432 PRINCIPLES OF CHEMISTRY 



or other animals, but experience has shown that many wild animals travel 

 long distances in search of salt springs, and that domestic animals 

 which in their natural condition do not require salt, willingly take 

 it, and that the functions of their organisms become much more regular 

 from their doing so. 



The, action of sulphuric acid on sodium chloride. If sulphuric acid 

 be poured over common salt, then even at the ordinary temperature, as 

 Glauber observed, an odorous gas, hydrochloric acid, is evolved. The 

 reaction which takes place consists in the sodium of the. salt and the 

 hydrogen of the sulphuric acid changing places. 



NaCl + H 2 S0 4 HC1 -f NaHS0 4 



Sodium chloride Sulphuric acid Hydrochloric acid Acid sodium sulphate 



At the ordinary temperature this reaction is not complete, but soon 

 ceases. When the mixture is heated, the decomposition proceeds 

 until, if there be sufficient salt present, all the sulphuric acid taken is 

 converted into acid, sodium sulphate. Any excess of acid will remain 

 -unaltered. If 2 molecules of sodium chloride (117 parts) be taken 

 per molecule of sulphuric acid (98 parts), then on heating the' mixture 

 to a moderate temperature only one-half (58'5) of the salt will suffer 

 change. Complete decomposition, 'after which neither hydrogen nor 

 chlorine is left in the residue, proceeds (when 117 parts of table salt 

 are taken per 98 parts of sulphuric acid) at a red heat only. Then 



+ H 2 S0 4 2HC1 + 



Table salt Sulphuric acid Hydrochloric acid Sodium sulphate 



This double decomposition is the result of the action of the acid 

 salt, NaHSO 4 , first formed, on sodium chloride, for the acid salt, 

 since it contains hydrogen, itself acts like an acid, NaCl + NaHS0 4 

 = HC1 + Na 2 SO 4 . By adding this equation to the first we obtain 

 the second, which expresses the ultimate reaction. Hence in the above 

 reaction, non-volatile or sparingly volatile table salt and sparingly 

 volatile sulphuric acid are taken, and as the result of their reaction, 

 after the hydrogen and sodium have exchanged places, there is obtained 

 non- volatile sodium sulphate and gaseous hydrochloric acid. The fact 

 of the latter being a gaseous substance forms the main reason for the 

 reaction proceeding to the very end. The mechanism of this kind of 

 double decomposition, and the cause of the course of the reaction, are 

 exactly the same as those we saw in the decomposition of nitre 

 (Chapter VI.) by the action of sulpliuric acid. The sulphuric acid ia 

 each case displaces the other, volatile, acid. 



