Chemical composition, definition

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... 

Definitions. In addition to showing varying degrees of chemical purity, limestone assumes a number of widely divergent physical forms, including marble, travertine, chalk, calcareous mad, coral, shell, ooHtes, stalagmites, and stalactites. AH these materials are essentially carbonate rocks of the same approximate chemical composition as conventional limestone (2—4). 

Clays (qv) are aluminosihcate minerals, some of which have definite chemical compositions. In regard to tar sands, however, clay is only a size classification and is usually deterrnined by a sedimentation method. According to the previous definition of fines, the fines fraction equals the sum of the silt and clay fractions. The clay fraction over a wide range of fines contents is a relatively constant 30% of the fines. 

Exempt colorants are made up of a wide variety of organic and inorganic compounds representing the animal, vegetable, and mineral kingdoms. Some, like -carotene and 2inc oxide, are essentially pure factory-produced chemicals of definite and known composition. Others, including annatto extract, cochineal extract, caramel, and beet powder are mixtures obtained from natural sources and have somewhat indefinite compositions. 

The presence of tubercles is usually obvious. Friable brown and orange nodular encrustations on mild steel and cast iron cooling water components are almost always tubercles (Figs. 3.12 through 3.14). The presence of a crust, shell, core, cavity, and corroded floor are definitive (Fig. 3.3). Careful analysis can provide considerable information concerning growth, chemical composition, and associated metal loss. 

Ocean sea water is roughly equivalent in strength to a 3 j % w/v solution of sodium chloride, but it has a much more complex composition, embodying a number of major constituents, and traces at least of almost all naturally occurring elements. For convenience, however, the concentration of salts in any sample of sea water is expressed in terms of the chloride content, either as chlorinity or as salinity. Both these units are again subject to arbitrary definition and do not conform simply to the chemical composition. 

The precipitate must be convertible into a pure substance of definite chemical composition this may be effected either by ignition or by a simple chemical operation, such as evaporation, with a suitable liquid. 

Definition.—The heat absorbed in producing a change of physical state or chemical composition of a system, at constant temperature and pressure, is called the latent heat of the given transition, and is measured by the number of calories absorbed during the transition of unit mass of the substance from the initial to the final state. 

Whether any of the formulation variables change the mechanical properties significantly upon aging cannot be answered due to the lack of definition of chemical composition parameters as discussed previously... 

A review is presented here of certification approaches, followed by several of the major agencies and individual developers of RMs for chemical composition, addressing some of the many associated scientific aspects that significantly impinge on the conduct and outcome of the analytical characterization exercises. These include definition of analytical methods selection of analytical methodologies, analysts and laboratories in-house characterization and cooperative inter-laboratory characterization. 

The three historical approaches to certification mentioned above were recently expanded to identily seven modes that are used at NIST for value assignment for chemical composition (May et al. 2000). These seven modes and the resulting values are summarized in Table 3.13. The basic principles of value assignment remain unchanged however, these modes now provide a well-defined link between the process used for value assignment and the definition of the assigned value (i.e. certified, reference, or information value). The terms described above provide a clear indication of the level of confidence that NIST has in the accuracy of the assigned value. The definition of a certified value implies that NIST must be involved in the measurement process for the value to be classified as a NIST certified value (see modes 1-3 in Table 3.13). Thus, modes 4 and 7, which do not involve NIST measure-... 

An ordered distribution of spheres of different sizes always allows a better filling of space the atoms are closer together, and the attractive bonding forces become more effective. As for the structures of other types of compound, we observe the validity of the principle of the most efficient filling of space. A definite order of atoms requires a definite chemical composition. Therefore, metal atoms having different radii preferentially will combine in the solid state with a definite stoichiometric ratio they will form an inter-metallic compound. 

Chemical composition. Only very definite patterns of linking polyhedra are consistent with a given composition. 

In addition to the minerals, there are also some rock-forming homogeneous materials that have neither the definite chemical composition nor the distinctive crystal structure characteristic of minerals. Such materials cannot, therefore, be considered as minerals and are known as mineraloids. Obsidian, for example, a natural material that has been widely used since prehistoric times for making lithic tools and decorative objects, is a mineraloid. Obsidian has neither a definite chemical composition nor a characteristic crystal structure and is not, therefore, a mineral. Copal and amber are other mineraloids that since antiquity have been treasured as semiprecious gemstones. 

Informed debate and decisions on such important matters as the depletion of the ozone layer, acid rain and the quality of waterways all depend on the data provided by analytical chemists. Forensic evidence also often depends on chemical measurements. National and international trade are critically dependent on analytical results. Chemical composition is often the basis for the definition of the nature of goods and tariff classification. In all of these areas not only is it important to get the right answer but it is essential that the user of the results is confident and assured that the data are truly representative of the sample and that the results are defendable, traceable and mutually acceptable by all laboratories. 

The chemical composition of petroleum also varies over a wide range. A broad functional definition of petroleum hydrocarbons is that hydrocarbons are composed primarily of many organic compounds of natural origin and low water solubility. 

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