CARBONIC ACID 151 



can be attained with pure water only after 

 elaborate and very difficult purification, yet 

 in the presence of carbonic acid it is the natural 

 condition. Of course the case above dis- 

 cussed is an extreme one. In nature the con- 

 centrations of dissolved substances are less, 

 the mixing less efficient, and the variations of 

 reaction a little greater. There is also, in 

 nature, likely to be a considerable excess of 

 bicarbonate over free carbonic acid present. 

 The cause of the greater constancy of re- 

 action in the case of the heterogeneous equi- 

 librium is simple enough. At the beginning of 

 the experiment the free carbonic acid of the 

 solution is exactly in equilibrium with that of 

 the atmosphere. Accordingly, when hydro- 

 chloric acid is poured in and reacts with so- 

 dium bicarbonate to form sodium chloride and 

 more carbonic acid, 



NaHC0 3 + HC1 = NaCl + H 2 + C0 2 



every bit of the latter escapes to the atmos- 

 phere, and the total amount of acid is just 

 what it was before. But a certain amount of 

 carbonic acid has been replaced by the equiv- 

 alent amount of hydrochloric acid. This 

 latter substance, however, is wholly in union 

 with sodium, as its salt. Thus the addition of 

 the strong acid diminishes the amount of 



