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VACETS Regular Technical Column

"Science for Everyone"

"Science for Everyone" was a technical column posted regularly on the VACETS forum. The author of the following articles is Dr. Vo Ta Duc. For more publications produced by other VACETS  members, please visit the VACETS Member Publications page or Technical Columns page.

The VACETS Technical Column is contributed by various members , especially those of the VACETS Technical Affairs Committe. Articles are posted regulary on vacets@peak.org forum. Please send questions, comments and suggestions to vacets-ta@vacets.org

September 26, 1994

HOT WATER FREEZES FASTER THAN COLD ???

If you live in the Midwest of the US or in Canada, on a freezing day in January, you put two pails of water outside. One has hot water (~95C) and one has cold water (~15C). Which one will freeze first? The answer is: if it is 18th century, the pail of hot water will probably freeze first; if it is Jan 1995 then the cold water probably will.

"A ha," someone may say, "I knew that the physical laws do change from time to time". Some others, "no way Jose, some thing must be wrong with the waters". And again someone else, "the cold water must freeze first in both cases"... Whoaa!!! Slow down, let me explain.

About 10 years ago, a friend of mine, who acted or seemed to act like he knew every subject in the universe, said that hot water would freeze faster than cold water. Being a nice guy and not wanting him to miss out an argument, I automatically took the opposite position. I don't remember who won the argument and didn't think about that for 10 years, until last week. While searching for a topic to write for this week's column, I remembered the 10 year old argument and decided to do and experiment to prove once and for all that my who-knew-it-all friend was wrong. After the experiment, I hate to say this but he was only half wrong. That who-knew-it-all friend of mine later went on to graduate school and got a Ph.D. in physics. So I guess that he did know a thing or two after all (I hate to admit this too).

The experiment was done at my favorite micro-biology-chemistry laboratory, my kitchen. The materials were tap-water (directly from the faucet), 2 identical glasses, and 6 foam cups. The stove and the freezer compartment of the refrigerator were used to heat and cool the water, respectively. The two glasses were filled with water, one with one cup (16 fluid ounces) of cold tap-water and one with one cup of boiling water. Each of the two triply stacked foam cups (i.e., each cup was made of three foam cups stacked together to get better side insulation) was also filled with a cup of water, one cold and one hot. All four containers were put in the freezer together. After one hour, the cold water in the glass started to freeze and it was completely frozen in five hours. The originally hot water in the glass started to freeze after two hours and was completely frozen in five and half hours. (Excellence, my friend was wrong here.) For the two triply stacked foam cups, the time was much longer. The cold water started to freeze after two hours. The hot water started to freeze after two and half hours. Four hours, five hours... it was time for bed. The results came the next day. After 14 hours, the originally hot water was completely frozen. The other cup was two hours later. Dang! Something must have gone wrong here. Did anyone try to screw up the experiment by taking the cup of cold water out of the freezer for few hours during the night? No one in the famous micro-biology-chemistry lab admitted of doing it, so the results were accepted, reluctantly. It is interesting to note that after letting the ice melt, the water in the four containers were re-measured and it was found that the cold water in both the glass and the cup lost few percents of it masses, the hot water in the glass lost about 10% and the hot water in the cup lost about 15%. The reason to let the ice melt before measuring was that there was not a mili-gram balance in the lab and a measuring cup was used instead.

The conclusion of the experiment was that my friend was wrong about the water in the glasses and was somewhat right (terrible, isn't it?) about the water in the foam cups.

Was the above experiment correctly and scientifically done? Correctness, may be. Scientific, may be not. Don't ask the exact time as how many seconds or mili-seconds it took for each container to completely freeze because the only instrument to measure time used in the experiment was the clock on the wall. And don't ask how or why the water froze, or the exact temperature of the air inside the freezer.

It seems difficult to believe that the hot water would freeze first (for the two foam cups). The answer can be found if one believes that the evaporation process does happen (yes, even in the freezer) and it takes away heat and carries off the water in the containers. The process is more profound for the hot water than the cold water. The cooling of the water in the containers is partly by heat conduction and partly by evaporation. The proportions depend on the walls of the containers and on the temperatures. If equal amounts of water are used at the beginning, more rapid evaporation from the hotter water may diminish its mass enough to compensate for the the greater temperature range it must cover to reach freezing and it may complete the freezing process before the cold water.

The glasses are good heat conductors and the heat conduction process was the dominant one. Therefore, the cold water freezes first. For the foam cups, the main cooling process is evaporation and it carries off enough water in the hot cup that even the cold water starts to freeze first, the hot water will complete the freezing faster.

So now everyone understands why the pail hot water would freeze faster than the cold one in 18th century and slower than the cold one in 20th century, don't you? No? It is because the pails in 18th century would be likely made of thick wood which is excellent insulator while the pails today are made of metal or thin plastic which is good heat conductor. Understand now? Yes, yes, yes...

Aaach oooh... I think I caught a cold from opening and closing the freezer's door 100 times on the week end. Aaach ooh!!!

-----------------------

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


Duc Ta Vo, Ph.D.
ducvo@lanl.gov

For discussion on this column, join vacets-tech@vacets.org


Copyright © 1996 by VACETS and Duc Ta Vo

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