120 PRINCIPLES OF CHEMISTRY. 



gen, and the peculiar condition in which it is present, which impart 

 to acids their peculiar properties, are demonstrated by the fact that the 

 acid properties disappear as soon as the hydrogen is replaced by a 

 metal. Thus, the acid characteristics of hydrochloric acid, HC1, 

 vanish when it is acted on by sodium, or by the basic substance 

 caustic soda (sodium hydroxide, NaOH), both of which cause a re- 

 placement of the acid hydrogen by sodium. These actions can be 

 represented by the equations : 



HC1 -f Na NaCl + H. 



HC1 + NaOH NaCl -f H 2 O. 



In both cases sodium chloride, NaCl (common salt), is formed, which 

 possesses neither acid nor basic properties. 



Neutral substances. All substances having neither acid nor basic 

 properties are neutral. Water, for instance, is a neutral substance, 

 having no acid or alkaline taste, and no action on red or blue litmus. 

 Many neutral substances, to some extent even water, appear to possess 

 the characteristic properties of both classes, acids and bases ; of neither 

 class, however, to a very great extent. 



Salts. Salts are acids in which hydrogen has been replaced by 

 metals or by basic radicals. There are several general methods by 

 which salts may be obtained : 



1. By the action of an acid on a metal. This is illustrated in the 

 preparation of hydrogen from sulphuric or hydrochloric acid and 



zinc or iron. 



Zn + H 2 S0 4 = ZnS0 4 + H 2 . 

 Fe + 2HC1 = FeCl 2 + H 2 . 



2. By the action of an acid on an oxide or hydroxide of a metal. 

 This is of wider application than the previous method. 



ZnO + H 2 SO 4 = ZnSO 4 + H 2 0. 

 MgO + 2HC1 = MgCl 2 -f H 2 O. 

 NaOH + HC1 = NaCl + H 2 O. 



3. By the action of an acid on a salt of a volatile acid. This finds 

 most extensive and useful application in the case of carbonates, which 

 are decomposed by nearly all other acids, and are found ready-formed 

 in nature or can be easily made. 



MgCO s + H. 2 S0 4 = MgS0 4 + H 2 + C0 2 . 

 CaC0 3 + 2HC1 = CaCl 2 + H 2 O + CO 2 . 



Other volatile acids whose salts are decomposed by acids are sulphur- 

 ous, nitrous, hydrogen sulphide, hydrocyanic, etc. The manufacture 



