Unit 5: Phase Change

Our existence on Earth depends on the abundance of water found here. This water can be found in three different phases: solid, liquid, and gas. Until recently, it was thought Earth was the only place in our solar system with liquid lakes and seas. However, NASA’s Cassini-Huygens mission has discovered the presence of lakes and seas on Titan, one of Saturn’s moons. Similar to liquids on Earth, the liquids in these lakes and seas undergo changes in phase. Unlike Earth, however, these bodies do not consist of water. They are made of hydrocarbons such as methane. In a picture taken by a space probe in 2007, a large lake of liquid methane was detected. In a picture of the same area taken in 2009, the lake did not appear to be there. Upon further testing, a solid was detected. The solid was either solid methane or solid ground that was underneath the lake. NASA scientists wondered if the lake evaporated or froze.

Taking on the role of student chemists working for the Universal Space Agency (a fictional agency), your students will investigate the mystery of the methane lake on Titan. One team of scientists at the Universal Space Agency claims that the lake evaporated while the other team of scientists claims that the lake froze. The apparent disappearance of the lake serves as the anchor phenomenon for the unit, and the students’ assignment is to determine what happened to the lake. Motivated to understand phase change more deeply, students gather evidence from the Phase Change Simulation, from several articles, and from physical investigations of phase change. They learn that the molecules of a substance move differently when that substance is in different phases. They also learn how the kinetic energy of molecules and the attraction between the molecules affects the way in which the molecules move. Students use this newfound understanding of molecules, kinetic energy, and attraction, as well as evidence about the conditions on Titan, to explain what they think happened to Titan’s mysterious lake.