HYDROCHLORIC ACID. CALCULATIONS 127 



from these objections, all the other acids are more expensive than 

 sulphuric acid. 



The Molecular View of the Interaction of Sulphuric Acid 

 and Salt. One who has used the above-described methods for 

 making hydrogen chloride without reflection would not realize 

 the complexity of the machinery by which the result is achieved. 

 The means are apparently very simple. Yet the mechanical 

 features of this experiment, when laid bare, are extremely curious 

 and interesting. A single fact will show the possibilities which 

 are concealed in it. 



If we take a saturated solution of sodium-hydrogen sulphate in 

 water and add to it a concentrated solution of hydrogen chloride in 

 water (concentrated hydrochloric acid), we shall perceive at once 

 the formation of a copious precipitate. This is composed entirely 

 of minute cubes of sodium chloride: 



NaHS0 4 + HC1 -> H 2 S0 4 + NaCl J, . (2) 



(An arrow pointing downwards is used in equations to indicate 

 that the substance to which it refers removes itself from the 

 reaction in the form of a precipitate.) Now this action is nothing 

 less than the precise reverse of (1), yet it proceeds with equal 

 success. In fact, this chemical interaction is not only reversible 

 (p. 103), but can be carried virtually to completion in either 

 direction. It is only in presence of a large amount of water, 

 sufficient to keep both the hydrogen chloride and the salt all in 

 solution, that it stops midway in its career and is valueless for 

 securing a complete transformation in either direction: 



NaHS0 4 + HC1 <=> H 2 S0 4 + NaCl. 



In an action which is reversible, if the products remain as per- 

 fectly mixed and accessible to each other as were the initial sub- 

 stances, their interaction will continually undo a part of the work 

 of the forward direction of the change. Hence, in such a case the 



