OXIDIZING SUBSTANCES 223 



with age, because hydrogen sulphide in the air turns the white lead 

 into the black lead sulphide, PbS. In picture restoring the latter 

 is oxidized to lead sulphate, which is white, by treatment with 

 hydrogen peroxide solution. 



PbS + 4H 2 2 -> PbS0 4 + 4H 2 O. 



4. The following reaction is used as a test for hydrogen peroxide. 

 Wjieix- a -solution of_potassium dichromate K 2 Cr 2 7 is. acidified 

 with fiiilphinjfl acid and a drop of the mixture is added to aqueous 

 hydrogen peroxide, an unstable substance possessing a deep, 

 brilliant blue color is formed. By this test the presence of hydro- 

 gen peroxide in rain water can often be demonstrated. 



HYPOCHLOROUS ACID HOC1 



Pure Hypochlorous Acid. A pure solution of the acid may 

 be made by dissolving chlorine monoxide C1 2 in water. Chlorine 

 monoxide is a brownish-yellow, explosive gas, made by passing 

 chlorine gas over warmed mercuric oxide : 



2C1 2 +;HgO -> HgCl 2 + CM). 



C1 2 + H 2 0-+2HOC1. 



As an acid, hypochlorous acid is very weak, being very little 

 decomposed into its ions, H + and (OC1)~. 



It is unstable, exposure to sunlight being sufficient to cause it 

 to give up oxygen, which rises in bubbles 

 through the solution (Fig. 64) : 



2HOC1 -> 2HC1 + 2 1 . 



Heat is given out in the action, and the stable 

 hydrochloric acid remains. 



It is a most active oxidizing agent, because 

 of this tendency to give up oxygen with libera- 

 tion of energy. Thus, its solution oxidizes 

 organic colored substances, producing color- 

 less or less strongly colored ones : ^ 

 Ci 6 H 10 N 2 2 + 2HOC1 -4 2C 8 H 5 N0 2 + 2HC1. 



indigo isatin 



