28 CHEMISTRY OF FARM PRACTICE 



which water has been subtracted. They are quite gen- 

 erally oxides, that is, compounds in which oxygen is com- 

 bined with one other element. There are two classes: 



(1) Acid Anhydrides. For example: Sulphuric anhy- 

 dride, SOs, produced by subtracting water, H 2 0, from 

 sulphuric acid, H 2 SO4; nitric anhydride, N2O5, produced by 

 subtracting water from two molecules of nitric acid, HNOs. 



(2) Basic Anhydrides. For example: Calcium oxide, 

 CaO, produced by subtracting water from the base, cal- 

 cium hydroxide, Ca(OH) 2 ; sodium oxide, Na2O, produced 

 by subtracting water from two molecules of the base NaOH. 



27. Dissociation. If an acid or a base or a salt is dis 

 solved in water and a current of electricity is passed through 

 the solution, it will be found that the compound will tend 

 to separate into two parts, one of which will accumulate 

 at the + electrode, where the current enters the solution, 

 while the other portion will condense about the elec- 

 trode, where the current leaves the solution. As opposite 

 kinds of electric charges attract each the other, the portion 

 attracted to the -f- electrode is called the electro-negative 

 part of the compound, and the portion attracted towards 

 the electrode is called the electro-positive portion of the 

 compound. These portions, because they move through the 

 solution, are termed ions. In the case of acids it is found 

 that the replaceable or acid hydrogen of the acid is con- 

 densed about the electrode, and is therefore electro- 

 positive, while the remainder of the acid, the acid radical 

 portion, moves toward the + electrode and is therefore 

 electro-negative. These are some examples: 



HC1, H(N0 3 ), H(C 2 H 3 2 ). 



When salts are thus electrolyzed, the metallic part of 

 the salt is found to be electro-positive, and the acid radical 

 part is electro-negative : 



+ -+-+- + - + 



ZnCl 2 , PbS, Na 3 (PO 4 ), Na(H 2 PO 4 ), Na 2 (HPO 4 ). 



