## Phase Diagrams

Although the introductory example of H 2O mentioned changes of state caused by varying the temperature, it is known that variation of pressure can also produce such changes. In laboratory experiments, these two environmental factors—temperature and pressure—can each be varied or held constant; they are referred to as independent variables. Figure 1 assigns these variables to axes to form a plot that describes the physical condition at each point in the graph. The vertical axis is the pressure measured in atmospheres (atm).

Figure 1. The phase diagram for water. A temperature‐pressure graph showing the various states of matter is a phase diagram. Phase refers to a single homogeneous physical state. Different phases have either different compositions or different physical states. In the preceding figure, there are three phases with the same composition in the solid, liquid, and gaseous states of matter.

Begin studying how both temperature and pressure determine the state of H 2O by taking some ice at a temperature of –10°C and pressure of 5 atmospheres, labeled S in Figure 2. If the pressure is held constant but the temperature is increased, the substance heats up along the dashed line marked L, melting to a liquid at point m, about –0.01°C. Alternatively, if you decrease the pressure on the initial solid S, while holding the temperature constant at –10°C, the conditions change downward along path G, and the ice vaporizes abruptly when the pressure has fallen to the point marked n, about 3 × 10 –3 atm. Such a direct change from a solid to a gas is called sublimation; notice that there was no intervening liquid state.

Figure 2. Changing the phase of solid water.