91 lines
4.3 KiB
Plaintext
91 lines
4.3 KiB
Plaintext
From: sichase@csa2.lbl.gov (SCOTT I CHASE)
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Subject: Re: Hot water
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Followup-To: rec.martial-arts
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Date: 25 Jul 92 06:12:10 GMT
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Organization: Lawrence Berkeley Laboratory - Berkeley, CA, USA
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Lines: 75
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Distribution: na
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Message-ID: <24887@dog.ee.lbl.gov>
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References: <1992Jul20.174611.28999@uwm.edu> <59140@mimsy.umd.edu> <1992Jul22.095347.16@antioc.antioch.edu>
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Reply-To: sichase@csa2.lbl.gov
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NNTP-Posting-Host: 128.3.254.197
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News-Software: VAX/VMS VNEWS 1.3-4
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In article <1992Jul22.095347.16@antioc.antioch.edu>, mbaya@antioc.antioch.edu writes...
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>
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>and is it also true that Hot/boiling water will freeze faster than
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>cold water? I know I heard this somewhere a long time ago. Why does it
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>do this?
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>
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Yes, under some conditions. This is in the sci.physics FAQ. Here is
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the appropriate text:
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********************************************************************************
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Item 10.
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Hot Water Freezes Faster than Cold! updated 11-May-1992
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----------------------------------- original by Richard M. Mathews
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You put two pails of water outside on a freezing day. One has hot
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water (95 degrees C) and the other has an equal amount of colder water (50
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degrees C). Which freezes first? The hot water freezes first! Why?
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It is commonly argued that the hot water will take some time to
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reach the initial temperature of the cold water, and then follow the same
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cooling curve. So it seems at first glance difficult to believe that the
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hot water freezes first. The answer lies mostly in evaporation. The effect
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is definitely real and can be duplicated in your own kitchen.
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Every "proof" that hot water can't freeze faster assumes that the
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state of the water can be described by a single number. Remember that
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temperature is a function of position. There are also other factors
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besides temperature, such as motion of the water, gas content, etc. With
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these multiple parameters, any argument based on the hot water having to
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pass through the initial state of the cold water before reaching the
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freezing point will fall apart. The most important factor is evaporation.
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The cooling of pails without lids is partly Newtonian and partly by
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evaporation of the contents. The proportions depend on the walls and on
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temperature. At sufficiently high temperatures evaporation is more
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important. If equal masses of water are taken at two starting
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temperatures, more rapid evaporation from the hotter one may diminish its
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mass enough to compensate for the greater temperature range it must cover
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to reach freezing. The mass lost when cooling is by evaporation is not
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negligible. In one experiment, water cooling from 100C lost 16% of its mass
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by 0C, and lost a further 12% on freezing, for a total loss of 26%.
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The cooling effect of evaporation is twofold. First, mass is
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carried off so that less needs to be cooled from then on. Also,
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evaporation carries off the hottest molecules, lowering considerably the
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average kinetic energy of the molecules remaining. This is why "blowing on
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your soup" cools it. It encourages evaporation by removing the water vapor
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above the soup.
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Thus experiment and theory agree that hot water freezes faster than
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cold for sufficiently high starting temperatures, if the cooling is by
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evaporation. Cooling in a wooden pail or barrel is mostly by evaporation.
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In fact, a wooden bucket of water starting at 100C would finish freezing in
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90% of the time taken by an equal volume starting at room temperature. The
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folklore on this matter may well have started a century or more ago when
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wooden pails were usual. Considerable heat is transferred through the
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sides of metal pails, and evaporation no longer dominates the cooling, so
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the belief is unlikely to have started from correct observations after
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metal pails became common.
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References:
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"Hot water freezes faster than cold water. Why does it do so?",
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Jearl Walker in The Amateur Scientist, Scientific American,
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Vol. 237, No. 3, pp 246-257; September, 1977.
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"The Freezing of Hot and Cold Water", G.S. Kell in American
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Journal of Physics, Vol. 37, No. 5, pp 564-565; May, 1969.
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--------------------
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Scott I. Chase "The question seems to be of such a character
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SICHASE@CSA2.LBL.GOV that if I should come to life after my death
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and some mathematician were to tell me that it
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had been definitely settled, I think I would
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immediately drop dead again." - Vandiver
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