168 CHEMISTRY. 



is added in making aqua regia forces the hydrochloric acid 

 to give up its chlorine, which at once unites with the gold. 



226. Compounds of Chlorine with Oxygen. Chlorine unites 

 with oxygen in several properties, forming anhydrides. 

 These form with water four acids : hypochlorous, IIC1O ; 

 chlorous, HC1O 2 ; chloric, HC1O 3 ; and perchloric, HC1O 4 , as 

 shown in the table on p. 35. 



Two of these acids are formed on passing chlorine gas 

 into a solution of potassium hydrate; thus: 



^i , . Potassium Potassium Potassium Potassium w 

 ine ' hydrate. chloride. hypochlorite. chlorate. 



C1 8 + 8KHO = 6KC1 + KC10 + KC1O 3 + 4H 2 O 

 You see that part of the oxygen of the potassium hydrate 

 oxidizes the chlorine and combines with the potassium. Ac- 

 cording to the strength of the potassium solution you ob- 

 tain more of the hypochlorite or of the chlorate, the weaker 

 solution giving more of the former. Hypochlorous acid is a 

 powerful bleaching agent ; combined with calcium it makes 

 the so-called chloride of lime, of which you will learn more 

 later. Potassium chlorate, you remember, was used in the 

 preparation of oxygen gas. It is also used in medicine to 

 a limited extent. The anhydrides corresponding to the 

 first three acids named are unstable gases ; all have a red or 

 yellow color and a pungent odor. We will not describe them 

 further, but will notice the preparation of one of them hy- 

 pochlorous anhydride. It is best prepared by the action of 

 chlorine upon dry mercuric oxide: 



Mercuric oxide. Chlorine. 2 dride" 8 Mercuric chloride. 



2HgO + C1 4 = C1 2 + HgCl 2 



227. Other Compounds of Chlorine. Chlorine also forms com- 

 pounds with carbon and with nitrogen, but they are far too rare and unin- 

 teresting to describe here. Chloride of nitrogen is one of the most danger- 

 ously explosive substances known, and must never be prepared by students 

 11 for fun." So we will not tell you how to make it. 



