Thermodynamics

cen84959_ch03.qxd 4/26/05 4:40 PM Page 113distance between the molecules approaches zero, thus preventing the moleculesfrom piling up on top of each other. Even though the molecules in asolid cannot move relative to each other, they continually oscillate abouttheir equilibrium positions. The velocity of the molecules during these oscillationsdepends on the temperature. At sufficiently high temperatures, thevelocity (and thus the momentum) of the molecules may reach a pointwhere the intermolecular forces are partially overcome and groups of moleculesbreak away (Fig. 3–5). This is the beginning of the melting process.The molecular spacing in the liquid phase is not much different from thatof the solid phase, except the molecules are no longer at fixed positions relativeto each other and they can rotate and translate freely. In a liquid, theintermolecular forces are weaker relative to solids, but still relatively strongcompared with gases. The distances between molecules generally experiencea slight increase as a solid turns liquid, with water being a notableexception.In the gas phase, the molecules are far apart from each other, and a molecularorder is nonexistent. Gas molecules move about at random, continuallycolliding with each other and the walls of the container they are in. Particularlyat low densities, the intermolecular forces are very small, and collisionsare the only mode of interaction between the molecules. Molecules inthe gas phase are at a considerably higher energy level than they are in theliquid or solid phases. Therefore, the gas must release a large amount of itsenergy before it can condense or freeze.3–3 ■ PHASE-CHANGE PROCESSESOF PURE SUBSTANCESThere are many practical situations where two phases of a pure substancecoexist in equilibrium. Water exists as a mixture of liquid and vapor in theboiler and the condenser of a steam power plant. The refrigerant turns fromliquid to vapor in the freezer of a refrigerator. Even though many homeowners consider the freezing of water in underground pipes as the mostChapter 3 | 113FIGURE 3–4In a solid, the attractive and repulsiveforces between the molecules tend tomaintain them at relatively constantdistances from each other.© Reprinted with special permission of KingFeatures Syndicate.INTERACTIVETUTORIALSEE TUTORIAL CH. 3, SEC. 3 ON THE DVD.(a) (b) (c)FIGURE 3–5The arrangement of atoms in different phases: (a) molecules are at relatively fixed positions in a solid,(b) groups of molecules move about each other in the liquid phase, and (c) molecules move about at randomin the gas phase.