This seemingly simple question continues to generate considerable controversy. Takamasa Takahashi, a physicist at St. Norbert College in De Pere, Wis., attempts a definitive answer:
"Cold water does not boil faster than hot water. The rate of heating of a liquid depends on the magnitude of the temperature difference between the liquid and its surroundings (the flame on the stove, for instance). As a result, cold water will be absorbing heat faster while it is still cold; once it gets up to the temperature of hot water, the heating rate slows down and from there it takes just as long to bring it to a boil as the water that was hot to begin with. Because it takes cold water some time to reach the temperature of hot water, cold water clearly takes longer to boil than hot water does. There may be some psychological effect at play; cold water starts boiling sooner than one might expect because of the aforementioned greater heat absorption rate when water is colder.
"To the first part of the question--'Does hot water freeze faster than cold water?'--the answer is 'Not usually, but possibly under certain conditions.' It takes 540 calories to vaporize one gram of water, whereas it takes 100 calories to bring one gram of liquid water from 0 degrees Celsius to 100 degrees C. When water is hotter than 80 degrees C, the rate of cooling by rapid vaporization is very high because each evaporating gram draws at least 540 calories from the water left behind. This is a very large amount of heat compared with the one calorie per Celsius degree that is drawn from each gram of water that cools by regular thermal conduction.
"It all depends on how fast the cooling occurs, and it turns out that hot water will not freeze before cold water but will freeze before lukewarm water. Water at 100 degrees C, for example, will freeze before water warmer than 60 degrees C but not before water cooler than 60 degrees C. This phenomenon is particularly evident when the surface area that cools by rapid evaporation is large compared with the amount of water involved, such as when you wash a car with hot water on a cold winter day. [For reference, look at Conceptual Physics, by Paul G. Hewitt (HarperCollins, 1993).]
"Another situation in which hot water may freeze faster is when a pan of cold water and a pan of hot water of equal mass are placed in a freezer compartment. There is the effect of evaporation mentioned above, and also the thermal contact with the freezer shelf will cool the bottom part of the body of water. If water is cold enough, close to four degrees C (the temperature at which water is densest), then near-freezing water at the bottom will rise to the top. Convection currents will continue until the entire body of water is 0 degrees C, at which point all the water finally freezes. If the water is initially hot, cooled water at the bottom is denser than the hot water at the top, so no convection will occur and the bottom part will start freezing while the top is still warm. This effect, combined with the evaporation effect, may make hot water freeze faster than cold water in some cases. In this case, of course, the freezer will have worked harder during the given amount of time, extracting more heat from hot water."
Robert Ehrlich of George Mason University, in Fairfax, Va., adds to some of the points made by Takahashi:
"There are two ways in which hot water could freeze faster than cold water. One way [described in Jearl Walker's book The Flying Circus of Physics (Wiley, 1975)] depends on the fact that hot water evaporates faster, so that if you started with equal masses of hot and cold water, there would soon be less of the hot water to freeze, and hence it would overtake the cold water and freeze first, because the lesser the mass, the shorter the freezing time. The other way it could happen (in the case of a flat-bottomed dish of water placed in a freezer) is if the hot water melts the ice under the bottom of the dish, leading to a better thermal contact when it refreezes."
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Add CommentIt's true cold water will not boil faster than hot water; of course hot water has a "head start". So should we cook with hot water? NO! This is an "Old Wives Tale" that was meant to remind people to use cold water - you see, hot water that's been sitting in a hot water tank for perhaps days can be far less fresh than cold water! So for better tasting food (and coffee) always use cold water.
Reply | Report Abuse | Link to thisHot water freezes faster than cold water because when hot water is boiled it takes away something called gas bubbles.Then it causes an easier way for it to catch to the same tempature as the cold water when the cold water has gas causing it to slow down because the gas bubbles causes the waters temperature to get higher than it actually decreasing.
Reply | Report Abuse | Link to thisIm adding a Question. What about water in a pipe or tube not touching any surface area? will a hot water line freeze before a cold water line?
Reply | Report Abuse | Link to thisThere is no room for expansion if the water isn't dripping, and not moving , so is there a difference under these conditions?
Even hot water in a container (where no evaporation will occur) will freeze quicker than cold water in a container. This is called the Mpemba effect and it is not just because of evaporation or dissolved gasses. In fact, dissolved gasses make no significant difference.
Reply | Report Abuse | Link to thisThe main problem with this question is that there are so many variables that come in to play, such as starting temperature and surroundings. But under most circumstances, the answer should not be "Not usually, but possibly" it should be "Usually, but sometimes doesn't". This article is only considering one factor.
Another factor is:
Convection Currents. Go here to learn more about it: http://www.desy.de/user/projects/Physics/General/hot_water.html#Convection
My son is in 7th grade, and did this experiment for his science fair project. With no walk in freezer, he used a digital weather thermometer with a remote digital readout and clock and a wide range to measure the water temperature. He set up his experiment so that the probe measured 1cm into the water, and used a regular ice tray on a wire rack (to avoid contact with the freezer surface as a variable). He used a total volume of 450ml with each trial (boiling vs room temperature start points). To collect his data, he used a digital camera, and sat for hours snapping a photo every one minute for each trial (to ensure accurate documentation, and be there at the moment the water reached solid). Plotting his data points revealed the boiling water freezes solid faster than the room temperature water (multiple trials got similar results). Water that was boiled, then cooled to room temp before freezing did not freeze faster than plain room temp water. One observation was at 0˚C, water forms a layer of ice on the surface. Another observation was that the boiling water took an average of 39 minutes longer to reach 0˚C than room temp water. Before the boiling water reached the 0˚C point, it cooled to a lower average temperature than the room temperature water before the ice layer formed. The room temp water reached 0˚C, formed the ice layer, and stalled in its further cooling. Probably because the ice layer forms an insulation layer between the water and the freezer air. It's possible that because the boiling water got cooler before this ice insulating layer formed, it had less heat to lose, and froze faster as a result.
Reply | Report Abuse | Link to thisI am not going to claim to know all the basis to it but I have a theory. Also, would be nice if I had equipment to test this but I do not. Seems to me that the basis for this phenomenon can only be done by a controlling your base factors. Water shouldnt not be out of hot or cold tap water to make it a fair variable... It should be distilled to assure that you are getting a same medium for the base of the experiment... It was mentioned in another comment about water that sits overnight and whatnot in water heaters and that is accurate. Also, you should have same potency, direction and room size for both samples. First 2 are obvious to why they are needed... the third would be a buffer affect that your surrounding space would have in regards to dropping the room and sample temperature at desired rates. If the room is bigger it will get cold slower.
Reply | Report Abuse | Link to thisNow my theory to why it will freeze faster. To me it comes down to the transfer rate of heat and the spacing of the water molecules in respect to inertia. When your hot water is affected by the removing of heat it will start to cool right away due to the fact of course it is way hotter then cold and hot water molecules should be spaced farther apart leaving more access to the removal of heat. Inertia would leave us to believe once you start a movement or a transfer of energy that it will tend to stay in motion at a rate that I am sure can be mathematically defined.
The cold sample however will need to room temperature to drop to the water temperature before it will start to affect it. This causes a sort of insulation affect in the sense that it will take more time for it to affect the cold water because one at this point no transfer is occuring and 2 tighter molecules more resistance and yes they will condense more before they expand to ice, just not in a way that is openly visible to normal perception. Transfer of heat will then be in progress but do to delay caused by the lack of earlier reaction it will have a much slower transfer rate. Your formula should look something like Freeze time= Freeze Difference/Freeze Rate. With the exeption of the need of a few readings here and there for The Freeze Rate and the the molecular compaction and whatnot you should be able to create a proper temp verses rate of freezing formula. I unfortunately do not have the scope to record molecular compaction and shifting at the change of temp or the nec area to do this experiment. Formula should be something like:
time = Temp/Y*Ri oversimplified but that is just the idea :).
I didnt mean to simplify it so drastically but I am very tired and I got the explination out. Basically to run this experiment you would need to pick your Y Variable which would be your break up of the time you have whilst you are waiting for both to achieve 1Degree Celcius. Then you would need to take your rate reading and compare time overall and time for specific temperatures on both to see if there was a difference in rate at specific temp. Also, also pay close attention to the temp of your hot water when it makes it to the original temp of the cold. Might find that there is a slight difference there in regards to the hot water cooling down throught there way faste then the cold water temp did. As I said before this is all speculation for me and I wish I had more time and equipment to go through it. Enjoy, thoughts appreciated
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