The boiling point of a liquid depends on the external air pressure. When water is placed under vacuum, the boiling point decreases and the water boils. Boiling, however, is an endothermic process—as the water boils, the temperature decreases, and the water soon freezes!
In this demo, pressure changes cause a sample of acetone freezes while still boiling.
Click here for the complete instructions, courtesy of Flinn Scientific