Compound definite composition

Because the metal structure is locked by these atoms, the resulting compound is often much harder than the original metal, and some of the compounds are therefore of industrial importance (see under iron). Since there is a definite ratio of holes to atoms, filling of all the holes yields compounds with definite small atom-metal atom ratios in practice, all the holes are not always filled, and compounds of less definite composition non-stoichiometric compounds) are formed. 

It has been assumed that the cineol and phosphoric acid enter into combination in molecular proportions, forming a solid compound of definite composition. Helbing and Passmore have described a method in which the compound is weighed, and assume that the percentage of cineol in the phosphoric compound is 61 1 per cent, based on the formula CjQHjgO. H3PO4. 

Let us begin by looking again at the kinds of evidence we already have for the existence of atoms—the evidence from chemistry. We shall consider, in turn, the definite composition of compounds, the simple weight relations among compounds, and the reacting volumes of gases. Each behavior provides experimental support for the atomic theory. 

Compounds are found to have definite composition, no matter how prepared. For example,... 

The atomic theory provides a ready explanation for the definite composition of chemical compounds. It says that compounds are composed of atoms, and every sample of a given compound must contain the same relative number of atoms of each of its elements. Since the atoms of each element have a characteristic weight, the weight composition of a compound is always the same. Thus, the definite composition of compounds provides experimental support for the atomic theory. 

Chromic hydroxide, Cr(OH)3, is a compound with low solubility in water. It is usually hydrated and does not have the definite composition represented by the formula. It is quite soluble either in strong acid or strong base. 

If you mix sulfur and iron filings in a certain proportion and then heat the mixture, you can see a red glow spread through the mixture. After it cools, the black solid lump which has been produced, even if crushed into a powder, does not dissolve in carbon disulfide and is not attracted by a magnet. The material has a new set of properties it is a compound, called iron(II) sulfide. It has a definite composition, and if, for example, you had mixed more iron with the sulfur originally, some iron(II) sulfide and some leftover iron would have resulted. The extra iron would not have become part of the compound. 

The law of definite composition states that every chemical compound is made up of elements in a definite ratio by mass. 

Finely divided nickel absorbs a large volume of hydrogen at high temperatures. Even at ordinary temperatures, considerable occlusion of hydrogen occurs on to the metal surface and no definite composition of any hydride formed is known. The metal activates molecular hydrogen to its atomic state, contributing to its catalytic action in hydrogenation of unsaturated compounds. 

Tellurium Sulphates.—By heating a mixture of powdered tellurium and concentrated sulphuric acid until a dry residue remained, Berzelius obtained a white, earthy mass, which after analysis he concluded to be a disulphate, Te02.2S03 or Te(S04)2.8 The composition of the product, however, is somewhat uncertain on heating it yields a more definite compound of composition 2Te02..S03, which can be regarded as a basic tellurium sulphate or as a pyrotelluryl sulphate. The latter compound... 

By then the French chemist, Joseph Louis Proust, had discovered that whenever elements form compounds these are always of a very definite composition —- the Law of Definite Composition. Water molecules, for example, always contain the same number of hydrogen and oxygen atoms. And Dalton had found that when two elements combine in different ways they do this in simple proportions — the Law of Multiple Proportions. One atom of carbon and one atom of oxygen make carbon monoxide one atom of carbon and two atoms of oxygen make carbon dioxide. 

Stock - An uncured rubber compound of a definite composition from which a given article is manufactured. 

The anhydrous acetates of the rare earths have recently been prepared. Moeller et al. [355] obtained them for La, Dy, Ho, Er, Yb and Y by the azeotropic distillation of a mixture of hydrated acetates with N,N -dimethylformamide (DMF) and benzene. In the case of Ce, Pr, Nd, Sm, Eu and Gd the same method gave a monosolvated acetate, M(C2Hs02)3 DMF. However, the anhydrous acetates of Ce, Pr, Nd, Sm, Eu and Gd can be prepared [355] by vacuum desolvation of the monosolvated compounds. A direct desolvation of the acetates in vacuum at —150° C was attempted by Witt and Onstott [389] after dissolution of the rare earth oxides in 50 per cent acetic acid, and anhydrous acetates of definite composition were obtained for La, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu and Y. 

Hydrogen chloride gas, like the bromide and iodide, has a curious power of forming solid compounds of definite composition with certain anhydrous salts of oxy-acids, especially the sulphates, phosphates, and phosphites of di- and trivalent (mainly transitional and B) metals.443 Many of these are formed at the ordinary temperature, and do not decompose below 200°, where the HCl compounds lose their HCl, but those ofHBr and HI usually have the anion of the oxy-acid reduced, with liberation of the halogen. These compounds have as many molecules of halogens hydride to one metal atom as the latter has valencies thus salts M" P04 have three molecules, but M SO4, M HP04, and M (HP02)2 only two molecules of the halide to 1 M. 

Within this group, only lead and zirconium have borates of definite composition. The PbO 2B203 4H20 compound is formed by evaporation of acetic acid from lead acetate-boric acid mixtures (206). It dehydrates in three stages the first water molecule is lost at 50-95°C, two more between 130 and 170°C, and the remainder at over 300°C. The Pb0-B203-H20 system has been studied at 50, 75, and 100°C (385, 386), and the compounds detected were 1 2 3 at 50°C 4 5 2.5, 1 2 3, and 3 10 9 at 75°C and 4 5 2.5, 1 2.5 2, and 3 10 9 at 100°C. Postulated structures for some of these borates are outlined in Section III,D. 

Sometimes compounds of definite composition may be formed, such as KC6 or CdBr, but in other cases intercalated phases may be non-stoichiometric such as Li2TiS2 (O < x < 1). Most interaction reactions... 

Solid phases of binary systems, like the liquid phases, are very commonly of variable composition. Here, as with the liquid, the stable range of composition is larger, the more similar the two components are. This of course is quite c-ontrary to the chemists notion of definite chemical composition, definite structural formulas, etc., but those notions are really of extremely limited application. It happens that the solid phases in the system water—ionic compound are often of rather definite composition, and it is largely from this rather special case that the idea of definite compositions in solids has become so firmly rooted. In such a system, there are normally two solid phases ice and the crystalline ionic compound. Ice can take up practically none of any ionic compound, so that it has practically no range of compositions. And many ionic crystals... 

Water and ionic compounds are very different types of substances, and it is not unnatural that they do not form solids of variable composition. The reason why water solutions of ionic substances exist is that the water molecules can rotate so as to be attracted to the ions this is not allowed in the solid, where the ice structure demands a fairly definite orientation of the molecule. But as soon as we think about solid phases of a mixture of similar components, we find that almost all the solid phases exist over quite a range. Such phases are often called by the chemists solid solutions, to distinguish them from chemical compounds. This distinction is valid if we mean by a chemical compound a phase which really exists at only a quite definite composition. But the chemists, and particularly the metallurgists, are not always careful about making this distinction for this reason the notation is misleading, and we shall not often use it. 

Fig. XVII-7.—Phase equilibrium diagram for the system Cu-Mg, in which the two metals are insoluble in each other, forming intermetallie compounds of definite composition.

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