Thermodynamics Table of Contents

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 possibleto have the three phases in equilibrium. This is called the triple point, defined as the state inwhich 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 forthe tri point because there is no distinct change from the liquid phase to the vapor phase above the critical point.