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Hot water can freeze more quickly than cold water. However, it does not always happen, nor has science explained exactly why it can happen.
Although Aristotle, Bacon, and Descartes all described hot water freezing faster than cold water, the notion was mostly resisted until the 1960's when a high school student named Mpemba noticed that hot ice cream mix, when placed into the freezer, would freeze before ice cream mix that had been cooled to room temperature before being placed in the freezer. Mpemba repeated his experiment with water rather than ice cream mixture and found the same result: the hot water froze more quickly than the cooler water. When Mpemba asked his physics teacher to explain the observations, the teacher told Mpemba his data must be in error, because the phenomenon was impossible.
Mpemba asked a visiting physics professor, Dr. Osborne, the same question. This professor replied that he did not know, but he would test the experiment. Dr. Osborne had a lab tech perform Mpemba's test. The lab tech reported that he had duplicated Mpemba's result, "But we'll keep on repeating the experiment until we get the right result." Well, the data is the data, so when the experiment was repeated, it continued to yield the same result. In 1969 Osborne and Mpemba published the results of their research. Now the phenomenon in which hot water may freeze faster than cold water is sometimes called the Mpemba Effect.
Why Hot Water Sometimes Freezes Faster Than Cold Water
There is no definitive explanation for why hot water may freeze faster than cold water. Different mechanisms come into play, depending on the conditions. The main factors appear to be:
- Evaporation - More hot water will evaporate than cold water, thus reducing the amount of water remaining to be frozen. Mass measurements lead us to believe this is an important factor when chilling water in open containers, though it isn't the mechanism that explains how the Mpemba Effect occurs in closed containers.
- Supercooling - Hot water tends to experience less of a supercooling effect than cold water. This makes it more likely to become solid when it reaches the freezing point of water.
- Convection - Water develops convection currents as it cools. Water density usually decreases as temperature increases, so a container of cooling water typically is warmer on top than on the bottom. If we assume water loses most of its heat across its surface (which may or may not be true, depending on the conditions), then water with a hotter top would lose its heat and freeze faster than water with a cooler top.
- Dissolved Gases - Hot water has less capacity to hold dissolved gases than cold water, which may affect its rate of freezing.
- Effect of the Surroundings - The difference between the initial temperatures of two containers of water may have an effect on the surroundings that could influence the rate of cooling. One example would be warm water melting a pre-existing layer of frost, permitting a better cooling rate.
Test It Yourself
Now, don't take my word for this! If you are doubtful that hot water sometimes freezes more quickly than cold water, test it for yourself. Be aware the Mpemba Effect will not be seen for all experimental conditions, so consult the references in this post to see what might work best for you (or try making ice cream in your freezer, if you'll accept that as a demonstration of the effect).