18.5 Phase Changes

The temperature inside the freezer compartment of a refrigerator is typically about $- 20 ° C$ . Suppose you were to remove a few ice cubes from the freezer, place them in a sealed container with a thermometer, then heat them, as FIGURE 18.5a shows. We’ll assume that the heating is done so slowly that the inside of the container always has a single, well-defined temperature.

The figure consists of two parts labeled ("a") and (b) that illustrate ice melting to water in a square box and a graph that measures changes in temperature.

FIGURE 18.5 Full Alternative Text

FIGURE 18.5b shows the temperature as a function of time. After steadily rising from the initial $- 20 ° C$ , the temperature remains fixed at $0 ° C$ for an extended period of time. This is the interval of time during which the ice melts. As it’s melting, the ice temperature is $0 ° C$ and the liquid water temperature is $0 ° C$ . Even though the system is being heated, the liquid water temperature doesn’t begin to rise until all the ice has melted. If you were to turn off the flame at any point, the system would remain a mixture of ice and liquid water at $0 ° C$ .

NOTE In everyday language, the three phases of water are called ice, water, and steam. That is, the term “water” implies the liquid phase. Scientifically, these are the solid, liquid, and gas phases of the compound called water. To be clear, we’ll use the term “water” in the scientific sense of a collection of $H_{2} O$ molecules. We’ll say either “liquid” or “liquid water” to denote the liquid phase.

The thermal energy of a solid is the kinetic energy of the vibrating atoms plus the potential energy of the stretched and compressed molecular bonds. Melting occurs when the thermal energy gets so large that molecular bonds begin to break, allowing the atoms to move around. The temperature at which a solid becomes a liquid or, if the thermal energy is reduced, a liquid becomes a solid is called the melting point or the freezing point. Melting and freezing are phase changes.

A system at the melting point is in phase equilibrium, meaning that any amount of solid can coexist with any amount of liquid. Raise the temperature ever so slightly and the entire system becomes liquid. Lower it slightly and it all becomes solid. But exactly at the melting point the system has no tendency to move one way or the other. That is why the temperature remains constant at the melting point until the phase change is complete.

You can see the same thing happening in Figure 18.5b at $100 ° C$ , the boiling point. This is a phase equilibrium between the liquid phase and the gas phase, and any amount of liquid can coexist with any amount of gas at this temperature. Above this temperature, the thermal energy is too large for bonds to be established between molecules, so the system is a gas. If the thermal energy is reduced, the molecules begin to bond with each other and stick together. In other words, the gas condenses into a liquid. The temperature at which a gas becomes a liquid or, if the thermal energy is increased, a liquid becomes a gas is called the condensation point or the boiling point.

NOTE Liquid water becomes solid ice at $0 ° C$ , but that doesn’t mean the temperature of ice is always $0 ° C$ . Ice reaches the temperature of its surroundings. If the air temperature in a freezer is $- 20 ° C$ , then the ice temperature is $- 20 ° C$ . Likewise, steam can be heated to temperatures above $100 ° C$ . That doesn’t happen when you boil water on the stove because the steam escapes, but steam can be heated far above $100 ° C$ in a sealed container.