In the univalent oxides, 2 equivalents of H are replaced by 2 univalent atoms. Thus, H2O=water. Ag2O= argentic oxide. In the bivalent oxides, 2 equivalents of H are replaced by I bivalent atom. Thus, In the trivalent oxides, 6 equivalents of H are replaced by 2 trivalent atoms, which bind together 3 molecules of H2O. Thus, H6O3 = 3H2O. = auric oxide. Al2O3, aluminic oxide, Fe2O3, ferric oxide, Cr2O3, chromic oxide, Mn2O, manganic oxide, and U2O3, uranic oxide, are formed in the same way as the trivalent oxides; 2 of the metallic equivalents in each case being neutralized by the metal itself, as in their chlorides and hydrates. All these metals, except Al, form regular bivalent oxides, whose names take the ending -ous. Hg, besides its bivalent oxide, HgO, forms a mercurous oxide, Hg2O. 12. Acids. The oxyacids belong to the water type, and may be regarded as formed by replacing a part of the H of one or more molecules of H2O by an equivalent of a non-metallic element or radical. Nitrous acid is formed by replacing 3 atoms of H in 2 molecules of H2O by 1 of the trivalent N. Thus, — HH3O2=2H2O. HNO2 = nitrous acid. Nitric acid may be formed by replacing 1 atom of H in I molecule of H2O by the univalent radical NO2. Thus, HHO= H2O. HNO2O= HNO3 (nitric acid). In chlorous acid, 1 atom of H is replaced by the univalent radical CIO. Thus, HHO=H2O. If the atom of H be replaced by the univalent CIO2, we get HCIO, chloric acid. When 2 molecules of water are soldered together by the bivalent SO, we get H2SOO2 or H2SO, sulphurous acid. If the bivalent SO2 solders the 2 molecules of H2O, we get H2SO2O2 or H2SO4, sulphuric acid. When 3 molecules of H2O are soldered by the trivalent P or As, we get H¿PO, phosphorous acid, and H.AsО, arsenious acid. If the 3 molecules of H2O are soldered by the trivalent PO or AsO, we get HPO4, phosphoric acid, or H ̧AsO4, arsenic acid. Boracic acid, HBO2, is formed by replacing 3 atoms of H in 2 molecules of H2O by the trivalent B. Silicic acid, H4SiO4, is formed by replacing 4 atoms of H in 4 molecules of H2O by the quadrivalent Si. Carbonic acid, H2CO3, is formed by replacing 2 atoms of H in 2 molecules of H2O by the bivalent CO. 13. Anhydrides. The anhydrides belong to the water type, and are formed by replacing all the H of one or more molecules of H2O by an equivalent of a non-metallic element or radical. Thus, 14. Salts. When the H of a base is replaced by a nonmetallic element or radical, or the H of an acid by a metallic element or radical, a salt is formed. Thus, KHO gives KCIO (potassic hypochlorite). CaH2O2 gives CaSO2O2 = CaSO4 (calcic sulphate). Again, The nitrates of the metals are analogous to the chlorides, with NO in place of the Cl. The sulphates are analogous to the oxides, with SO4 in place of the O. Thus, The chlorates are like the nitrates; and the carbonates like the sulphates, with CO in place of SO4. The sulphides are like the oxides. 15. Substitution. "When cotton-wool is dipped in strong nitric acid (rendered still more active by being mixed with twice its volume of concentrated sulphuric acid), and afterwards washed and dried, it is rendered highly explosive; and, although no important change has taken place in its outward aspect, it is found on analysis to have lost a certain amount of hydrogen, and to have gained from the nitric acid an equivalent amount of nitric peroxide, NO2, in its place.” C12H20O10 (cotton) becomes C12H14 (NO2)6O10 (gun-cotton). Under the same conditions, glycerine undergoes a like change, and is converted into the explosive nitro-glycerine. C&H16O6 (glycerine) becomes C&H10 (NO2)&O (nitro-glycerine). So also the hydro-carbon, naphtha, called benzole, is changed into nitro-benzole. C&H (benzole) becomes C6H5(NO2) (nitro-benzole). "The last compound is not explosive, and the explosive nature of the first two is, in a measure, an accidental quality, and is evidently owing to the fact, that into an already complex structure there have been introduced, in place of the indivisible atoms of hydrogen, the atoms of a highly unstable radical, rich in oxygen. The point of chief interest for our chemical theory is that this substitution does not alter, at least profoundly, the outward aspect of the original compound. Every one knows how closely gun-cotton resembles cotton-wool. In like manner, nitro-glycerine is an oily liquid, like glycerine; and nitrobenzole, although darker in color, is a highly aromatic, volatile fluid, like benzole itself. Products like these are called substitution products; and they certainly suggest the idea that each chemical compound has a certain definite structure, which may be preserved even when the materials of which it is built are, in part, at least, changed. If, in the place of firm iron girders, we insert weak wooden beams, a building, while retaining all its outward aspects, may be rendered wholly insecure. And so the explosive nature of the products we have been considering is not at all incompatible with a close resemblance, in outward aspects and internal structure, to the compounds from which they are derived." 16. Isomorphism. -"Closely associated with the facts of the last section, which find their chief manifestation in substances of organic origin, are the phenomena of isomorphism, which are equally conspicuous among artificial salts and native minerals. There seems to be an intimate connection between chemical composition and crystalline form, and two substances which, under a like form, have an analogous composition, are said to be isomorphous." The following minerals are isomorphous : CaCO3 Calcite, or calcic carbonate, Diallogite, or manganous carbonate, MnCO "The most cursory examination of these symbols will show that they differ from each other only in the fact that one metallic atom has been replaced by another. It is not, however, every metallic atom which can thus be put in without altering the form. This is a peculiarity which is confined to certain groups of elements, which for this reason are called groups of isomorphous elements. Moreover, as a rule, there is a close resemblance between the members of any one of these groups in all their other chemical relations. These facts, like those of the last section, tend to show that the molecules of every substance have a determinate structure, which admits of a limited substitution of parts without undergoing essential change, but which is either destroyed or takes a new shape when, in place of one of its constituents, we force in an unconformable element. A well-known class of artificial salts called the alums affords even a more striking illustration of the principles of isomorphism than the simpler examples we have chosen." "We should 17. Isomerism, Allotropism, and Polymorphism. infer from the doctrine of chemical types that the same atoms might be grouped together in different ways, so as to form different molecules, which, in their aggregation, would present essentially distinct qualities. Hence we should expect to find distinct substances having the same composition, and, in fact, our science, organic chemistry especially, is rich in examples of this kind. Such substances are said to be isomeric, and the phenomenon is called isomerism." Thus, common sugar and gum arabig have exactly the same composition, C12H22O11, and are isomeric. When, however, the differences are not sufficiently great to justify a distinct name, the two bodies are said to be different allotropic states of the same substance, and the phenomenon is called allotropism. Thus, there are three varieties of tartaric acid which differ in their action on polarized light, but which are in almost every other respect identical. Sometimes a substance crystallizes in fundamentally different forms, as calcic carbonate in the minerals calcite and aragonite. This phenomenon is called polymorphism, and is invariably accompanied by a marked difference of properties. |