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 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. The pressure and temperature axes on this phase diagram of water are not drawn to constant scale in order to illustrate several important properties. A typical phase diagram for a pure substance is shown in. 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.

Use phase diagrams to identify stable phases at given temperatures and pressures, and to describe phase transitions resulting from changes in these properties.

Explain the construction and use of a typical phase diagram.

By the end of this section, you will be able to:

Materials cha pter ten phase diagrams