I saw this one in elementary too. I think, given the explanation, the question is SUPPOSED to be, which /cools off/ faster. In that case hot water DOES cool faster according to the difference in temperature between the liquid and the air–basic themodynamics.
Hi Aubri; welcome to the Straight Dope. I’ve heard the same myth that was reported in the article (Which freezes faster, hot water or cold water?); namely, that ice cube trays filled with hot tap water will freeze quicker than ice cube tray filled with cold tap water.
I had suspected that the myth resulted from people who boiled water prior to freezing it (makes crystal clear cubes, I understand). I assume the evaporation rate of 212 degree water is pretty substantial.
You are correct, of course, that the cooling rate of hot water is greater than that of cold water. Wouldn’t the concept of “cooling rate” be a little advanced for elementary-school aged students, though?
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I’d heard this old chestnut a few years ago,so in the unflagging determination of a Straight Doper I performed a
not so unscientific investigation into one of the Great Mysteries Of Our Time.
For this epic undertaking I utilized an otherwise serviceable Amana ™ Refrigerator/Freezer. (It’s electric)
Additionally, I obtained the necessary H2O via an ordinary tap which dispenses this most excellent solvent via our Water Works. I must admit, given the urgency of the situation and other time constraints I elected to use a microwave oven (Brand name uncertain, I’m too lazy too check)to raise the temperature of the H20 to what a reasonable observer might agree as “boiling.”
To make an otherwise long story even longer, suffice to say that I found “hot” water does indeed reach a frozen state quicker than “cold” water, I suspect for reasons of evaporation. The net result, boys and girls, is smaller cubes.
If we were to repeat the experiment using, say, sealed Nalgene ™ bottles under similar conditions, perhaps we’d find out what really happens. Perhaps someone can loan me a walk in freezer?? 
I have a PhD in physics, yet my dad tried to tell me that zambonis use hot water on hockey rinks because hot water has a different molecular structure than cold water, which makes it freeze faster. “Mmmm,” I said.
It’s conceivable that zambonis might use hot water to partially melt and smooth the existing ice, I suppose…
Zambonis use a blade, HOT water and a squeegie to smooth out the ice rink.
just in case you wanna know.
http://www.zamboni.com/how.html
Nice link, fenrir. However, that page does not explain why they use hot water.
I think Karen’s skepticism wasn’t that zamboni’s use hot water, but that hot water has a different molecular structure.
Question: does phase state count as “molecular structure”?
There are, of course, two separate questions here:
- Are there certain conditions under which water at a given temperature may freeze faster than water at a lower temperature?
The answer to this question appears to be yes, but only under very specific conditions… The warmer water must be at or near the boiling point, the cooler water must be fairly close in temperature and both volumes of water must be in relatively shallow open containers. Under these conditions the warmer water may freeze faster due to evaporation. This, however, results in unequal volumes of frozen water and doesn’t prove anything except that a smaller volume of water freezes faster than a larger volume of water.
BTW, I think there may be another slightly different origin to this myth – which has been around for quite some time. Once upon a time there was no such thing as a frostless freezer, and all freezers had a thick layer of frost. An ice cube tray (metal and relatively conductive of heat in those days) filled with hot water would melt through some of the frost and therefore be closer to the cooling element of the freezer. More cooling = faster freezing.
2. Does hot water freeze faster than cold water?
Due to the observed ice cube tray phenomenon, it is a fairly common belief that hot water freezes faster than cold water. I can even remember back in the 70s when a plumber fixing a frozen pipe in our house made this very claim to my father. My father said, “bullshit” and the plumber replied with something along the lines of “don’t tell me my business unless you’re some kind of scientist.” My father was, at that time, chairman of the chemistry department at MIT, so we all had a good laugh at that one.
Anyway, the simple fact is this: at some point the warmer water will cool down to the starting temperature of the cooler water. At this point it becomes, for all intents and purposes, equal to the original volume of cooler water (unless you believe that warming water changes its fundamental nature). Unless one believes that two equal volumes of water at the same temperature will cool at different rates simply because one volume of water was previously at a higher temperature, there is no way hot water can freeze faster than cold water.
Well, as a matter of fact the hot water pipes do freeze sooner than the cold water pipes; on many occasions the hot water pipes froze and the cold water ones never did. And I don’t know why and I only know that I hope I never again live where any pipes freeze and the only ice I see is in the hockey rink or the glass of scotch in my hand. And maybe the plumber wasn’t a scientist but I betcha he saw a lot of frozen hot water pipes and no frozen cold water ones. So, how come?
Different amounts of flow. And, of course, disused hot-water pipes are no hotter than the cold-water pipes are.
here’s another link about hot and cold. I have to say a few years ago a roommate and i had the same arguement.Tried the experiment and the hot did freeze faster than the cold.
http://hepweb.rl.ac.uk/ppUK/PhysFAQ/hot_water.html
Regarding pipes:
First, it is a common practice to leave a faucet dripping to prevent freezing. Could it be that people leave the cold water dripping and not the hot water?
Also, as mentioned, the water in the hot water pipes will cool to ambient if not used, such as overnight. Thus, the pipes can freeze even though it’s the “hot” pipes. Demonstration of this principle is that when you turn on the hot water, it takes a few moments to get hot from the faucet. In winter, the water will run out cold until the new hot water is there. (I hate that.)
Third, I do not know this is the case, but could perhaps the hot water lines use smaller pipes than the cold water lines? This might be done because you have to heat in the water tank and thus hot water is a scarcer commodity than cold. Again, merely a possibility. Smaller pipes will freeze easier than larger ones.
It may have to do with the amount of dissolved gas. Cold water can hold more dissolved gas than hot water. Since it’s the same water coming in the house, though, that gas would have to go somewhere. Possibly small bubbles in suspension in the water, that eventually make their way up to the top of the pipe (assuming the pipe is running up & down, not side to side).
Another possibility is that there are fewer dissolved solids in hot water. I drain about 5 gallons of water from my hot water heater twice a year, and gets lots (well, maybe half a cup to a cup) of white sediment. Some kind of calcium compound, I’m told.
Is it possible that near-boiling water will freeze quicker in a freezer because the excessive heat causes the compressor to work a lot harder?
Well, to be technically correct, I think it’s best to say that the compressor works longer, not harder.
In in very small way, there will be some effect of the evaporator having to remove more heat from the icebox. But we’re talking very small things here.