The previously described properties of solids, liquids, and gases, as well as the processes relating to their change from one to another, all point toward the importance of temperature and pressure in determining the stable state of matter for a given substance. A phase diagram provides a graphical way to summarize the conditions of those parameters that dictate the phase the substance will primarily find itself in once equilibrium is established. Figure 7.4 is an example of a phase diagram of the substance water. It is important to note that many phase diagrams (like the one shown in Figure 7.4) are not shown to scale but are meant to convey important information about a substance. Analysis of Figure 7.4 shows that line BD is essentially the vapor pressure curve for water’s liquid phase. Notice that when the pressure on a sample of water is 760 mm of Hg, the vapor pressure of the water matches that at 100°C, and the water will boil. However, if the pressure is raised, the boiling point temperature increases, and if the pressure is less than 760 mm of Hg, the boiling point decreases along the BD curve down to point B.
As line BD is essentially the vapor pressure curve for liquid water, line AB is the vapor pressure curve for solid water. Point B is one position where the vapor pressure (or VP) of the solid is equal to the vapor pressure of the liquid. As the pressure is increased, the temperature at which this remains true goes down, as seen by the negative slope of the line emanating upward from point B. This differs from most other substances, which display a positive slope in this condition, and has to do with the density of solid water (ice) being less than the density of liquid water (discussed earlier in this chapter). Recall too that when the VP of a solid is equal to the VP of a liquid, melting and freezing take place in equilibrium with each other. Consequently, the normal melting/freezing point for water (i.e., at 760 mm of Hg) is shown on the diagram to be 0.0°C. Again, this point is affected by pressure along line BC so that if the pressure is decreased, the melting/freezing point is slightly higher up to point B, or 0.01°C.
Point B on the diagram is unique in that not only are melting and freezing in equilibrium with each other but so are all other phase changes. Point B, commonly referred to as the triple point, describes the only combination of temperature and pressure where all three phases are simultaneously stable. Combinations of temperature and pressure that fall on the lines in these diagrams describe stable two-phase regions. All other combinations represent stable single-phase regions for the substance.
Another noteworthy point on a phase diagram is the end of the vapor pressure curve for the liquid. The phase diagram for water in Figure 7.4 cannot show this because of the figure’s scale, but it is shown on many other diagrams. As earlier discussed in reference to the properties of liquids, this point, called the critical temperature, is the temperature above which a substance cannot exist as a liquid. That explains why the vapor pressure curve comes to an end. If the liquid cannot exist, there can be no vapor pressure measured for it. Note that if a substance is at its critical temperature, it can exist as a liquid, but in order to be so, it must also be at a high enough pressure. The pressure needed to liquefy a gas at its critical temperature is referred to as its critical pressure. The combination of critical temperature and critical pressure, often seen in phase diagrams, is called its critical point.