128 SMITH'S INTERMEDIATE CHEMISTRY 



reaction must, and does, come to a standstill while as yet only 

 partly accomplished; but this was not the case with action? 

 (1) and (2). Let us examine the means by which the premature 

 cessation of each was avoided. 



In equation (1) the salt dissolved to some extent in the sulphuric 

 acid, NaCl (solid) ^NaCl (dslvd.), and so, by intimate contact 

 of the two kinds of molecules in the resulting solution, the prod- 

 ucts HC1 and NaHS0 4 were formed. On the other hand, the 

 hydrogen chloride, being practically insoluble in sulphuric acid, 

 escaped as fast as it was formed: HC1 (dslvd.) <= HC1 (gas). 

 Hence, in that case, almost no reverse action was possible, and the 

 double decomposition went on virtually to completion. With 

 all the sodium-hydrogen sulphate in the bottom of the flask, and 

 most of the hydrogen chloride in the space above, the two products 

 might as well have been in separate vessels so far as any efficient 

 re-interaction was concerned. This plan, in which water is pur- 

 posely excluded, forms therefore the method of making hydrogen 

 chloride. 



In equation (2), on the other hand, the hydrogen chloride was 

 taken in aqueous solution, and was mixed with a concentrated 

 solution of sodium-hydrogen sulphate. The acid was, therefore, 

 kept permanently in full contact with the sodiXim-hydrogen sul- 

 phate. It had in this case, every opportunity to interact with 

 the latter and no chance of escape. Every molecule of each 

 ingredient could reach every molecule of the other with equal 

 ease. Furthermore, the sodium chloride, produced as a result 

 of their activity, is not very soluble in concentrated hydrochloric 

 acid (far less so than in water), and so it came out as a precipi- 

 tate: NaCl (dslvd.) + NaCl (solid). But this was almost the 

 same as if it had gone off as a gas. It meant that the greater part 

 of the salt was in the solid form. In this form, it was no longer 

 able to interact effectively molecule to molecule with the other 

 product, the sulphuric acid. Hence, there was little reverse 

 action to impede the progress of the primary one. Thus (2) is 



