172 



we have met with. But there are well-known acids correspond- 

 ing to them, namely, chloric acid HClOs, hydrogen sulphide [28, 

 hydrogen peroxide H 2 02, and carbonic acid H^COs. 



In general, then, all positive radicals combine with OH to 

 give bases, all negative radicals combine with H to give acids. 

 In general, also, each positive radical will combine with any 

 negative radical to give a salt. In a few exceptional cases only, 

 the compound cannot be formed, presumably because it is un- 

 stable under ordinary conditions. 



Reaction Formulae. The formulae of acids, bases, and salts 

 are written in a uniform manner to show the behavior of the 

 substances represented, when in solution. Thus, the radical 

 written first is usually the positive one which, when a solution is 

 electrolyzed, is attracted by the negative wire, and the negative 

 radical follows. Again, in a compound like calcium hydroxide, 

 the formula Ca0 2 H 2 would conceal the existence of the hydroxyl 

 group. So the radicals are written in brackets, with the coefficient 

 outside Ca(OH) 2 . Thus we write also Cu(N0 3 ) 2 , and not 

 CuN 2 6 , and (NH 4 ) 2 S0 4 , not N 2 H 8 SO 4 . These formulas are all 

 reaction formulae. That is, they indicate, not simply the com- 

 position, but also the parts into which the compounds decompose, 

 and from which they are formed in double decompositions (p. 132, 

 and p. 173, below). 



Properties Common to Acids, Bases and Salts in Solution. 



- There are four of these properties, all of which have come up 

 previously. 



1. Displacement. A simple radical belonging to an acid, 

 base, or salt in solution can be displaced by another element, and 

 is thereby obtained in the free state. Thus we have already seen 

 (p. 51) that hydrogen is liberated from acids by the addition of 

 the more active metals: 



