The AFU and Urban Legend Archive
Science
 hot water freezes faster




From: sichase@csa2.lbl.gov (SCOTT I CHASE)
Subject: Re: Hot water
Date: 25 Jul 92 06:12:10 GMT

In article <1992Jul22.095347.16@antioc.antioch.edu>, mbaya@antioc.antioch.edu writes... >
>and is it also true that Hot/boiling water will freeze faster than
>cold water? I know I heard this somewhere a long time ago. Why does it
>do this?
>
Yes, under some conditions. This is in the sci.physics FAQ. Here is the appropriate text:

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.

References: 

        "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.

Scott I. Chase                  "The question seems to be of such a character
SICHASE@CSA2.LBL.GOV            that if I should come to life after my death
                                and some mathematician were to tell me that it
                                had been definitely settled, I think I would
                                immediately drop dead again."      - Vandiver



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