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." �
Still feeling skeptical? Fred W. Decker, a meteorologist at Oregon State University in Corvallis, encourages readers to settle the question for themselves:
"You can readily set up an experiment to learn which freezes earlier: water that is initially hot, or water that is initially cold. Use a given setting on an electric hot plate and clock the time between start and boiling for a given pot containing, say, one quart of water; first start with the water as cold as the tap will provide and then repeat it with the hottest water available from that tap. I'd wager the quart of water initially hot will come to a boil in much less time than the quart of water initially cold.
"The freezing experiment is harder to perform, because it ideally requires a walk-in cold storage chamber that is set to a temperature below freezing. Take into the chamber two quart-volume milk bottles filled with water, one from a hot tap and the other from a cold tap outside the chamber. Time them to freezing, and I would wager again that the initially colder water will freeze sooner than the initially hot water."
[We would add that, if you don't want to suffer in a walk-in freezer, you can conduct a reasonably good version of the above experiment in the freezer compartment of your refrigerator; just don't check the water too often-in which case it will never freeze-or too infrequently, in which case you may miss the moment when one container is frozen but not the other.]
Decker concludes that "much folklore results from trying to answer such a question under conditions that do not make 'all other things equal,' which the foregoing experiments do.