Is It True that Hot Water Freezes Faster than Cold Water or that Cold Water Boils Faster than Hot Water?
This question continues to generate controversy. Takamasa Takahashi, a physicist at St. Norbert College in De Pere, Wis., attempts a definitive answer:
«Cold water does not boil faster than warm water. The rate of heating of a liquid depends on the size of the temperature difference between the liquid and its environment (the fire on the stove, for instance). As a result, cold water will be absorbing heat faster while it’s still cold; after it gets up to the temperature of hot water, then the heating rate slows down and from that point it takes as long to bring it to a boil since the water that was warm to begin with. Because it requires cold water some time to make it to the temperature of warm water, cold water clearly takes longer to boil than hot water does. There may be some psychological effect at play; cold water starts boiling earlier than you 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 response is’Not generally, but possibly under specific conditions.’ It requires 540 calories to vaporize one g of water, whereas it takes 100 calories to deliver 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 with rapid vaporization is quite high because each evaporating gram draws at least 540 calories from the water left behind. This is a very large amount of heat as well as the 1 calorie per Celsius degree that is drawn from every gram of water which cools by routine thermal conduction.
«It all depends on how fast the cooling occurs, and it turns out that hot water will not freeze before chilly water but will freeze before lukewarm water. Water at 100 degrees C, for instance, will freeze until water warmer than 60 degrees C but not until water cooler than 60 degrees C. This phenomenon is particularly evident when the surface region that cools by rapid evaporation is substantial compared with the total amount of water involved, like when you wash a vehicle with warm water on a chilly winter day. [For reference, consider Conceptual Physics, by Paul G. Hewitt (HarperCollins, 1993).]
«Another situation where hot water can freeze quicker is when a pan of cold water along with a pan of hot water of equivalent mass are set in a freezer . There is the effect of evaporation mentioned previously, and also the thermal contact with an freezer will cool the bottom region of the entire body of water. If water is cold enough, near four degrees C (the temperature at which water is densest), subsequently near-freezing water at the bottom will rise into the surface. Convection currents will last until the entire body of water is 0 degrees C, at which point all the water eventually freezes. If the water is initially hot, cooled water at the base is thicker than the hot water at the top, therefore no convection will occur and the bottom part begins freezing while the top is still warm. This effect, combined with the evaporation effect, can create hot water freeze faster than cold water in some cases. In cases like this, of course, the freezer will have worked harder during the given time period, extracting more heat from warm water.»
Robert Ehrlich of George Mason University, in Fairfax, Va., adds to some of the things created by Takahashi:
«There are two methods by which warm water can freeze faster than cold water. One way [clarified in Jearl Walker’s novel The Flying Circus of Physics (Wiley, 1975)] is determined by the fact that warm water disappears faster, so if you started with equivalent masses of warm and cold water, then there will soon be less of the warm water to freeze, and hence it might overtake the chilly water and freeze first, since the lower the bulk, the shorter the freezing time. Another way it may occur (in the case of a flat-bottomed dish of water placed in a freezer) is when the hot water melts the ice under the base of the dish, then resulting in a better thermal contact once it refreezes.»
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