122 PRINCIPLES OF CHEMISTRY. 



for this is that such salts are partially decomposed or hydrolyzed by 

 water. Thus, in the case of ferrous sulphate, 



FeS0 4 -f 2H 2 = Fe(OH) 2 + H 2 SO 4 , 



the small quantity of free acid formed affects litmus-paper, Fe(OH) 2 

 being neutral. Sodium carbonate is acted on thus : 



Na 2 C0 3 + H 2 = NaHC0 3 + NaOH, 



the free alkali causes litmus to turn blue, while NaHCO 3 is neutral. 



Acid salts are acids in which there has been replaced only a portion 

 of their replaceable hydrogen atoms. For instance : KHSO 4 , K 2 H PO 4 , 

 KH 2 PO 4 . While acid salts have generally an acid reaction to litmus, 

 there are many exceptions to this rule. Indeed, the reaction may be 

 neutral or even alkaline, as, for instance, in the case of the ordinary 

 sodium phosphate, Na 2 HPO 4 , which is slightly alkaline to litmus. 



Basic salts are salts containing a higher proportion of a base than 

 is necessary for the formation of a normal salt. Instances are basic 

 mercuric sulphate, HgSO 4 .(HgO) 2 , basic lead nitrate, Pb(NO 3 ) 2 . 

 Pb(OH) 2 . According to modern views basic salts are looked upon 

 as derived from bases by replacement of part of their hydrogen by acid 

 radicals. In the base lead hydroxide, Pb(OH) 2 , one of the hydrogen 

 atoms may be replaced by the radical of nitric acid, when basic lead 



nitrate, Pb<TT 3 ' is formed. 



In bismuth hydroxide, Bi(OH) 3 , one, two, or three hydrogen atoms 

 may be replaced by nitric acid, when the salts Bi(^ /QTT\ 



and Bi(NO 3 ) 3 are formed. The first two compounds are basic salts, 

 while the third one is the normal salt. 



Double salts are salts formed by replacement of hydrogen in an 

 acid by more than one metal. For instance : Potassium-sodium sul- 

 phate, KNaSO 4 . 



Residue, radical, or compound radical, are expressions for un- 

 saturated groups of atoms known to enter as a whole into different 

 compounds, but having no separate existence. For instance: The 

 bivalent oxygen combines with two atoms of the univalent hydrogen, 

 forming the saturated compound H 2 O, water. If we take from this 

 H 2 O one atom of H, there is left the group of atoms HO (generally 

 written OH), consisting of an atom of oxygen in which but one point 

 of attraction is actually saturated, the second one not being pro- 

 vided for. 



This group, OH, is a residue or radical, and is known to enter into 



