208 PRINCIPLES OF CHEMISTRY 



directly obtained, 111 is by the double decomposition of an acid and the 

 peroxides of certain metals, especially those of potassium, calcium, and 

 barium. 20 Among these peroxides, that of barium is the most 

 conveniently obtained, it being enough, as we saw when speaking of 

 oxygen (Chap. III.), to heat the anhydrous oxide of barium to a red heat 

 in a current of air or oxygen ; or, better still, to heat it with potassium 

 chlorate, and then to wash away the potassium chloride also formed. 21 

 Barium peroxide gives hydrogen peroxide by the action of acids in the 

 cold. 22 The process of decomposition is very clear in this case ; the 

 hydrogen of the acid replaces the barium of the peroxide, a barium salt 

 of the acid being formed, while the hydrogen peroxide formed by the 



19 The formation of hydrogen peroxide from barium peroxide by a method of double 

 decomposition is an instance of a number of indirect methods of prepa/Tafaon. A sub- 

 stance A does not combine with B, but AB is obtained from AC in its action on HP (see 

 Introduction) when CD is formed. Water does not combine with oxygen, but as a hydrate 

 of acids it acts on the compound of oxygen with barium oxide, because this oxide gives a 

 salt with an acid anhydride ; or, what is the same, hydrogen with oxygen does not directly 

 form hydrogen peroxide, but when combined with a haloid (for example, chlorine), under 

 the action of barium peroxide, BaOo, it leads to the formation of a salt of barium and H._,(\>. 

 It is to be remarked that the passage of barium oxide, BaO, into the peroxide, BaO,>, is 

 accompanied by the evolution of 121000 heat units per 16 parts of oxygen by weight 

 combined, and the passage of H.,O into the peroxide H._>O._> does not proceed directly, 

 because it would be accompanied by the absorption of 22000 units of heat by 10 parts 

 by weight of oxygen combined. Barium peroxide, in acting 011 an acid, evidently evolves 

 less heat than the oxide, and it is this difference of heat that is absorbed in the hydrogen 

 peroxide. Its energy is obtained from the energy evolved in the formation of the salt of 

 barium. 



20 Peroxides of lead and manganese, and other analogous peroxides (see Chapter III., 

 Note 9), do not give hydrogen peroxide under these conditions, but yield chlorine 

 with hydrochloric acid. 



21 The impure barium peroxide obtained in this manner may be easily purified. For 

 this purpose it is dissolved in a dilute solution of nitric acid. There will always remain 

 a certain quantity of an insoluble residue, from which the solution is separated by filtra- 

 tion. The solution will contain not only the compound of the barium peroxide, but also 

 a compound of the barium oxide itself, a certain quantity of which always remains un- 

 combined with oxygen. The acid compounds of the peroxide and oxide of barium are 

 easily distinguishable by their stability. The peroxide gives an unstable compound, and 

 the oxide a stable salt. By adding an aqueous solution of barium oxide to the resultant 

 solution, the whole of the peroxide contained in the solution may be precipitated as a 

 pure aqueous compound. The first portions of the precipitate will consist of impurities 

 for instance, oxide of iron. The barium peroxide separates out, and is collected on a 

 filter and washed ; it then forms a substance having an entirely definite composition, 

 BaOo,8H 2 O, and is very pure. Pure hydrogen peroxide should always be prepared from 

 such purified barium peroxide. 



22 In the cold, strong sulphuric acid with barium peroxide gives ozone; when diluted 

 with a certain amount of water it gives oxygen (see Note 6), and hydrogen peroxide is 

 only obtained by the action of very weak sulphuric acid. The acids hydrochloric, 

 hydrofluoric, carbonic, and hydrosilicofluoric, and others, when diluted with water also 

 give hydrogen peroxide with barium peroxide. Professor Scho'ne, who investigated 

 hydrogen peroxide with great detail, showed that it is formed by the action of many of 

 the above-mentioned acids on barium peroxide. 



