HYDROGEN DIOXIDE. 155 



The solution is colorless and without odor, and has a slightly acidu- 

 lous taste, producing a peculiar sensation and soapy froth in the 

 mouth. It is liable to deteriorate by age, especially on exposure to 

 heat and light. 



Pyrozone is the trade name under which a 50 per cent, hydrogen peroxide 

 is sold, but diluted pyrozone also is found in the market. 



Glycozone is hydrogen dioxide dissolved in glycerin instead of in water. 



Hydrogen dioxide, owing to its instability and tendency to decom- 

 pose into water and oxygen, is an excellent oxidizing agent. It is 

 frequently used in preference to other such agen-ts, because by its use 

 no other products are introduced into solutions than water and oxy- 

 gen. Toward a few substances, which themselves are unstable and 

 easily give up oxygen, it also acts as a reducing agent. For example, 

 silver oxide is reduced to metallic silver thus : 



Ag 2 + H 2 2 = 2Ag -f 2 + H 2 0. 



When hydrogen dioxide decomposes into water and oxygen, heat is liberated. 

 The thermal equation is 



H 2 O 2 = H 2 O + O -f 22,926 cal. 



that is, when 33.76 grammes of hydrogen dioxide corresponding to the formula, 

 H 2 2 , decomposes, 22,926 calories of heat energy are liberated. This is in 

 addition to the heat that is produced when the liberated oxygen unites with 

 other substances. In this way the great activity of hydrogen dioxide as an 

 oxidizer is accounted for. 



Tests 1 for solution of hydrogen dioxide. 



(Use the commercial solution after diluting about five times with water.) 



1. To a beaker half full of water, add 1 or 2 c.c. of solution of 



potassium iodide (see Reagents) and about 2 c.c. of the hydrogen 



dioxide solution. Is any yellow color produced ? Then add a few 



drops of starch solution (for which, see Index). A deep blue color 



is produced by the action of the starch on the iodine liberated from 



potassium iodide by the oxidizing action of the hydrogen dioxide : 



2KI -f H 2 2 = 2KOH + 21. 

 The action is more intense if the water is first acidified with 5 or 10 



1 Tests are reactions to which a substance may be subjected for the purpose of recognition. 

 Acids turn blue litmus red, and we call that a test for acids. Carbon dioxide gas gives a 

 milky appearance to lime-water, which is a test for the gas. Some tests are much more strik- 

 ing than others, indeed, they are so characteristic that they tell at once the nature of the sub- 

 stance tested. Such tests might be called decisive, in distinction to others which are only cor- 

 roborative, and to which several substances may respond. 



