# Phase Diagrams

A phase diagram is a graphical representation of phase transitions under different pressures and temperatures. By analyzing the phase diagram of a substance, we can predict the state of that substance at a given temperature and pressure. For the MCAT, you should be familiar with the phase diagram and what exactly it represents, and also be able to predict some of the related trends from a given phase diagram.
In a phase diagram, there are three sections representing the solid, the liquid, and the gas phases. The phase diagram of a substance enables us to determine the phase of that substance at a given temperature and pressure. The phase diagram is actually plotted using experimentally determined values at different temperatures and pressures. Let's take a look at the phase diagram (Figure 7-5) of water.

Phase diagram of water (diagram not drawn to scale)

As you can see in the phase diagram of water shown above, there are three regions in the graph representing solid, liquid, and gas phases. The segment BC divides the solid and the liquid phases of water. This segment represents the equilibrium region between the solid and the liquid phases. Because water is denser than ice, the segment BC is slightly slanting toward the left. For most phase diagrams (Figure 7-6), the segment BC will be slightly slanting toward the right (positive slope), since the liquid phase is usually less dense than the solid phase.
The segment BD divides the liquid and the gas phases in the graph. It also denotes the vapor pressures at different temperatures. The intersecting point of the curves represented by point B is called the triple point. This point represents the temperature and pressure at which the three phases of a substance are in equilibrium. How many triple points are likely to exist for a substance? The answer is one. There is only one pair of temperature-pressure combination for a substance at which the three states will be in equilibrium. Figure 7-6 shows a phase diagram representing the majority of other compounds that we see in nature.

Phase diagram of most substances (diagram not drawn to scale)

(Notice the positive slope of segment BC, unlike that of water which has a negative slope)
Substances in their gas phase can usually be liquefied by increasing the pressure. But it reaches a point where this conversion is not possible. The temperature above which liquid phase cannot be achieved regardless of the applied pressure is called the critical temperature. The vapor pressure at the critical temperature is called the critical pressure.