1.5 class periods

3-2 Phase Changes

In this section students learn about the five phase changes of matter: melting, freezing, vaporization, condensation, and sublimation. Thee phase changes are physical changes, and the identity of the substance involved remains the same. Students discover that phase changes are produced when energy is added to or taken away from the substance. When heat is added to or removed from a substance between phase changes, the temperature of the substance increases or decreases respectively.

The themes that can be focused on in this section are energy, unity, and diversity, and stability.

Energy: Point out that the only difference between water in the solid phase and water in the liquid phase is the amount of energy possessed by the particles of water. Explain that the addition or heat energy to a substance will raise its temperature while it is in that particular phase, but that the addition of heat energy to a substance as its melting point or boiling point is used to overcome the forces that hold the particles together and thus produces a change in phase.

Unity and diversity: Explain that the basic properties of matter, such as weight, volume, and density, are physical properties. A substance may change from a liquid to a gas. It may expand because of a temperature change. It may appear to be quite different, but the basic particles remain unchanged. A substance is still the same substance regardless of its phase.

Stability: Stress that when a substance undergoes a physical change, its physical properties are altered, but the substance remains the same kind of matter. You may be able to pick up an ice cube and transport it across a room with no spillage. You could not do that with water in the liquid stage unless it was in a container. And youd be hard pressed to see water in its gaseous phase. Nevertheless, the particles of which all three phases are composed remain the same.

Performance Objectives 3-2

1. Identify the phase change in matter.
2. Explain how adding or taking away energy will produce a phase change.
3. Discuss the relationship of heat, energy, and phase change.

Science Terms 3-2

Melting, p.70

Melting point, p.70

Freezing, p. 71

Freezing point, p.71

Vaporization, p. 72

Evaporation, p.72

Boiling, p.72

Boiling point, p.72

Condensation, p.73

Sublimation, p.74

Show students a block of ice or some ice cubes.

• What phase of matter is the ice?
• What will happen if the ice sits at room temperature for a long time?
• What phase of matter is melted ice?

Have students observe Figure 3-13, p.70, as you read the caption aloud.

• What are some other examples of melting?
• What is added to cause the substance to melt?
• What do you think happens to the particles of a substance when the substance is heated and melts?
• Where does the energy come from?

Point out that the temperature at which a solid changes into a liquid is called the melting point. Tell students that substances have their own individual melting points. Explain that ice has a melting point of 0¥ C. Write that fact on the chalkboard.

• What do you think happens to the particles of a liquid when heat is removed from the liquid?
• What do you notice about the melting point of ice and the freezing point of water?

Have students observe Figure 3-15,p.71, and read the caption. Have them discuss the following questions.

• Why is it not a good idea to leave bottled beverages outside in cold weather?
• What damage can be done to a car if antifreeze is not added to the water in the radiator?
• Why do many potholes appear in roadways during the winter?

Tell students that a change of phase from liquid to a gas is called vaporization.

• What must happen to the particles of a liquid in order for them to change into a gas?
• What kind of energy?
• What do you predict would happen to body temperature if body perspiration did not evaporate?

Point out that gases also change into liquids.

• What are some examples of a gas changing into a liquid?
• What happens to the particles of a gas as they change into the liquid phase?
• How is this energy loss accomplished?

Tell students that some solids do not pass through the liquid phase and then into the gas phase. Certain solid pass directly from a solid phase into a gas phase by a process called sublimation.

• How can substances be made to change phase?
• What is a melting point? A freezing point?
• What is the difference between evaporation and condensation?
• Describe the changes in heat energy and particle arrangement as dry ice sublimes.
• Suppose you place several ice cubes in a glass of water that is at room temperature. What happens to the ice cubes over time? What happens to the temperature of the water in the glass? What happens to the level of water in the glass? (Assume that you do not drink any of the liquid and that evaporation does not occur to any great extent.)