4C20.10 - Super Cooling Water
The vacuum pump method works extremely well for this demonstration.
Some advanced prep is necessary: Place the watch glass in the Petri dish and fill the watch glass with water. Place the thermocouple probe into the center of the water in the watch glass. Open the gas ballast on the vacuum pump. Start the vacuum pump and pump on the water until it freezes. This cycle will draw the air out of the water sample and a great deal of this will splash into the Petri dish. Close the gate valve, keep the water sample under vacuum, and allow it to warm up to room temperature again.
Once the above prep is done, keep the demo under vacuum and move it into the classroom. Position the water sample and the digital thermometer so that you can see them both clearly with the video camera. When ready, start the vacuum pump and open the gate valve fully. Monitor the temperature. When the temperature approaches +5 degrees C close the gate valve until the second (Red) arrow mark on the gate valve is pointed straight up. (The closing of the gate valve is necessary to reduce the pumping speed. Too fast and the water sample will freeze before it is supercooled.) It will take several minutes from this point, but the temperature should go to somewhere between -5 and -9 degrees C before the water sample freezes. Observe that when this happens the temperature of the frozen water sample instantly rises to 0.0 degrees C.
NOTE: THIS DEMO ONLY WORKS ABOUT 10 % OF THE TIME DUE TO THE CLASSROOM ENVIRONMENT AND THE MACHINE ROOM VIBRATIONS THAT ARE INTRODUCED INTO THE CLASSROOM DURING NORMAL EVERY DAY OPERATION.
It helps if you boil and filter the water that you are trying to super cool to get rid of the air and the dust particles that act as nucleation sites. Put about 5 cc of that water in the test tube and lower into the dry ice alcohol bath. Observe the temperature over time and if things are very quiet and you don't shake the test tube you should get down to -4 degrees C. Shake the test tube and the water should freeze instantly and the temperature should come up to 0 degrees C.
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- Emily Townsend, "Supercooled Water", Tap-L Conversations, Dec. 27, 2005.
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