154 NON-METALS AND THEIR COMBINATIONS. 



In no case is it possible to obtain, as might appear from the 

 above equation, pure hydrogen dioxide directly, as a considerable 

 quantity of water has to be present in order to effect the de- 

 composition. The aqueous solution, if quite pure, can be concen- 

 trated by evaporation at a temperature not exceeding 60 C. (140 

 F.) until it has a strength of 50 per cent. If this be further heated 

 in vacuo at a gradually increased temperature, a nearly pure hydro- 

 gen dioxide distils over at a temperature of 85 C. (185 F.). 



Pure hydrogen dioxide is a colorless, oily liquid, of a specific 

 gravity 1.45. It is soluble in water, alcohol, and ether, which latter 

 extracts it from its aqueous solutions. 



Hydrogen dioxide decomposes slowly at ordinary temperature, 

 more rapidly on exposure to light and at higher temperatures ; at 

 100 C. the decomposition is often explosively rapid. Many inert 

 subtances, in powder, cause its decomposition, and it is for this 

 reason that even dust particles from the air act decomposingly, 

 especially during evaporation. The presence of very small quanti- 

 ties of certain substances retards the decomposition. Traces of free 

 acids, as also boro-glycerin, have been used for this purpose. It has 

 been found that a very small quantity of acetanilide is an excellent 

 preservative and it is now added to commercial hydrogen dioxide 

 solution. 



Catalytic action. There are a number of instances in chemistry 

 where a substance, which apparently undergoes no change itself, 

 causes by its presence an increase in chemical change in other sub- 

 stances, or induces a change where, without it, there would be no 

 chemical action. This is known as catalytic or contact action and 

 the process is called catalysis. Examples of this action are seen in 

 the decomposition of hydrogen dioxide by dust particles, or finely 

 divided platinum, the influence of manganese dioxide on potassium 

 chlorate in the preparation of oxygen, and the explosion of a mixture 

 of hydrogen and oxygen when platinum black is introduced into it. 



Hydrogen dioxide possesses bleaching, caustic, and antiseptic 

 properties. It is used as a bleaching agent for hair, wool, teeth, and 

 other articles, and as an antiseptic in surgical and in dental opera- 

 tions. It effervesces with pus, as also with saliva, in consequence of 

 the liberation of oxygen. 



Solution of hydrogen dioxide, Aqua hydrogenii dioxidi, should 

 contain about 3 per cent, by weight, of pure dioxide, corresponding 

 to about 10 volumes of available oxygen in 1 volume of the solution. 



