Ask A Scientist© Chemistry Archive

 
name        Nathan
status      student
age         20s

Question -  This is actually an inquiry on a earlier answer of yours.
Regarding hot vs cold water making ice first. There is a school of thought
which says something to do with dissolved gasses in the water affecting
the thermal conductivity of the water, bla, bla, unfortunately thats all I
understood, but Im trying to recover a fifty dollar bet on the topic so
any relevant info would be great.
ps this opinion was that of a chem prof at U of NewBrunswick, Canada
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Since NEWTON is really for K-12 students and educators I don't
usually answer questions from college students trying to settle
bets, but this question is so entrenched, and I gave a dumb
answer for it once which should have been removed long ago,
I'll give this a shot.

This is an ancient question, sort of an "urban
legend" of science education.

Let me say first that I personally have never measured
the freezing rate of hot vs cold water, and have seen no publications
documenting it in the literature, or heard of any specific references.
The only "paper" I've seen so far is a syndicated kids' newspaper feature,
"Beakman and Jax" (on which Beakman's World was based). "Beakman"
clained that the phenomenon, often reported, is simply untrue!
You might try to find the Beakman and Jax web site and check out
what they have to say. I believe that the author's name is Jok Church.
Personally, I really don't know one way or the other, so there's
no way I'd bet $ on it. But to my mind, there must be at
least 5 effects variously competing/working together to determine
th final outcome:

(1)ordinary laws of heat transfer, which say that cold objects
cool down more rapidly than hot objects to any given temperature
(in this case the freezing point). I believe that Church's claim
was that this one is more important than anything else if I
understood him correctly.
(2)evaporative cooling. As a substance evaporates, the surface
cools because the escaping particles carry kinetic energy. Since
the vapor pressure of hot water is greater than that of cold water,
this would tend to help hot water cool more quickly than cold water.
This is closely related to...
(3)low humidity in the freezer. The freezer is very dry, so evaporation
should be happening, and again the hot water will evaporate more
rapidly than the cold water. This in turn leaves less water behind
in the hot water tray than in the cold water tray at any moment in time,
and therefore there is less water present for ordinary cooling by
heat transfer in the hot tray than in the cold tray.
(4) the presence of dissolved gases in the water. Hot water will have
fewer dissolved gases (like CO2) than cold water, because the water will
be "degassed" in the process of heating it. The presence of solute
particles in the water should have the effect of decreasing its thermal
conductivity, therefore making it take longer to freeze cold water
than the relatively degassed hot water (your professor's view, I guess).
This is closely related to...
(5)again, the presence of dissolved gases in the water.
The presence of a solute in solution thermodynamically tends to
increase its melting temperature (check your freshman chem text).
So the degassed hot water should freeze at a lower temp than the
non-degassed cold water.

So here we have (1), which says that cold wins, and (2-5),
each of which say that hot has a shot at the finish line.
Probably the truth is some combination/competition. But personally
until I see some experimental data, I'm from Missouri on this one
(so says the theoretical chemist).

best regards, and be careful what you bet your $ on-
prof. topper
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