278 . % PRINCIPLES OF CHEMISTRY 



gr. 1'3. At a very low temperature it condenses into a blue liquid boiling 

 below 0, 54 but then partially decomposing into NO + NO 2 . Nitrous an- 

 hydride evinces a remarkable capacity for oxidising. Ignited bodies burn 

 in it, nitric acid absorbs it, and then acquires the property of acting on 

 silver and other metals, even when diluted. Potassium iodide is 

 oxidised by this gas just as it is by ozone (and by peroxide of hydrogen, 

 chromic and other acids, but not by dilute nitric acid nor by sulphuric 

 acid), with the separation of iodine. This iodine may be recognised 

 (see Ozone, Chap. IV.) by its turning starch blue. The smallest traces 

 of nitrites may be easily discovered by this method. If, for example, 

 starch and potassium iodide are added to a solution of potassium 

 nitrite (there will be no change, there being no free nitrous acid), and 

 then sulphuric acid be added, then the nitrous acid (or its anhydride) 

 immediately set free evolves iodine, which communicates a blue colour 

 to the starch. Nitric acid does not act in this manner, but in the 

 presence of zinc the coloration takes place, which proves the formation 

 of nitrous acid in the deoxidation of nitric acid. 55 Nitrous acid (or 

 even a mixture of HNO 2 -l-NO) acts directly on ammonia, forming 

 nitrogen and water, HNO 2 + NH 3 =N 2 + 2H 2 0. 56 



As nitrous anhydride easily splits up into NO 2 + NO, so with warm 

 water it, like NO 2 , gives nitric acid and nitric oxide, according to the 

 equation 3N 2 O 3 + H 2 O=4NO + 2NHO 3 . 



Being in a lower degree of oxidation than nitric acid, nitrous acid 



studied. The brown colour of the vapours of nitrous anhydride probably depends on 

 the presence of NO. 2 . 



If nitrogen peroxide be cooled to 20, and half its weight of water be added to it drop 

 by drop, then the peroxide is decomposed, as we have already said, into nitrous and nitric 

 acids ; the former does not then remain as a hydrate, but straightway passes into the 

 anhydride, and, therefore, if the resultant liquid be slightly warmed vapours of nitrous 

 anhydride, N 2 O 5 , are evolved, and condense into a blue liquid, as Fritzsche showed. 

 This method of preparing nitrous anhydride evidently gives the purest product. 



54 According to Thorpe, N 2 O 3 boils at +18. According to Geuther, at +3'5, and its 

 sp. gr. at = 1-449. 



55 In its oxidising action nitrous anhydride gives nitric oxide, N<jO 5 = 2NO + O. Thus 

 its analogy to ozone becomes still closer, because in ozone it is only one-third of the 

 oxygen that acts in oxidising ; from O-, there is obtained O, which acts as an oxidiser, and 

 common oxygen O 3 . In a physical aspect the affinity between N. 2 O 3 and O 3 is expressed 

 by both substances being of a blue colour when in the liquid state. 



56 This reaction is taken advantage of for converting the amides, NHoR (where R is 

 an element or a complex group) into hydroxides, RHO. In this case NPL>R + NHOo forms 

 2N + H.,O + RHO ; NHo is replaced by HO, the radicle of ammonia by the radicle of water. 

 This reaction is employed for transforming many nitrogenous organic substances having 

 the properties of amides into their corresponding hydroxides. Thus aniline, CgHj'NH.^, 

 which is obtained from nitrobenzene, C 6 H 5 'NO.,, (Note 37), is converted by nitrous anhy- 

 dride into phenol, C 6 H 5 'OH, which occurs in the creosote extracted from coal tar. Thus 

 the H of the benzene is successively replaced by NCX>, NH.,, and HO a method which is 

 suitable for other cases also. 



