates from the mass of the active substances. Thus, for example, 

 if a solution of common salt be mixed with a solution of magnesium sulphate, double 

 decomposition ensues, but not completely, because all the substances remain in the solu- 

 tion. In this case the decomposition must result in the formation of sodium sulphate and 

 magnesium chloride, substances which are soluble in water ; nothing is disengaged, and 

 therefore the decomposition 2NaCl>MgSO4 = MgCl 2 + Na 2 SO 4 cannot proceed to the end. 

 However, the sodium sulphate formed in this manner may be separated by freezing the 

 mixture. The complete separation of the sodium sulphate will naturally not take place, 

 owing to a portion of the salt remaining in the solution. Nevertheless, this kind of 

 decomposition is made use of for the preparation of sodium sulphate from the residues 

 left after the evaporation of sea-water, which contain a mixture of magnesium sulphate 

 and common salt. Such a mixture is found at Stassf urt in a natural form. It might be 

 said that this form of double decomposition is only accomplished with a change of tem- 

 perature ; but this would not be true, as may be concluded from other analogous cases. 

 Thus, for instance, a solution of copper sulphate is of a blue colour, while a solution of 

 copper chloride is green. If we mix the two salts together the green tint is distinctly 

 visible, so that by this means the presence of the copper chloride in the solution of copper 

 sulphate is clearly seen. If now we add a solution of common salt to a solution of copper 

 sulphate, a green coloration is obtained, which indicates the formation of copper chloride. 

 In this instance it is not separated, but it is immediately formed on the addition of 

 common salt, as it should be according to Berthollet's doctrine. 



The complete formation of a metallic chloride from common salt can only occur, 

 judging from the above, when it separates from the sphere of action. The salts of silver 

 are instances in point, because the silver chloride is insoluble in water; and therefore 

 jf we add a solution of sodium chloride to a solution of a silver salt, silver chloride and 

 the sodium salt of that acid which was in the silver salt are formed. 



