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Name:  william
Status:  educator
Age:  50s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
I have heard that hot water freezes faster than cold water, at least under certain circumstances. Is this true? Under what circumstances? and What is the explanation of why this happens if it is true?


Replies:
A warm ice-cube tray melts ice crystals between the tray and the freezer shelf that it sits on. This improves the thermal contact, and so increases the rate at which heat leaves the tray.

Tim Mooney


In my case two pots were filled with about the same amount of water from the same tap. One was heated to a rolling boil and then both were placed in a refrigerator. I confidently predicted that the colder water would freeze first. I was wrong. Decisively wrong!

I believe the reason is that boiling removes air disolved in the water. Exactly why disolved air keeps the water from freezing, however, I still don't understand (50 years later). Maybe now that I'm retired, I'll spend some time trying to underestand...

Best, Dick Plano...

PLEASE NOTE: This is a claim in Paul Hewitt's Conceptual Physics Textbook. I have heard it discussed at several conferences, and the battle is between less mass (due to evaporation) and Newton's Law of cooling. It would be a great help to the high school physics community if a clear answer could be made. ---Nate Unterman


The reason cannot be solely due to Newton's Law of cooling. Mathematical analysis of the law indicates that a warmer substance should take more time to cool. Evaporation, however, does have the added effect of removing some of the molecules. Evaporation is a fairly quick cooling process. Also, by removing a significant number of molecules, the actual freezing process takes less time.

To verify this, Try to freeze equal amounts of hot and cold water under two circumstances. Use identical amounts of water in four identical containers. You will need two containers of hot water and two containers of lukewarm water. Have one of each open. Also have one of each sealed with a piece of plastic wrap IN CONTACT with the surface. The open containers have conduction/convection, as well as evaporation to provide cooling. The closed containers, if the wrap is in contact with the surface, do not allow evaporation.

I would expect the open containers to freeze faster than the corresponding closed containers. I would expect the order to be open hot, open lukewarm, closed lukewarm, closed hot.

If this is the result, then the following would be my conclusion: The evaporation removes enough mass and provides enough cooling to almost catch up with the lukewarm water. When the freezing time comes, the less mass of the originally hot water freezes faster.

If the cooler water is almost freezing already, then I do not expect the hot water to freeze more quickly. The cold water would not have to cool off before the freezing process begins.

Kenneth Mellendorf



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