Activity Overview: Changes in states of matter from gas to liquid to solid and back are all achieved by cooling and heating. This activity focuses on the very smallest level of thermal energy rather than the macroscopic level of cooling and heating in the built environment.

If you are introducing the concept of melting point and boiling point, freezing point and dew point (condensation point) for state changes, use water as a reference point. Water's state changes are easily recognizable and are probably familiar to most if not all of your students.

In order to prepare for the investigation, identify the state change points of water listed below for students or display them for the class to reference. Further explain that due to the equilibrium of thermal energy, a substance will remain the same temperature until the state change has occurred completely. In other words, a liquid stays at 0°C until all of it is melted and then the temperature begins to rise. Similarly, a liquid stays at 100°C until all of it has evaporated.

• Melting/freezing 0°C (32°F)
• Condensing/boiling 100°C (212°F)

Materials: student-friendly thermometer, 2 cups, ice, liquid water, freezer

Procedure:

As a class or in groups, test the temperature of water during state changes. If a burner or heat source is not available or unsafe, that's not a problem. The state change between liquid and solid is cold, but less dangerous than an open flame or boiling water.

First, pour liquid water into one cup. Take a measurement of the temperature of the water. Then, place ice into the other cup. Try to take a measurement of the temperature of the ice. Depending on the thermometer probe, this may be difficult. If you cannot get a good reading on the temperature of the ice. Pour enough water to cover the ice in the cup. Wait a couple of minutes then take a measurement of the temperature of the icy water.

Then, place the cup with water in the freezer. Leave the icy water out on a table or counter. Take a new temperature measurement of each every 15 minutes. As ice appears on the surface of the water in the freezer, the temperature should approach 0°C and stay there until the entire cup of water is frozen when it will continue to descend. Similarly, as the ice begins to melt, the icy water in the cup should remain at 0°C until it is entirely melted and then the temperature will continue to rise.

In an ideal experiment, your results will match those outlined in this activity procedure. However, experimental measurements depend on the devices used to make the measurements, and may vary depending on the quality of your instruments.