cen84959_ch03

cen84959_ch03 - Chapter 3 PROPERTIES OF PURE SUBSTANCES e...

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Chapter 3 PROPERTIES OF PURE SUBSTANCES | 111 W e start this chapter with the introduction of the con- cept of a pure substance and a discussion of the physics of phase-change processes. We then illus- trate the various property diagrams and P- v -T surfaces of pure substances. After demonstrating the use of the property tables, the hypothetical substance ideal gas and the ideal-gas equation of state are discussed. The compressibility factor , which accounts for the deviation of real gases from ideal-gas behavior, is introduced, and some of the best-known equa- tions of state such as the van der Waals, Beattie-Bridgeman, and Benedict-Webb-Rubin equations are presented. Objectives The objectives of Chapter 3 are to: Introduce the concept of a pure substance. Discuss the physics of phase-change processes. Illustrate the P - v , T - v , and P - T property diagrams and P - v - T surfaces of pure substances. Demonstrate the procedures for determining thermodynamic properties of pure substances from tables of property data. Describe the hypothetical substance “ideal gas” and the ideal-gas equation of state. Apply the ideal-gas equation of state in the solution of typical problems. Introduce the compressibility factor, which accounts for the deviation of real gases from ideal-gas behavior. Present some of the best-known equations of state.

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3–1 PURE SUBSTANCE A substance that has a fixed chemical composition throughout is called a pure substance . Water, nitrogen, helium, and carbon dioxide, for example, are all pure substances. A pure substance does not have to be of a single chemical element or compound, however. A mixture of various chemical elements or compounds also qualifies as a pure substance as long as the mixture is homogeneous. Air, for example, is a mixture of several gases, but it is often considered to be a pure substance because it has a uniform chemical composition (Fig. 3–1). However, a mixture of oil and water is not a pure substance. Since oil is not soluble in water, it will collect on top of the water, forming two chemically dissimilar regions. A mixture of two or more phases of a pure substance is still a pure sub- stance as long as the chemical composition of all phases is the same (Fig. 3–2). A mixture of ice and liquid water, for example, is a pure sub- stance because both phases have the same chemical composition. A mixture of liquid air and gaseous air, however, is not a pure substance since the composition of liquid air is different from the composition of gaseous air, and thus the mixture is no longer chemically homogeneous. This is due to different components in air condensing at different temperatures at a specified pressure. 3–2 PHASES OF A PURE SUBSTANCE We all know from experience that substances exist in different phases. At room temperature and pressure, copper is a solid, mercury is a liquid, and nitrogen is a gas. Under different conditions, each may appear in a different phase. Even though there are three principal phases—solid, liquid, and
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