MAE 241 Introduction to Thermodynamics Exam 1 Practice

The idea being tested is the state where a pure substance is on the phase-change boundary between liquid and vapor at a given pressure. When a substance is at this boundary, it is saturated with respect to its phases, and the temperature at which the liquid and vapor are in equilibrium is fixed for that pressure. That fixed temperature is the temperature at which the phase change occurs, so it describes the condition of the substance being at the point where liquid can turn to vapor (or vapor can condense) without a change in temperature. In many thermodynamics contexts, this is called the saturation temperature; some texts use the phrasing “saturated temperature” to describe the same idea. The key point is that the temperature corresponds to the equilibrium between phases for that pressure, not the pressure itself or the end of phase distinction that happens at the critical point.