ACIDS, BASES, AND SALTS 179 



ence of nitrogen peroxide NO 2 , while a careful analysis would 

 show that it contained also carbon dioxide CO 2 , nitrogen N 2 , 

 nitrous oxide N 2 O, water vapor H 2 0, and perhaps still other prod- 

 ucts. 



The contrast, when the substances are dissolved in water before 

 being brought in contact, is very great. A pale green precipi- 

 tate is immediately produced, which rapidly settles out and proves, 

 on examination, to be a carbonate of copper. Evaporation of 

 the solution gives us ammonium nitrate. 



(NH 4 ) 2 C0 3 + Cu(N0 3 ) 2 -> CuC0 3 J, + 2NH 4 NO 3 . 



The reaction is essentially a double decomposition (although, 

 strictly speaking, the precipitate is not the normal carbonate of 

 copper, but a basic salt, see p. 192), similar in character to those 

 already discussed. 



The differences in properties between dry hydrogen chloride 

 and hydrochloric acid (p. 131) furnish another good example of 

 the fundamental changes involved in the addition of water as a 

 solvent. Carefully dried hydrogen chloride has practically none 

 of the properties of an acid. It does not affect litmus, it does 

 not conduct the electric current. It is true that it does react 

 with the more active metals (such as Na or K), hydrogen being 

 displaced and the chloride of the metal formed. But it is probable 

 that even this property would disappear if we could succeed in 

 eliminating the last traces of moisture from our reacting products. 

 A solution of sulphuric acid acts on sodium so vigorously that the 

 mixture explodes, but pure hydrogen sulphate H 2 SO4, when very 

 carefully dehydrated, is so inert that metallic sodium floats in it 

 without the slightest evidence of any interaction. 



The rules that have been derived in the preceding sections re- 

 garding the behavior of acids, bases, and salts in solution will be 

 found to clear the ground wonderfully in the development of 

 subsequent chapters, for the substances which we shall meet with, 

 that are not included in the above groups, are very few indeed. 



