This question continues to generate considerable controversy. Takamasa Takahashi, a physicist at St. Norbert College in De Pere, Wis., attempts a definitive answer:
“Cold water doesn’t boil quicker than hot water. The rate of heating of a liquid is dependent upon the magnitude of the temperature difference between the liquid and its environment (the fire on the stove, for example ). Because of this, cold water will soon be absorbing heat quicker while it is still cold; after it becomes up to the warmth of hot water, the heating rate slows down and from that point it takes just as long to bring it to a boil since the water that was hot to start with. Because it requires cold water some time to make it to the temperature of warm water, cold water obviously takes longer to boil than hot water does. There may be some psychological effect at play; chilly water begins boiling earlier than you might expect because of the aforementioned higher 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 maybe under specific conditions.’ It takes 540 calories to vaporize 1 gram of water, whereas it takes 100 calories to bring 1 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 quite high because every evaporating g draws at least 540 calories from the water left behind. This is a really large amount of heat as well as the 1 calorie per Celsius degree that is drawn from every gram of water that cools by regular thermal conduction.
“It all is dependent upon how fast the cooling occurs, and it ends up that hot water will not freeze before chilly water but will freeze until 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 especially evident when the surface region which cools by accelerated evaporation is large 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 may freeze faster is when a bowl of cold water along with a pan of warm water of equivalent mass are placed in a freezer . There’s the effect of evaporation mentioned above, and also the thermal contact with an freezer will cool the base part of the entire body of water. If water is cold enough, close to four degrees C (the temperature at which water is densest), subsequently near-freezing water at the base increases 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 in the base is thicker than the warm water at the top, therefore no convection will occur and the base part will start freezing while the surface remains warm. This effect, combined with the evaporation effect, can make hot water freeze faster than cold water sometimes. In this case, of course, the freezer will probably have worked harder throughout the specified amount of time, extracting more heat from warm water.”
Robert Ehrlich of George Mason University, in Fairfax, Va., adds to some of the things made by Takahashi:
“There are just two ways in which hot water could freeze faster than cold water. One way [clarified in Jearl Walker’s book The Flying Circus of Physics (Wiley, 1975)] depends on the fact that warm water evaporates faster, so that if you began with equal masses of warm and cold water, then there will soon be less of the warm water to freeze, and therefore it might hamper the chilly water and freeze , because the lower the mass, the shorter the freezing time. The other way it may occur (in the case of a flat-bottomed dish of water set in a freezer) is if the hot water melts the ice under the bottom of the dish, then resulting in a superior thermal contact once it refreezes.”