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Trivia: Hot Water Freezes Faster Than Cold Water

Hot water freezes faster than cold water. It was (re)discovered by a Tanzanian high school student named Erasto Mpemba, so the phenomenon is called the Mpemba effect.

Counter-intuitive, isn't it? Read more about why hot water freezes faster than cold water here. [Update 9/28/07: technically, hot water freezes first (it actually forms ice at a higher temperature than cold water), but cold water freezes faster (it takes less time to reach the supercooled state, from which it forms ice) - to say the least, it's complicated... ]

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It is NOT wrong or poor observations or bad conclusions. It WILL happen under CERTAIN circumstances, for one or more of the following reasons:

1. Good conduction and good contact
One theory is that frost on a container can slow down the cooling process. If hot water is placed in the freezer in a small container that is a good conductor of heat (or cold), the warmth of the container can melt any frost that collects on its surface. This includes the ice on the bottom surface. When this ice refreezes, it creates a good connection between the container and the surface, allowing much better conduction of cold than a container of cold water that has frost on its surface, including its bottom. As a result, heat is drawn out of the warmer container more rapidly than the one with cold water in it.

2.Convection and insulating surface
Just as a layer of frost on the surface of a container can slow down the conduction of heat from the water, a layer of ice on the upper surface of the water can insulate the water from the from the colder air currents. Since water becomes less dense between 37oF and 32oF (3oC and 0oC), it will float to the top and then finally freeze. This thin layer of ice will then act as an insulator protecting the water below from freezing rapidly and will slow down the freezing process of cold water. In the case of warm water, the convection currents will cause that ice to melt, allowing the water to cool more rapidly.

It is thought that evaporation is one factor that allows warm water to freeze faster than cold water. There is more evaporation from warm or hot water than from cold water. Thus the evaporation not only carries off some of the water, resulting in slightly less water to freeze in the warm water container, but it also causes the temperature of the warm container to drop due to heat lost.

4. Poor conduction and evaporation
If the container is made of something like wood, which is a poor conductor of heat (or cold), then a great part of the cooling will be caused by evaporation instead of conduction. This could be an important factor in explaining how hot water freezes faster than cold. For example, Mpemba used wooden buckets when he was making his ice cream and noticed the phenomenon.

5. Dissolved gasses
One more possible factor concerns that fact that water always contains dissolved gases such as oxygen and carbon dioxide. These impurities have the effect of lowering its freezing point. When water is heated, gases are driven out because their solubility in water is less at higher temperatures. Thus, when the hot water cools, it has less dissolved gas than water which was not heated, so it has a higher freezing point and freezes first.

I hope this provides as full an answer as possible.
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This experiment works because the hot water rapidly melts the ice on which it is sat, forming a very good liquid coupling from the ice to the vessel. This then freezes quickly into a tight join, which conducts the heat away from the hot liquid MUCH more. The water in the "hot" vessel with the coupled base loses heat maybe 1000 times faster than the "cold" vessel sitting on a nice insulating layer of fluffy snow, and can in this instance, overtake it and freeze first.

People who run barefoot over hot coals (1200 degrees C) look ever so impressive and empowered, but I bet they can't walk over a hotplate at a mere 70 degrees C if it has a layer of oil over it - same insulation / conduction phenomenon.

BTW that wikipedia entry is a blatant attempt to technobamboozle laypeople. "Continuum mechanics", "partial differential equations" and CFD's trouble with turbulent flow has absolutely nothing to do with this; they are mathematical techniques used to describe or solve known heat transfer problems, they are not physical phenomenon that we don't understand.
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