From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.engr.chem
Subject: Re: Hot H2O Vs Cold H2O  Which freezes faster??
Date: 20 Feb 1996 21:25:50 GMT

In article <schne042-2002960758030001@x209-65.bae.umn.edu>,
schne042@gold.tc.umn.edu (Mike Schneider) says:

> So for cold water delta T/
>delta t = 8.6862 and for hot water delta T/ delta t = 38.2659.  So hot
>water freezes faster and cold water freezes sooner.

Hot water does not freeze - cold water does. The rate you are looking
for in the dictionary would be better derived from a handbook.

Hot water cools faster than cold water because they both follow
approximately the rate equation delta T = -k*dT/dt. The rate is
approximate primarily because of differences in convection, but
there is no doubt about average rate being higher for the hot
liquid.

The rate of interest in this discussion is the rate of freezing,
which is more or less an isothermal process occurring at 0 degC.
It is here that the differences in rate can be expected between
previously heated water and water which has not been recently
heated.

Now, if you junior physical chemists are feeling your oats and
would like to stop bickering over which end of the semantic egg
to crack, why don't you have a bash at applying your skills
to the following mathematics and experiments.

Assume that the difference in freezing rate is a function of
the gases dissolved in the water which are liberated on freezing.
If this is the case, then water that has been recently BOILED
should be reasonably gas free and freeze significantly faster
than ordinary tap water. At the other extreme, one might expect
soda water to freeze relatively slowly.

The mechanism proposed by the noted experimental chemist Ico
involved the formation of an advancing bubble front at the
water/ice interface. Imagine in the first case a minute layer
of microbubblets at the interface and in the other extreme,
look at the gas layer between the film of ice which often
first forms across the top of a cube.

In the first case, as the ice advances the thickness of
the layer of bubbles increases progressively. Given that the
thermal conductivity of air is very much less than that of
water or ice the thin set of bubbles acts as a thermal
barrier inhibiting the flow of heat in the direction of the
ice.

Ico suggested that a simple experiment be performed to test
the theory as a one-dimensional effect by freezing water in
a clear plastic tube arranged in a thermal gradient to produce
an advancing ice/water interface where the length of the bubble
layer or resultant clear air space could be observed and measured
as a function of time.

Ico also suggested that the experimental results might
easily be calculated using handbook solubility and thermal
conductance data by assuming that the liquid film portion
of the bubbles would be a small portion of the total bubble
volume crossection.  He also suggested that the relative
rate of linear freezing would represent a sort of limit to
to the ratios expected for more complex 3-D freezing scenarios.

I do not know if Ico has published the results of his
calculations but it is unlikely that he has conducted the
experiments since he is apparently quite old now.

Jerry

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.engr.chem
Subject: Re: Hot H2O Vs Cold H2O  Which freezes faster??
Date: 25 Feb 1996 00:57:16 GMT

In article <saunders-2302961757230001@putter.che.wisc.edu>,
saunders@castor.che.wisc.edu (Brian Saunders) says:

>In article <4gjb8j\$g3f@geraldo.cc.utexas.edu>, glhurst@onr.com (Gerald L.
>Hurst) wrote:
>
>>There may be other effects. I do not know.
>
>People have claimed that the freezer has dry enough air that the hot
>liquid will evaporate more than the cold liquid.  This would result in
>some additional heat being removed by the energy needed for vaporization
>(thus speeding the rate it cooled), and more importantly, would result in
>their being less water to freeze than the liquid that started cold.

What you say is true. However there have also been reports of
experiments run in closed containers which also gave results
suggesting that the hot water cooled and froze in a shorter
period than the cold. Also, recently, an experimenter reported
the loss of water to evaporation was not significant in magnitude
in a set of experiments which again gave the unexpected result.

Jerry (Ico)