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rates of freezing
Name: N/A
Status: N/A
Age: N/A
Location: N/A
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Date: N/A
Question:
I should know the answer to this question.
So should the physics and chemistry teachers
at our school, but they disagree.
I have heard two, perfectly good explanations
with different "facts". Here goes.....
DOES hot water freeze faster than cold
water? (You were expecting something really
good were not you?) If so, why? If not, why?
Somebody's gonna have to eat crow. (I am glad
that I did not express my opinion - but I will
bet I am right!)
Replies:
First, I think scientifically you should qualify your 'hot' or
'cold' description by temperature, in any scale of your choice
(i.e. Fahrenheit, Celsius, or Kelvin). Given that, however, I will
assume that your 'hot' water is higher in temperature than your
'cold' water. Assuming no dissolved impurities which might other-
wise affecting the freezing (melting) point of said water, I will
place my bet and say that since the 'hot' water, by definition
contains more energy and therefore more molecular motion, it would
freeze more slowly (i.e. would require more cooling to get down
to the freezing point) than the water you describe as 'cold'. The
'cold' water , containing less energy (by definition) and therefore
less molecular motion should require less cooling to get to the
freezing point. I would like to hear both sides of the opinions of
your faculty.
rickru
Yes, hot water in a freezer freezes fast than cold water, as every
competent houseperson (formerly,"housewife") knows. The reason is that
the surface of the freezer usually has a layer of ice on it. Ice is an
excellent insulator, and therefore limits the heat transfer to the freezing
surface. Putting hot water in your pan (or ice-cube tray) has the effect
of melting the ice-layer on the surface, providing for better heat transfer
to the surface.
There is also the fact that there is increased evaporation from the
warm water, which reduces the mass of water that must be cooled, as noted
in item 3.40 of Jearl Walker's book "The Flying Circus of Physics." That
book is the first place to look for answers to questions of the type that
you posed.
Jack L. Uretsky
Jack L. Uretsky, You will recall that the question was does hot water
freeze faster than cold water. I don't recall it mentioning a freezer, nor a
coating of ice, nor allowing for escape of some of the targeted hot water to
freeze. I maintain my answer as correct, given the question that was
asked. Naturally if heat is allowed to escape from the system, either
thru melting of a supposed coating of ice or warming of some freezer
space, than your assertion might change the scenario. In a laboratory
with a controlled experiment where all the heat is accounted for,
I believe you will find my answer to be the correct one , i.e.
cold water will freeze faster than an equivalent quantity of hot
water.
Rick
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Update: June 2012
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