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Question:
I should know the answer to this question. So should the physics and chemistry teachers at our school, but they disagree. I have heard two, perfectly good explanations with different "facts". Here goes..... DOES hot water freeze faster than cold water? (You were expecting something really good were not you?) If so, why? If not, why? Somebody's gonna have to eat crow. (I am glad that I did not express my opinion - but I will bet I am right!)



Replies:
First, I think scientifically you should qualify your 'hot' or 'cold' description by temperature, in any scale of your choice (i.e. Fahrenheit, Celsius, or Kelvin). Given that, however, I will assume that your 'hot' water is higher in temperature than your 'cold' water. Assuming no dissolved impurities which might other- wise affecting the freezing (melting) point of said water, I will place my bet and say that since the 'hot' water, by definition contains more energy and therefore more molecular motion, it would freeze more slowly (i.e. would require more cooling to get down to the freezing point) than the water you describe as 'cold'. The 'cold' water , containing less energy (by definition) and therefore less molecular motion should require less cooling to get to the freezing point. I would like to hear both sides of the opinions of your faculty.

rickru


Yes, hot water in a freezer freezes fast than cold water, as every competent houseperson (formerly,"housewife") knows. The reason is that the surface of the freezer usually has a layer of ice on it. Ice is an excellent insulator, and therefore limits the heat transfer to the freezing surface. Putting hot water in your pan (or ice-cube tray) has the effect of melting the ice-layer on the surface, providing for better heat transfer to the surface. There is also the fact that there is increased evaporation from the warm water, which reduces the mass of water that must be cooled, as noted in item 3.40 of Jearl Walker's book "The Flying Circus of Physics." That book is the first place to look for answers to questions of the type that you posed.

Jack L. Uretsky


Jack L. Uretsky, You will recall that the question was does hot water freeze faster than cold water. I don't recall it mentioning a freezer, nor a coating of ice, nor allowing for escape of some of the targeted hot water to freeze. I maintain my answer as correct, given the question that was asked. Naturally if heat is allowed to escape from the system, either thru melting of a supposed coating of ice or warming of some freezer space, than your assertion might change the scenario. In a laboratory with a controlled experiment where all the heat is accounted for, I believe you will find my answer to be the correct one , i.e. cold water will freeze faster than an equivalent quantity of hot water.

Rick



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