Courses: P3, P13, P15?, Astr1?, Astr2?
By reducing the pressure in a small flask of water, the water will first be made to boil, and then, after a minute or two, it will freeze.
Discuss air pressure holding the water together against its internal pressure. Once all the energetic particles are removed, only cold molecules remain.
Setup: A cold trap must be used to prevent water vapor from entering the pump and condensing there. The cold trap uses LN2 -- this can confuse students into thinking the liquid nitrogen is somehow involved in cooling the water. We may want to rethink the setup so the cold trap is a significant distance away from the water flask -- perhaps even hidden behind the lecture bench.
