Triple Point

Thermodynamics CHANGE OF PHASE Sublimation If the initial pressure of the ice at 0°F is 0.0505 psia, heat transfer to the ice first results in an increase in temperature to 20°F. At this point, however, the ice passes directly from the solid phase to the vapor phase in the process known as sublimation. Sublimation is a special term used for cases in which the transition between the solid phase and the vapor phase occurs directly, without passing through the liquid phase. Further heat transfer would result in superheating the vapor. Triple Point Finally, consider an initial pressure of the ice of 0.08854 psia. Again, as a result of heat transfer, the temperature will increase until it reaches 32°F. At this point, however, further heat transfer may result in some of the ice becoming vapor and some becoming liquid because it is possible to have the three phases in equilibrium. This is called the triple point, defined as the state in which all three phases may be present in equilibrium. Figure 8 is a pressure-temperature diagram for water that shows how the solid, liquid, and vapor phases may exist together in equilibrium. Along the sublimation line, the solid and vapor phases are in equilibrium, along the fusion line, the solid and liquid phases are in equilibrium; and along the vaporization line, the liquid and vapor phases are in equilibrium. The only point at which all three phases may exist in equilibrium is the triple point. The temperature and pressure for the triple point of water are 32.02°F and 0.08865 psia. The vaporization line ends at the critical point because there is no distinct change from the liquid phase to the vapor phase above the critical point. Rev. 0 Page 37 HT-01