Substances, Compounds, and Elements

A mixture can be separated by physical means because each component retains its properties (Eigures 1-11 and 1-12). Eor example, a mixture of salt and water can be separated by evaporating the water and leaving the solid salt behind. To separate a mixture of sand and salt, we could treat it with water to dissolve the salt, collect the sand by filtration, and then evaporate the water to reclaim the solid salt. Very fine iron powder can be mixed with powdered sulfur to give what appears to the naked eye to be a homogeneous mixture of the two. Separation of the components of this mixture is easy, however. The iron may be removed by a magnet (see Eigure 1-11), or the sulfur may be dissolved in carbon disulfide, which does not dissolve iron. 

A T-bone steak is a heterogeneous mixture of white fat, bone, and red meat. Each of these macroscopic items is in turn heterngenecus. For example, the meat is composed of blood vessels, protein structures, fine tendons, etc. 

0 Like any mixture, it can be separated by physical means, such as removing the iron with a magnet. 

In any mixture, (1) the composition can be varied and (2) each component of the mixture retains its own properties. 

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Recognize various forms of matter homogeneous and heterogeneous mixtures, substances, compounds, and elements... 

Enthalpies of reaction can also be calculated from individual enthalpies of formation (or heats of formation), AHf, for the reactants and products. Because the temperature, pressure, and state of the substance will cause these enthalpies to vary, it is common to use a standard state convention. For gases, the standard state is 1 atm pressure. For a substance in an aqueous solution, the standard state is 1 molar concentration. And for a pure substance (compound or element), the standard state is the most stable form at 1 atm pressure and 25°C. A degree symbol to the right of the H indicates a standard state, AH°. The standard enthalpy of formation of a substance (AHf) is the change in enthalpy when 1 mol of the substance is formed from its elements when all substances are in their standard states. These values are then tabulated and can be used in determining A//°rxn. 

Very limited experimental data for critical constants and acentric factor are available for inorganic compounds and elements which are solids at room temperature. Thus, the estimates for these substances should be considered rough approximations in the absence of experimental data. 

This is his third volume to be entitled Physical Properties of Chemical Compounds. The earlier volumes appeared as Numbers 15 and 22 of the Advances in Chemistry Series. The present work includes data on the physical properties of 434 aliphatic compounds and 22 miscellaneous compounds and elements. Of these, 22 are tables of improved values for compounds included in volume two (Advances No. 22). This brings the total number of substances treated up to 1421. This volume also includes a combined index to the tables in all three volumes. 

This entry has dealt with Iwo types of chemical composition—elements and compounds. Many materials, including the great majority of those found in nature, are mixtures of compounds and often elements. Practically all biochemical materials and rocks are complex mixtures. Obviously the first step in the determination of the composition of such substances is their separation into the individual compounds, and elements if any, which they contain. 

HOMOGENEOUS, t Latin, "the same kind" l. This term, in its strict sense, describes the chemical constitution ol a compound or element. A compound is homogeneous since it is composed ol one and only one group of atoms represented by a lornnilu. For example, pure waier is homogeneous because ii contains no other substance than is indicated by its formula. tUO Homogeneity is a characteristic property of compounds and elements (collectively culled substances) as opposed to mixtures. The term is often loosely used to describe a mixture or solution composed of two or... 

There are unlimited combinations of compounds and elements that result in glaze formations. The elimination or addition of one substance can radically change the product. In Activity 4.3, a glaze and variations of the glaze will be prepared. Then the prepared glazes will be applied to bisque ware clay slabs, which will be fired. The results obtained from the different glazes will be compared. 

In section 3.4, you learned how to name ionic and covalent compounds. You also learned how to write their formulas. In Chapter 4, you will learn how compounds and elements interact in nature, in the laboratory, and in everyday life. These interactions are responsible for the tremendous variety of substances and materials found on Earth. 

Alchemical compounds and elements were were also associated with other aspects of occult knowledge, such as astrology. The most basic substances were directly linked in western alchemy with celestial objects. 

As the temperature of a substance increases, the particles vibrate more vigorously, so the entropy increases (Figure 15-14). Further heat input causes either increased temperature (still higher entropy) or phase transitions (melting, sublimation, or boiling) that also result in higher entropy. The entropy of a substance at any condition is its absolute entropy, also called standard molar entropy. Consider the absolute entropies at 298 K listed in Table 15-5. At 298 K, any substance is more disordered than if it were in a perfect crystalline state at absolute zero, so tabulated values for compounds and elements are always positive. Notice especially that g of an element, unlike its A// , is not equal to zero. The reference state for absolute entropy is specified by the Third Law of Ther-... 

You might be wondering why the calculation of AH from AHf" values by this method works. The answer lies in the definition of standard enthalpy of formation, which requires AHf" values for elements in their reference states to be zero. Although the standard enthalpies (i.e. energies) of compounds and elements cannot be measured or calculated, the difference between the standard enthalpies of two substances is equal to the difference in the corresponding standard enthalpies of formation. This is the reason why equation (13.15) strongly resembles equation (13.2). 

Product(s) In a chemical reaction, substances (elements and/or compounds) called reactants are changed into other substances (compounds and/or elements) called products, the final form taken by the major reactant(s) of a reaction. Product(s) are formed during chemical reactions as reagents are consumed. Products have lower energy than the reagents and are produced during the reaction according to the second law of thermodynamics. 

So what are compounds and elements Just more of the anatomy of matter. Matter is made up of either pure substances or mixtures of pure substances, and substances themselves are made up of either elements or compounds. (Chapter 2 dissects the anatomy of matter. And, as with all matters of dissection, it s best to be prepared — with a nose plug and an empty stomach.)... 

Many of the metals that are most useful to us are not especially abundant in that portion of the lithosphere to which we have ready access. Consequently, the ocourence and distribution of concentrated deposits of these elements often play a role in international politics as nations compete for access to these materials. Deposits that contain metals in economically exploitable quantities are known as ores. UsueiUy, the compounds or elements that we desire must be separated from a large quantity of unwanted material and then chemically processed to make them useful. About 2.3 X 10 kg (25 tons) of materials are extracted from the lithosphere and processed annually to support each person in our country. Because the richest sources of many substances are becoming exhausted, it may be necessary in the future to process larger volumes of lower-quality raw materials. Consequently, extraction of the compounds and elements we need could cost more in both energy and environmental impact. 

Some 10 years ago, the U.S. Environmental Protection Agency (EPA) focussed on 129 substances and elements from 65 pollutant classes. These compounds and elements are frequently referred to as priority pollutants 114 organic compounds are included and the breakdown into nine general groups is presented in Table 1. 

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