This simple question continues to generate controversy. Takamasa Takahashi, a physicist at St. Norbert College in De Pere, Wis., tries a definitive answer:
“Cold water doesn’t boil quicker than hot water. The rate of heating of a liquid depends on the size of the temperature difference between the liquid and its environment (the flame on the stove, for example ). Because of this, cold water will soon be absorbing heat faster while it’s still cold; after it becomes up to the warmth of hot water, the heating rate slows down and from that point it takes as long to deliver it to a boil since the water that was hot to begin with. As it requires cold water a while to reach the temperature of hot water, cold water clearly takes longer to boil than warm water does. There may be some psychological impact at play; chilly water starts boiling sooner than one might expect due to 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 response is’Not generally, but maybe under specific conditions.’ It takes 540 calories to vaporize one g of water, whereas it requires 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 very high because every evaporating g draws at least 540 calories in the water left behind. This is a really large quantity of heat as well as the one calorie per Celsius degree that is drawn from each gram of water that cools by regular thermal conduction.
“It all is dependent upon how quickly the cooling happens, and it ends up that hot water will not freeze until cold water but will freeze until lukewarm water. Water at 100 degrees C, by way of instance, 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 that cools by accelerated evaporation is large compared with the amount of water involved, like when you wash a car with warm water on a cold winter day. [For reference, look at Conceptual Physics, by Paul G. Hewitt (HarperCollins, 1993).]
“Another situation where warm water may freeze quicker is when a bowl of cold water along with a pan of hot water of equal mass are placed in a freezer . There is the effect of evaporation mentioned previously, and also the thermal contact with an freezer shelf will cool the bottom region of the entire body of water. If water is cold enough, near four degrees C (the temperature where water is densest), subsequently near-freezing water in the base increases into the surface. Convection currents will continue until the entire body of water is 0 degrees C, at which point each of the water eventually freezes. If the water is hot, cooled water in the base is thicker than the warm water on very top, so no convection will happen and the base part will start 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, naturally, 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 points made by Takahashi:
“There are two ways in which warm water could freeze faster than cold water. 1 way [described in Jearl Walker’s novel The Flying Circus of Physics (Wiley, 1975)] depends on the fact that hot water evaporates faster, so that if you began with equivalent masses of warm and cold water, then there would soon be of the hot water to freeze, and hence it might hamper the chilly water and freeze , because the lower the mass, the shorter the freezing time. Another way it could happen (in the case of a flat-bottomed dish of water set in a freezer) is if the warm water melts the ice below the bottom of the dish, resulting in a superior thermal contact when it refreezes.”
Read more here: http://test.malmass.co.id/?p=9937