Water Freezing and Temperature
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?
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.
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
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.
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Update: June 2012