PURPOSE: To freeze water by reducing the ambient pressure.

DESCRIPTION: Place about four drops of water onto a cellophane "watchglass" in the vacuum chamber as in the photograph at the left below. Evacuate the chamber with the vacuum pump. Almost immediately the water will begin to bubble (boil), as seen in the middle photograph below, and within about one minute the boiling will subside and the water will freeze. When the water freezes it is clearly seen to become opaque on the overhead projector screen, as seen in the photograph on the right below. This is a very dramatic demonstration.

SUGGESTIONS: Ask your students whether this ice is cold like regular ice. Then pass the ice around for them to feel it.

REFERENCES: (PIRA 4C31.20) Articles in the Demonstration Reference File describe this apparatus and the physics involved.

EQUIPMENT: Freezing by pumping chamber with cellophane watchglass, water and eyedropper, vacuum pump.

SETUP TIME: 5 min.