Oxide, in chemistry, a binary compound of oxygen and other elements. In general, oxides are the most important compounds with which the chemist has to deal, a study of their composition and properties permitting a valuable comparative investigation of the elements. It is possible to bring about the direct combination of oxygen with most of the elements (the presence of traces of water vapour is generally necessary according to the researches of H. B. Baker), and when this is not so, indirect methods are available, except with bromine and fluorine (and also with the so-called inert gases - argon, helium, &c.), which so far have yielded no oxides. Most of the elements combine with oxygen in several proportions, for example nitrogen has five oxides: N 2 0, NO, N 2 0 3, NO 2, N205; for classificatory purposes, however, it is advantageous to assign a typical oxide to each element, which, in general, is the highest having a basic or acid character. Thus in Group I. of the periodic system, the typical oxide is M 2 O, of Group II. MO, of Group III. M 2 O 3, of Group IV. MO 2, of Group V. M 2 O 5, of Group VI. M03.
Five species of oxides may be distinguished: (I) basic oxides, (2) acidic oxides, (3) neutral oxides, (4) peroxides, (5) mixed anhydrides and salts. Basic oxides combine with acids or acidic oxides to form salts; similarly acidic oxides combine with basic oxides to form salts also. The former are more usually yielded by the metals (some metals, however, form oxides belonging to the other groups), whilst the latter are usually associated with the non-metals. An oxide may be both acidic and basic, i.e. combine with bases as well as acids; this is the case with elements occurring at the transi tion between basigenic and oxygenic elements in the periodic classification, e.g. aluminium and zinc. Neutral oxides combine neither with acids nor bases to give salts nor with water to give a base or acid. A typical member is nitric oxide; carbon monoxide and nitrous oxide may also be put in this class, but it must be remembered that these oxides may be regarded, in some measure at least, as the anhydrides of formic and hyponitrous acid, although, at the same time, it is impossible to obtain these acids by simple hydration of these oxides. Peroxides may in most cases be defined as oxides containing more oxygen than the typical oxide. The failure of this definition is seen in the case of lead dioxide, which is certainly a peroxide in properties, but it is also the typical oxide of Group IV. to which lead belongs. All peroxides have oxidizing properties. Peroxides may be basic or acidic. Some basic oxides yield hydrogen peroxide with acids, others yield oxygen (these also liberate chlorine from hydrochloric acid), and may combine with lower acidic oxides to form salts of the normal basic oxide with the higher acidic oxide. Examples are Ba02-1-H2S04=BaS04+H202; 2Mn02-I-2H2S04=2MnS04-1-2H20+02; Mn02-{-4HC1=MnC12+ 2H 2 0 +C1 2; Pb02+S02=PbS04 (i.e. Pb0+503)Two species of basic peroxides may be distinguished: (I) the superoxides or peroxidates, containing the oxygen atoms in a chain, e.g. Na 0 0 Na, Bab, which yield hydrogen peroxide with acids; and (2) the polyoxides, having the oxygen atoms doubly linked to the metallic atom, e.g. 0: M n: 0,0: Pb: O, and giving oxygen with sulphuric acid, and chlorine with hydrochloric. L. Marino (Zeit. anorg. Chem., 1907, 5 6, p. 2 33) pointed out that manganese and lead dioxide behaved differently with sulphur dioxide, the former giving dithionate and the latter sulphate, and suggested the following formulae: O:Mn :0, OPb: as explaining this difference. A simpler explanation is that the manganese dioxide first gives a normal sulphite which rearranges to dithionate, thus: Mn0 2 +2S0 2 = Mn(S03)2-MnS206, whilst the lead dioxide gives a basic sulphite which rearranges to sulphate, thus: PbO-l-S0 2 = PbOS03 - > PbS04. Acidic peroxides combine with basic oxides to form "per" salts, and by loss of oxygen yield the acidic oxide typical of the element. Mixed anhydrides are oxides, which yield with water two acids, or are salts composed of a basic and acidic oxide of the same metal. Examples of mixed anhydrides are C10 2 and N02, which give chlorous and chloric acid, and nitrous and nitric acid: 2C102+ H20=HC102+HC103, 2N02+H20=HN02+HN03; and of mixed salts Pb203 and Pb304, which may be regarded as lead metaand ortho-plumbate: Pb0Pb02, 2PbOPb02.
Oxidation and Reduction
In the narrow sense "oxidation" may be regarded as the combination of a substance with oxygen, and conversely, "reduction" as the abstraction of oxygen; in the wider sense oxidation includes not merely the addition of oxygen, but also of other electro-negative elements or groups, or the removal of hydrogen or an electro-positive element or group. In inorganic chemistry oxidation is associated in many cases with an increase in the active valency. Ignoring processes of oxidation or reduction simply brought about by heat or some other form of energy, we may regard an oxidizing agent as a substance having a strong affinity for electro-positive atoms or groups, and a reducing agent as having a strong affinity for electro-negative atoms or groups; in the actual processes the oxidizing agent suffers reduction and the reducing agent oxidation.
Many substances undergo simultaneous oxidation and reduction when treated in a particular manner; this is known as selfor auto-oxidation. For example, on boiling an aqueous solution of a hypochlorite, a chlorate and a chloride results, part of the original salt being oxidized and part reduced: 3NaOC1= NaC103-2NaC1. Similarly phosphorous and hypophosphorous acids give phosphoric acid and phosphene, whilst nitrous acid gives nitric acid and nitric oxide: 4H3P03=3H3P04+PH3; 2H 3 PO 2 =H 3 PO 4 +PH 3 i 3HN02= HNO 3 +2NO--H20. In organic chemistry, a celebrated example is Cannizzaro's reaction wherein an aromatic aldehyde gives an acid and an alcohol: 2C E H 5 CHO+H 2 O =C6H5C02H+C6H5CH20H.
The important oxidizing agents include: oxygen, ozone, peroxides, the halogens chlorine and bromine, oxyacids such as nitric and those of chlorine, bromine and iodine, and also chromic and permanganic acid. The important reducing agents include hydrogen, hydrides such as those of iodine, sulphur, phosphorus, &c., carbon, many metals, potassium, sodium, aluminium, magnesium, &c., salts of lower oxyacids, lower salts of metals and lower oxides.
