Thermodynamics

cen84959_ch03.qxd 4/1/05 12:31 PM Page 123rays would never reach these ice layers, and the bottoms of many rivers,lakes, and oceans would be covered with ice at times, seriously disruptingmarine life.We are all familiar with two phases being in equilibrium, but under someconditions all three phases of a pure substance coexist in equilibrium(Fig. 3–23). On P-v or T-v diagrams, these triple-phase states form a linecalled the triple line. The states on the triple line of a substance have thesame pressure and temperature but different specific volumes. The tripleline appears as a point on the P-T diagrams and, therefore, is often calledthe triple point. The triple-point temperatures and pressures of various substancesare given in Table 3–3. For water, the triple-point temperature andpressure are 0.01°C and 0.6117 kPa, respectively. That is, all three phases ofwater coexist in equilibrium only if the temperature and pressure have preciselythese values. No substance can exist in the liquid phase in stableequilibrium at pressures below the triple-point pressure. The same canbe said for temperature for substances that contract on freezing. However,Chapter 3 | 123VAPORLIQUIDSOLIDFIGURE 3–23At triple-point pressure andtemperature, a substance exists inthree phases in equilibrium.TABLE 3–3Triple-point temperatures and pressures of various substancesSubstance Formula T tp , K P tp , kPaAcetylene C 2 H 2 192.4 120Ammonia NH 3 195.40 6.076Argon A 83.81 68.9Carbon (graphite) C 3900 10,100Carbon dioxide CO 2 216.55 517Carbon monoxide CO 68.10 15.37Deuterium D 2 18.63 17.1Ethane C 2 H 6 89.89 8 10 4Ethylene C 2 H 4 104.0 0.12Helium 4 (l point) He 2.19 5.1Hydrogen H 2 13.84 7.04Hydrogen chloride HCl 158.96 13.9Mercury Hg 234.2 1.65 10 7Methane CH 4 90.68 11.7Neon Ne 24.57 43.2Nitric oxide NO 109.50 21.92Nitrogen N 2 63.18 12.6Nitrous oxide N 2 O 182.34 87.85Oxygen O 2 54.36 0.152Palladium Pd 1825 3.5 10 3Platinum Pt 2045 2.0 10 4Sulfur dioxide SO 2 197.69 1.67Titanium Ti 1941 5.3 10 3Uranium hexafluoride UF 6 337.17 151.7Water H 2 O 273.16 0.61Xenon Xe 161.3 81.5Zinc Zn 692.65 0.065Source: Data from National Bureau of Standards (U.S.) Circ., 500 (1952).