As nitrous anhydride easily splits up into NO2 + NO, so, like NO2, with warm water it gives nitric acid and nitric oxide, according to the equation 3N2O3 + H2O = 4NO + 2NHO3.
Being in a lower degree of oxidation than nitric acid, nitrous acid and its anhydride are oxidised in solutions by many oxidising substances—for example, by potassium permanganate—into nitric acid.65
Nitric oxide, NO.—This permanent gas66 (that is, unliquefiable by pressure without the aid of cold) may be obtained from all the above-described compounds of nitrogen with oxygen. The deoxidation of nitric acid by metals is the usual method employed for its preparation. Dilute nitric acid (sp. gr. 1·18, but not stronger, as then N2O3 and NO2 are produced) is poured into a flask containing metallic copper.67 The reaction commences at the ordinary temperature. Mercury and silver also give nitric oxide with nitric acid. In these reactions with metals one portion of the nitric acid is employed in the oxidation of the metal, whilst the other, and by far the greater, portion combines with the metallic oxide so obtained, with formation of the nitrate corresponding with the metal taken. The first action of the copper on the nitric acid is thus expressed by the equation
2NHO3 + 3Cu = H2O + 3CuO + 2NO.
The second reaction consists in the formation of copper nitrate—
6NHO3 + 3CuO = 3H2O + 3Cu(NO3)2.
Nitric oxide is a colourless gas which is only slightly soluble in water ( 1 / 20 of a volume at the ordinary temperature). Reactions of double decomposition in which nitric oxide readily takes part are not known—that is to say, it is an indifferent, not a saline, oxide. Like the other oxides of nitrogen, it is decomposed into its elements at a red heat (starting from 900°, at 1,200° 60 per cent. give N 2 and 2N 2 O 3 , but complete decomposition into N 2 and O 2 only takes place at the melting point of platinum, Emich 1892). The most characteristic property of nitric oxide is its capacity for directly and easily combining with oxygen (owing to the evolution of heat in the combination). With oxygen it forms nitrous anhydride and nitrogen peroxide, 2NO + O = N 2 O 3 , 2NO + O 2 = 2NO 2 . If nitric oxide is