Going through a phase : Teaching about energy transformations in phase changes with infrared cameras

Students have difficulties understanding energy transformations involved in phase changes in physics and chemistry education. For example, with a predict-observe-explain set-up, we have found that students tend to intuit that when table salt is poured onto ice, the ice will melt and the temperature increase. They are surprised to see that although the ice melts (due to freezing-point depression), the temperature actually decreases. In this study, we explore how infrared cameras as a visualization technology can help students come to terms with such challenges. We have designed a teaching sequence for in-service science teachers on the topic of phase changes, with a focus on the central idea that it requires energy to break bonds between particles. In group discussions, students are encouraged to use this idea to explain how the temperature of water can be constant during phase change from solid to liquid, and from liquid to gas, and draw on their experiences that it feels cold when they walk out of the shower but hot when water is poured onto the stove in a sauna. With the help of an infrared camera, students can see how the temperature decreases as water evaporates from their body. With this technology, they can also see that the temperature of a piece of paper increases as moist air condenses on its surface, and that the temperature decreases when the water evaporates away in dry air. Through video analysis, we study students’ interactions with each other and the types of talk they engage in during the exercises. Early findings in a pilot study with secondary school students indicate that they tend to interpret condensation as release of energy due to particles colliding with a surface, rather than bond formation