480 CHEMISTRY [OXYGEN. Oxygen is the most abundant and the most important of all the elements. About one-fifth of the atmosphere consists of free oxygen ; it is the chief constituent by weight of water ; it is an important constituent of all animal and vegetable substances, and is contained in greater or less proportion in most mineral substances. Oxygen may be obtained from water in the manner al ready mentioned under hydrogen, by decomposing it by an electric current, and also by transmitting chlorine gas and steam through a porcelain tube heated to bright red ness 2C1 2 Chlorine. 20H 2 Water. = 0, + 4HC1; Oxygen. Hydrochloric acid. the oxygen is freed from hydrochloric acid and any ex cess of chlorine by passing it through a solution of sodium hydroxide. The most interesting method of obtaining oxygen, al though it is not a usual method of preparing it, is that by which it was first isolated by Priestley. When mercury is heated in contact with air to a temperature just below its boiling point, it gradually becomes covered with a red scale of mercuric oxide, HgO, and when this red scale is exposed to a considerably higher temperature it is broken up itrto oxygen and metallic mercury. The oxides of gold, platinum, and other metals which have but a slight affinity for oxygen, are very readily decomposed when heated, oxygen being evolved and the metal remaining. Many other metallic oxides, and espe cially those which may be regarded as formed on the type of hydrogen dioxide, which readily breaks up into water and oxygen, when more or less strongly heated, are re solved into oxygen and a lower oxide : lead dioxide, PbO 2 , barium dioxide, BaO 2 , and manganese dioxide, MnO 2 , for example. In the case of manganese dioxide, 3Mn0 2 give Mn 3 O 4 , the decomposition taking place at a red heat. Barium dioxide when strongly heated gives up one-half of its oxygen to produce barium monoxide, 2BaO :! = O a 4- 2BaO; and by passing moist air over less heated barium mon oxide it may be reconverted into the dioxide, which may be decomposed by a stronger heat. By repetitions of these processes it is possible to procure large quantities of oxy gen with the aid of a small quantity of barium dioxide, and it has been proposed to employ this method for the preparation of oxygen on the large scale, but in practice there have been found difficulties attending its use. Ordinarily when pure oxygen is required it is prepared by heating potassium chlorate, which ultimately furnishes potassium chloride and oxygen: 2KC1O 3 = 3O 2 + 2KC1. This decomposition requires a high temperature, and can only be effected in vessels of hard glass, but when the chlorate is mixed with about one-eighth of its weight of a metallic oxide, such as copper oxide, ferric oxide, or manganese dioxide, the oxygen is given off at a consider ably lower temperature, and with great rapidity ; in this case, however, it is impure, being always contaminated with small quantities of chlorine. When potassium chlorate is heated alone the salt fuses, but after a considerable evolution of oxygen has taken place the fused mass becomes pasty, and on examination it is found to consist of a mixture of potassium chloride and potassium perchlorate, KC10 4 , so that the first stage in the decomposition may be represented by the equation 2KC10 3 = O, + KC10 4 + KC1; Potassium chlorate. Oxygen. Potassium perchlorate. Potassium chloride. the potassium perchlorate is resolved on further heating into oxygen and potassium chloride. When a metallic oxide is mixed with the chloride, however, and heat applied, the latter does not fuse, and the formation of potassium perchlorate cannot be detected at any stage of the decom position. It is difficult to explain the manner in which the metallic oxide acts in promoting the decompo sition of the chlorate, since it is found to be un changed at the conclusion of the reaction; but it is a well-known fact that many bodies which, under ordinary circumstances, do not yield oxygen, readily part with this element when another substance having a tendency to combine or enter into reaction with oxygen is introduced into the sphere of action. Thus, when potassium dichro- mate is heated with concentrated sulphuric acid, oxygen is evolved, the yellow solution becoming green owing to the formation of chromium sulphate 30 2 Oxygen. 2K 2 O 2 O 7 Potassium dichromate. 4- 2Cr.,(S0 4 ) 3 Chromiun, sulphate. 10H 2 SO 4 Sulphuric acid. 80H 2 Water. But if the dichromate is dissolved in a considerable quan tity of water no change of this kind takes place even after prolonged heating ; when, however, a body like sulphur ous acid, H 2 SO 3 , which has a tendency to unite with oxy gen to form sulphuric acid, is added to the solution, it at once becomes green, the dichromate being decomposed as represented in the above equation, and the oxygen being fixed by the sulphurous acid. Hence it is not improbable that the metallic oxide acts in virtue of a tendency to form a higher oxide, which higher oxide is no sooner produced, however, than it is broken up into oxygen and the lower oxide. The lower oxide may again undergo conversion into the higher oxide and the latter be broken up into oxygen and lower oxide, so that a relatively small quantity of the metallic oxide may suffice to promote the decomposition of a relatively large quantity of chlorate. When a small quantity of cobalt sesquioxide, or a few drops of a solution of a cobalt salt, is added to a clear con centrated aqueous solution of bleaching powder, which is then gently heated, oxygen is evolved with great regularity, and the bleaching powder is completely resolved into oxy gen and calcium chloride 2CaCl 2 O = O 2 + 2CaCl 2 . Bleaching powder. Oxygen. Calcium chloride. The bleaching powder solution is not decomposed in this manner when heated alone, and there is no doubt that the cobalt oxide induces the decomposition by acting as a carrier of oxygen ; that is to say, it first takes oxygen away and passes to a higher state of oxidation, and then gives it up again, the higher oxide being an extremely unstable body. When silver oxide is added to a solution of hydrogen dioxide in water, metallic silver, water, and oxygen are pro duced Ag 2 O Silver oxide H 2 O 2 = Hydrogen dioxide. O 2 Oxygen. 2Ag Silver. OH 2 . Water. This reaction is especially interesting, as it affords experi mental evidence in favour of the assumption that the mole cule of oxygen consists of two atoms ; moreover, a very considerable quantity of heat is developed in the reaction, and since there is every reason for believing that the separation of an atom of oxygen from silver oxide, and of an atom of oxygen from hydrogen dioxide, are both opera tions which involve an expenditure of energy, there can be little doubt that the heat developed is due to the com bination of the atoms of oxygen to form molecules. In other words, this reaction affords evidence that oxygen atoms have a powerful affinity for each other. The green leaves of plants with the aid of sunlight are enabled to decompose water and carbon dioxide, and
evolve oxygen from them.