This straightforward question continues to generate controversy. Takamasa Takahashi, a physicist at St. Norbert College in De Pere, Wis., tries a definitive response:
“Cold water doesn’t boil quicker than warm water. The rate of heating of a liquid depends on the magnitude of the temperature difference between the liquid and its environment (the flame on the stove, for instance). Because of this, cold water will soon be consuming heat faster while it’s still cold; once 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 as the water that was hot to begin with. As it takes cold water some time to make it to the temperature of warm water, cold water clearly takes longer to boil than warm water does. There can be some psychological impact 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 generally, but possibly under certain conditions.’ It takes 540 calories to vaporize 1 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 warmer than 80 degrees C, the rate of cooling by rapid vaporization is quite high because every evaporating gram draws at least 540 calories in the water left behind. This is a very large amount of heat compared with the 1 calorie per Celsius degree that’s drawn from each gram of water that cools by routine thermal conduction.
“It all depends on how fast the cooling happens, and it ends up that hot water won’t freeze before cold water but will freeze before lukewarm water. Water at 100 degrees C, for example, will freeze until water warmer than 60 degrees C but not before water cooler than 60 degrees C. This phenomenon is particularly evident once the surface area which cools by rapid evaporation is substantial compared with the amount of water required, like if 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 in which warm water may freeze quicker is when a bowl of cold water and a pan of warm water of equivalent mass are placed in a freezer compartment. There is the effect of evaporation mentioned above, and also the thermal contact with the freezer will cool the base region of the entire body of water. If water is cold enough, near four degrees C (the temperature where water is densest), then near-freezing water at the base increases 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 in the base is denser than the warm water at the top, so no convection will happen and the base part begins freezing while the surface remains warm. This effect, together with the evaporation impact, may make hot water freeze faster than cold water in some cases. In this case, of course, the freezer will probably have worked harder throughout the specified amount of time, extracting more heat from hot water.”
Robert Ehrlich of George Mason University, in Fairfax, Va., adds to some of the things created by Takahashi:
“There are just two methods by which warm water can freeze faster than cold water. 1 way [described in Jearl Walker’s book The Flying Circus of Physics (Wiley, 1975)] is determined by the fact that hot water disappears faster, so if you started with equal masses of hot and cold water, there would soon be of the warm water to freeze, and therefore it might overtake the chilly water and freeze , since the lesser the mass, the briefer the freezing time. The other way it could happen (in the case of a flat-bottomed dish of water set in a freezer) is when the warm water melts the ice below the bottom of the dish, then resulting in a superior thermal contact when it refreezes.”
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