Chilling Out with Evaporation

Description

This is a companion activity to Activity #1: Chilling Out with Condensation, though is more appropriate for older students due to their understanding of the different phases of matter and molecule movement. See the end of the activity for tips on sharing with elementary-age students.

Materials

Beaker

Room-temperature water

Glass thermometer, preferably with large numbers for easy reading

Vacuum chamber

**If a vacuum chamber is unavailable, there are many examples of this activity done on YouTube. The user “MrGrodskiChemistry” has a clear one titled “Boiling cold water In a Vacuum Chamber” https://youtu.be/glLPMXq6yc0

Outline

1.      To work properly, a refrigerator must not only be able to condense gas into a liquid, it must also be able to evaporate liquid into a gas. Normally, we could do that by boiling the water, but we don’t want to add heat into our refrigerator, we want it removed! How can we boil a liquid without increasing the temperature?

2.      Fill your beaker halfway with room-temperature water, placing the thermometer into the water. Have your students note and record that value.

a.      If using the video, pause to note the temperature before continuing

3.      Place the beaker, water, and thermometer set-up into your vacuum chamber. Your vacuum chamber set-up might differ slightly from others, but the goal is to position everything in a way that students can not only see when the water boils, but also read the (stable) temeprature.

4.      Turn on the vacuum chamber: tell students that the vacuum chamber is removing air to change the pressure on the inside.

a.      If desired, tape an identical thermometer on the outside of the vacuum chamber to prove that the temperature of the chamber is not changing, only the pressure inside.

5.      As the pressure decreases, the water will begin to boil. As it does so, have students note the temperature. When the vacuum stops and the seal is broken, allowing air inside again, the pressure will equalize and it will cease to boil.

Extended Activity

·        Students may inquire as to whether you can increase the pressure to the point where the water becomes a solid without decreasing the temperature. Technically it can happen, though Earth doesn’t have the conditions for this to happen. However, if you continue to boil the water in the vacuum chamber, eventually you’ll remove heat from the system (because boiling releases energy), and the water will freeze.

o  https://van.physics.illinois.edu/QA/listing.php?id=1723