PERIODIC LAW) as follows: The typical oxides of Group I. are of the form those of Group II. of the form MO, those of Group III. are and so on, those of Group VI. being Types of Oxide.—Oxides are often classed as (I) acidic oxides; (2) basic oxides; (3) amphoteric ; (4) neutral; (5) sub oxides; (6) saline oxides; and (7) peroxides (either acidic or basic) ; an oxide may not fall clearly into any one of these classes, but have the characteristics of two.
(I) Acidic oxides are those which combine with bases or basic oxides to form salts. Many of the oxides of non-metals, e.g., carbon, nitrogen, phosphorus and sulphur, are of this type. Some acidic oxides are "mixed" anhydrides; thus nitrogen peroxide reacts with water to give two acids, nitrous and nitric, and with bases to form nitrites and nitrates.
(2) Basic oxides similarly react with acids or acidic oxides to form salts, and many of the oxides of metals fall into this class.
(3) Amphoteric oxides behave as acidic oxides towards bases and as basic oxides towards acids; thus, zinc and aluminium oxides dissolve in either acids or bases to give salts of the acid or zincates (aluminates) of the base, respectively.
(4) Neutral oxides are those which neither react to form salts nor combine with water to give acids or bases; carbon monoxide and nitrous oxide are examples, for, although they result from formic and hyponitrous acids, respectively, by loss of water, they do not combine with water to give these acids and are therefore not acidic anhydrides.
(5) Suboxides are oxides having less oxygen than those oxides which give rise to a definite series of salts, and in a sense they are neutral oxides, for on reacting with an acid they give the metal and a salt corresponding to a higher oxide. Few suboxides are definitely known, but that of lead, exhibits the behaviour just described—there are no such salts as or Pbel, and the action of acids upon it results in the production of metallic lead and a salt derived from PbO.
(6) Saline oxides of non-metals are not known. As indicated above, some metals form two oxides of which the lower is basic and the higher acidic ; two such oxides may therefore combine to give another oxide which is to some extent a salt. Many higher oxides of metals may thus be regarded as saline, e.g., 2 (7) Peroxides are, strictly, those oxides having more oxygen than the typical oxide as determined by the periodic classification.
This definition, however, fails in certain cases, for it would exclude lead peroxide, from the category, since this is regarded as typical of Group IV. to which lead belongs, whereas the oxide is certainly a peroxide. It is therefore customary, in the case of basic peroxides, to regard them as being those oxides which are more highly oxygenated than the oxide which gives rise to a regular series of salts, and since, in the case of lead, this is the oxide PbO, the oxide is a peroxide. In the case of acidic peroxides, permanganic anhydride Mn207, for instance, is the typical oxide of Group VII. and also gives rise (as an acid) to a definite series of salts (the permanganates), so no difficulty arises; and oxides such as and which are higher than the typical oxides and are acidic peroxides and give rise to perchromates and persulphates.
It is convenient to subdivide basic peroxides according to their chemical behaviour: those which react with water or with dilute acids to form hydrogen peroxide (a neutral peroxide)are classed as superoxides, and the others as polyoxides. Mendeleyev, to whom this idea is due, pointed out that this subdivision could be re lated to structural formulae, for the former class includes oxides in which the oxygen atoms are linked together, e.g., whereas in the latter class these atoms are all attached solely to the metal, e.g., 0=Mn=0. Such a subdivision can be extended to distinguish between acidic superoxides and poly oxides; thus, is a superoxide (compare above).
Peroxides are characterised by their oxidising properties; thus lead peroxide oxidises so that the resulting Pb0 and form lead sulphate and manganese dioxide oxidises hydro chloric acid to chlorine, being itself reduced to the manganous state : Mn02-1-4HC1= MnC124-C12+2H20.
Many oxides combine with water to form hydroxides (q.v.), and all hydroxides lose water on heating to give the corresponding oxide. The chemical properties of an hydroxide are almost identi cal with those of the corresponding oxide, but the latter is some what more inert especially if it has been very strongly heated. For further details see HYDROXIDE; OXIDATION and RE DUCTION; and H. B. Weiser, The Hydrous Oxides (New York, 1926.) (A. D. M.)