The whichfreezesfirst enigma has been debated since at least the 17th century, when Sir Francis Bacon wrote about it. Even today, Canadians claim that a bucket of hot water left outdoors in cold weather will freeze faster than a bucket of cold water. Scientists, however, are still unable to explain why Canadians leave buckets of water outdoors in cold weather.
But believe it or not, hot water can freeze faster than cold water. Sometimes. It depends on a lot of things.
Intuitively, it seems impossible because the hot water simply has further to go in its downhill race toward 32 degrees Fahrenheit. In order to chill by each degree Fahrenheit, every pound of water has to lose a certain amount of heat: one Btu (British thermal unit). So the more degrees the water must fall, the more heat must be taken out of it, and all other things being equal, that means a longer cooling time.
But according to Wolke's Law of Pervasive Perversity, all other things are never equal. It turns out that hot and cold water are different in more ways than just their temperatures.
When cornered and pressed for an explanation of how hot water could possibly freeze faster, chemists are likely to prattle about cold water holding more dissolved air, and that all dissolved substances lower the freezing temperature of water. That's true, but trivial. I've calculated that the amount of dissolved air in cold tap water would lower its freezing temperature by less than a thousandth of a degree Fahrenheit, and no hotcold race can be controlled that precisely. The dissolvedair theory just doesn't hold water.
A real difference between hot and cold water is that the hotter a substance is, the faster it radiates its heat away into the surroundings; that is, it cools off at a faster rate -- more degrees per minute. But that still doesn't mean the hot water will reach the finish line first, because no matter how fast it begins to cool off, the best it can do is catch up with the cold water. After that, they're neck and neck.
A more significant difference between hot and cold water is that hot water evaporates faster than cold water. If we start with identical containers of the same amounts of hot and cold water, there will be less water remaining in the hotwater container when it gets down to rugcuttin' time at 32 degrees. And less water, naturally, can freeze in less time.
So there!
But can that really make a significant difference? Well, water is a very unusual substance in many ways. One of those ways is that we have to remove an unusually large amount of heat from it before its temperature will go down very much; (scientists say that water has a high heat capacity.) That one Btu per pound is a huge amount of heat energy. So even if the hot container loses only slightly more water by evaporation than the cold container does, it may require a lot less cooling, and hot wins the race.
Now don't go running into the kitchen to try it with icecube trays, because there are simply too many other factors operating. According to Wolke's Law, the two trays can never be identical. They are not in exactly the same place at exactly the same temperature, and they are not necessarily being cooled at the same rate.
Worse yet, how are you going to tell exactly when the water freezes? At the first skin of ice on the surface? That doesn't mean that the whole tray-full has yet reached 32 degrees. And you can't peek too often, because opening the freezer door can cause unpredictable air currents that affect the evaporation rates.
Most frustrating of all, undisturbed water has the annoying habit of getting colder than 32 degrees before it freezes. (It supercools.) It may then refuse to freeze until some largely unpredictable outside influence perturbs it, such as a vibration, a speck of dust, or a scratch on the inside surface of its container. In short, you're running a race with a very fuzzy finish line.
But I know that won't stop you. So go ahead and measure out exactly equal amounts of hot and cold water, record their temperatures, and put them in identical (ha!) freezer trays.
Please don't tell me what happens.