Triple-point diagrams are the culmination of this chapter; they answer many questions about the relationship between solids, liquids, and gases. For instance, why does ice melt into water, and then boils into steam, while solid carbon dioxide sublimes directly to its gaseous state? If we plot a diagram of a substance using temperature and pressure as our axes, noting where it is a solid, liquid, or gas, we get something like the following:
This is the triple-point diagram for water. Notice at 760 mm Hg, or 1 atm, water is converted from a solid to a liquid at 0 °C, and boils at 100 °C, both as expected. At very low pressure, though, we can see that the liquid phase is absent altogether. Most conspicuously, though, there is a point at 4.58 mm Hg and about 0.01 °C where all three states--solid, liquid, and gas--exist in equilibrium. That is, they are all present at once!
Water is very unusual in that its solid form, ice, is less dense than its liquid form. This phenomenon can be seen in the negative-sloped line delineating the boundary between the solid and liquid phases; as pressure increases, water wants to go back to its less-dense state (liquid) by Le Chatlier's principle. Therefore, as pressure increases, the freezing point of water drops. The triple-point diagram for carbon dioxide, shown below, is more normal:
One other important concept is noted by the markings of pressure and temperature at the edges (top and right) of the diagrams. The temperature is called the critical temperature, and it is the maximum temperature at any pressure that the substance can be found in the liquid state. In other words, the liquid will boil if the temperature is past the critical level, no matter how high the pressure is. The critical pressure is the last pressure level at which the liquid can exist (the pressure at the critical temperature when the substance is still liquid).
Looking at carbon dioxide's diagram, we see why it sublimes at room temperature. The pressure must be increased above 3886 mm Hg (5.113 atm) before liquid carbon dioxide can exist. If water were reduced to a pressure less than 4.58 mm Hg, it would sublime from ice to steam as well.
This concludes our last "States of Matter" lesson. The next page features a summarization of the chapter, as well as review problems to test your knowledge of what you have learned. The next section, "Atoms and Molecules," will delve even more deeply into the behavior of matter, this time at atomic and molecular scales.