From: sichase@csa2.lbl.gov (SCOTT I CHASE)
\nSubject: Re: Hot water
\nFollowup-To: rec.martial-arts
\nDate: 25 Jul 92 06:12:10 GMT
\nOrganization: Lawrence Berkeley Laboratory – Berkeley, CA, USA
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\nReferences:
\nReply-To: sichase@csa2.lbl.gov
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In article , mbaya@antioc.antioch.edu writes…
\n>
\n>and is it also true that Hot\/boiling water will freeze faster than
\n>cold water? I know I heard this somewhere a long time ago. Why does it
\n>do this?
\n>
\nYes, under some conditions. This is in the sci.physics FAQ. Here is
\nthe appropriate text:<\/p>\n
********************************************************************************
\nItem 10.<\/p>\n
Hot Water Freezes Faster than Cold! updated 11-May-1992
\n———————————– original by Richard M. Mathews<\/p>\n
\tYou put two pails of water outside on a freezing day. One has hot
\nwater (95 degrees C) and the other has an equal amount of colder water (50
\ndegrees C). Which freezes first? The hot water freezes first! Why? <\/p>\n
\tIt is commonly argued that the hot water will take some time to
\nreach the initial temperature of the cold water, and then follow the same
\ncooling curve. So it seems at first glance difficult to believe that the
\nhot water freezes first. The answer lies mostly in evaporation. The effect
\nis definitely real and can be duplicated in your own kitchen. <\/p>\n
\tEvery “proof” that hot water can’t freeze faster assumes that the
\nstate of the water can be described by a single number. Remember that
\ntemperature is a function of position. There are also other factors
\nbesides temperature, such as motion of the water, gas content, etc. With
\nthese multiple parameters, any argument based on the hot water having to
\npass through the initial state of the cold water before reaching the
\nfreezing point will fall apart. The most important factor is evaporation.<\/p>\n
\tThe cooling of pails without lids is partly Newtonian and partly by
\nevaporation of the contents. The proportions depend on the walls and on
\ntemperature. At sufficiently high temperatures evaporation is more
\nimportant. If equal masses of water are taken at two starting
\ntemperatures, more rapid evaporation from the hotter one may diminish its
\nmass enough to compensate for the greater temperature range it must cover
\nto reach freezing. The mass lost when cooling is by evaporation is not
\nnegligible. In one experiment, water cooling from 100C lost 16% of its mass
\nby 0C, and lost a further 12% on freezing, for a total loss of 26%. <\/p>\n
\tThe cooling effect of evaporation is twofold. First, mass is
\ncarried off so that less needs to be cooled from then on. Also,
\nevaporation carries off the hottest molecules, lowering considerably the
\naverage kinetic energy of the molecules remaining. This is why “blowing on
\nyour soup” cools it. It encourages evaporation by removing the water vapor
\nabove the soup. <\/p>\n
\tThus experiment and theory agree that hot water freezes faster than
\ncold for sufficiently high starting temperatures, if the cooling is by
\nevaporation. Cooling in a wooden pail or barrel is mostly by evaporation.
\nIn fact, a wooden bucket of water starting at 100C would finish freezing in
\n90% of the time taken by an equal volume starting at room temperature. The
\nfolklore on this matter may well have started a century or more ago when
\nwooden pails were usual. Considerable heat is transferred through the
\nsides of metal pails, and evaporation no longer dominates the cooling, so
\nthe belief is unlikely to have started from correct observations after
\nmetal pails became common. <\/p>\n
References:
\n\t“Hot water freezes faster than cold water. Why does it do so?”,
\n\tJearl Walker in The Amateur Scientist, Scientific American,
\n\tVol. 237, No. 3, pp 246-257; September, 1977.<\/p>\n
\t“The Freezing of Hot and Cold Water”, G.S. Kell in American
\n\tJournal of Physics, Vol. 37, No. 5, pp 564-565; May, 1969.<\/p>\n
——————–
\nScott I. Chase “The question seems to be of such a character
\nSICHASE@CSA2.LBL.GOV that if I should come to life after my death
\n\t\t\t\tand some mathematician were to tell me that it
\n\t\t\t\thad been definitely settled, I think I would
\n\t\t\t\timmediately drop dead again.” – Vandiver<\/p>\n
