Thermodynamics

cen84959_ch03.qxd 4/25/05 2:47 PM Page 112112 | ThermodynamicsN 2AIRFIGURE 3–1Nitrogen and gaseous air are puresubstances.VAPORLIQUID(a) H 2 OINTERACTIVETUTORIALSEE TUTORIAL CH. 3, SEC. 1 ON THE DVD.VAPORLIQUID(b) AIRFIGURE 3–2A mixture of liquid and gaseous wateris a pure substance, but a mixture ofliquid and gaseous air is not.INTERACTIVETUTORIALSEE TUTORIAL CH. 3, SEC. 2 ON THE DVD.FIGURE 3–3The molecules in a solid are kept attheir positions by the large springlikeintermolecular forces.3–1 ■ PURE SUBSTANCEA substance that has a fixed chemical composition throughout is called apure 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 orcompound, however. A mixture of various chemical elements or compoundsalso 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 tobe 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, formingtwo chemically dissimilar regions.A mixture of two or more phases of a pure substance is still a pure substanceas 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 substancebecause both phases have the same chemical composition. A mixtureof liquid air and gaseous air, however, is not a pure substance since thecomposition of liquid air is different from the composition of gaseous air,and thus the mixture is no longer chemically homogeneous. This is dueto different components in air condensing at different temperatures at aspecified pressure.3–2 ■ PHASES OF A PURE SUBSTANCEWe all know from experience that substances exist in different phases. Atroom temperature and pressure, copper is a solid, mercury is a liquid, andnitrogen is a gas. Under different conditions, each may appear in a differentphase. Even though there are three principal phases—solid, liquid, andgas—a substance may have several phases within a principal phase, eachwith a different molecular structure. Carbon, for example, may exist asgraphite or diamond in the solid phase. Helium has two liquid phases; ironhas three solid phases. Ice may exist at seven different phases at high pressures.A phase is identified as having a distinct molecular arrangement thatis homogeneous throughout and separated from the others by easily identifiableboundary surfaces. The two phases of H 2 O in iced water represent agood example of this.When studying phases or phase changes in thermodynamics, one does notneed to be concerned with the molecular structure and behavior of differentphases. However, it is very helpful to have some understanding of the molecularphenomena involved in each phase, and a brief discussion of phasetransformations follows.Intermolecular bonds are strongest in solids and weakest in gases. Onereason is that molecules in solids are closely packed together, whereas ingases they are separated by relatively large distances.The molecules in a solid are arranged in a three-dimensional pattern (lattice)that is repeated throughout (Fig. 3–3). Because of the small distancesbetween molecules in a solid, the attractive forces of molecules on eachother are large and keep the molecules at fixed positions (Fig. 3–4). Notethat the attractive forces between molecules turn to repulsive forces as the