Thus, if we place a frozen sample in a vacuum with a pressure less than 0.20 kPa, ice will sublime. If we could zoom in on the solid-gas line in Figure 10.31, we would see that ice has a vapor pressure of about 0.20 kPa at −10 ☌. These temperature-pressure data pairs correspond to the sublimation, or deposition, points for water. The solid-vapor curve, labeled AB in Figure 10.31, indicates the temperatures and pressures at which ice and water vapor are in equilibrium. This unique state of matter is called a supercritical fluid, a topic that will be described in the next section of this module. The physical properties of water under these conditions are intermediate between those of its liquid and gaseous phases. Notice that the liquid-vapor curve terminates at a temperature of 374 ☌ and a pressure of 218 atm, indicating that water cannot exist as a liquid above this temperature, regardless of the pressure. For example, at 1 atm, the boiling point is 100 ☌. This “liquid-vapor” curve separates the liquid and gaseous regions of the phase diagram and provides the boiling point for water at any pressure. The curve BC in Figure 10.31 is the plot of vapor pressure versus temperature as described in the previous module of this chapter. Note that on the H 2O phase diagram, the pressure and temperature axes are not drawn to a constant scale in order to permit the illustration of several important features as described here. At 25 kPa and 200 ☌, water exists only in the gaseous state. A pressure of 50 kPa and a temperature of 50 ☌ correspond to the “water” region-here, water exists only as a liquid. For example, a pressure of 50 kPa and a temperature of −10 ☌ correspond to the region of the diagram labeled “ice.” Under these conditions, water exists only as a solid (ice). We can use the phase diagram to identify the physical state of a sample of water under specified conditions of pressure and temperature. A typical phase diagram for a pure substance is shown in Figure 10.30.įigure 10.31 The pressure and temperature axes on this phase diagram of water are not drawn to constant scale in order to illustrate several important properties. These diagrams indicate the physical states that exist under specific conditions of pressure and temperature, and also provide the pressure dependence of the phase-transition temperatures (melting points, sublimation points, boiling points). A phase diagram combines plots of pressure versus temperature for the liquid-gas, solid-liquid, and solid-gas phase-transition equilibria of a substance. Making such measurements over a wide range of pressures yields data that may be presented graphically as a phase diagram. Also described was the use of heating and cooling curves to determine a substance’s melting (or freezing) point. Considering the definition of boiling point, plots of vapor pressure versus temperature represent how the boiling point of the liquid varies with pressure. In the previous module, the variation of a liquid’s equilibrium vapor pressure with temperature was described. Describe the supercritical fluid phase of matter.
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