From: neufeld@helios.physics.utoronto.ca (Christopher Neufeld)
Subject: Re: ice_cubes
Organization: University of Toronto Physics/Astronomy/CITA
Date: Fri, 4 Sep 1992 14:26:19 GMT

In article <1992Sep3.214944.11525@newsgate.sps.mot.com> rrgd50@email.sps.mot.com (Chris Terwilliger) writes:
>Here's one I remember from long ago and recently started a war at work with...
>
>"Ice cubes freeze faster when you use hot water..."
>
   From the sci.physics FAQ list maintained by Scott I. Chase:



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	  	  FREQUENTLY ASKED QUESTIONS ON SCI.PHYSICS
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Item 10.

Hot Water Freezes Faster than Cold!		updated 11-May-1992
-----------------------------------		original by Richard M. Mathews

	You put two pails of water outside on a freezing day.  One has hot
water (95 degrees C) and the other has an equal amount of colder water (50
degrees C).  Which freezes first?  The hot water freezes first!  Why?  

	It is commonly argued that the hot water will take some time to
reach the initial temperature of the cold water, and then follow the same
cooling curve.  So it seems at first glance difficult to believe that the
hot water freezes first.  The answer lies mostly in evaporation. The effect
is definitely real and can be duplicated in your own kitchen. 

	Every "proof" that hot water can't freeze faster assumes that the
state of the water can be described by a single number.  Remember that
temperature is a function of position.  There are also other factors
besides temperature, such as motion of the water, gas content, etc. With
these multiple parameters, any argument based on the hot water having to
pass through the initial state of the cold water before reaching the
freezing point will fall apart.  The most important factor is evaporation.

	The cooling of pails without lids is partly Newtonian and partly by
evaporation of the contents.  The proportions depend on the walls and on
temperature.  At sufficiently high temperatures evaporation is more
important.  If equal masses of water are taken at two starting
temperatures, more rapid evaporation from the hotter one may diminish its
mass enough to compensate for the greater temperature range it must cover
to reach freezing.  The mass lost when cooling is by evaporation is not
negligible. In one experiment, water cooling from 100C lost 16% of its mass
by 0C, and lost a further 12% on freezing, for a total loss of 26%. 

	The cooling effect of evaporation is twofold.  First, mass is
carried off so that less needs to be cooled from then on.  Also,
evaporation carries off the hottest molecules, lowering considerably the
average kinetic energy of the molecules remaining. This is why "blowing on
your soup" cools it.  It encourages evaporation by removing the water vapor
above the soup. 

	Thus experiment and theory agree that hot water freezes faster than
cold for sufficiently high starting temperatures, if the cooling is by
evaporation.  Cooling in a wooden pail or barrel is mostly by evaporation. 
In fact, a wooden bucket of water starting at 100C would finish freezing in
90% of the time taken by an equal volume starting at room temperature. The
folklore on this matter may well have started a century or more ago when
wooden pails were usual.  Considerable heat is transferred through the
sides of metal pails, and evaporation no longer dominates the cooling, so
the belief is unlikely to have started from correct observations after
metal pails became common. 

	"Hot water freezes faster than cold water.  Why does it do so?",
	Jearl Walker in The Amateur Scientist, Scientific American,
	Vol. 237, No. 3, pp 246-257; September, 1977.

	"The Freezing of Hot and Cold Water", G.S. Kell in American
	Journal of Physics, Vol. 37, No. 5, pp 564-565; May, 1969.
   

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-- 
 Christopher Neufeld....Just a graduate student  | Confidence is the feeling
 neufeld@helios.physics.utoronto.ca    Ad astra  | you have before you
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