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... ]
Most of the time the explanation for the alleged observation is as simple as the fact that given two exactly identical "quantities" of hot and cold water, there is less hot water than cold water.
In other words, it is not true that hot freezes before cold ... what is true is that people sometimes make poor observations and draw incorrect conclusions.
For what it's worth, water that's been heated up AND ALLOWED TO COOL BACK DOWN to a comparable temperature will freeze faster than water of that temperature that was never heated, but that's only because heating the water removes dissolved gases. It also provides you with far less cloudy ice.
But I stand by that the Mpemba effect exists, although physicists are unsure of the reason why. Here's an article from Physics World for all you non-believers, and I quote:
In 1995 German physicist David Auerbach at the Max Planck Institute for Fluid Dynamics in Göttingen looked at the role of supercooling in the Mpemba effect. But what he found only made things more complicated. He observed that hot water froze at a higher temperature than cold and therefore in a sense froze "first". However, the cold water took less time to reach its supercooled state and so seemed to freeze "faster". To add to the confusion, earlier researchers had reported the opposite: that initially hot water could be supercooled to lower temperatures than cold water. In 1948 Noah Dorsey of the US National Bureau of Standards argued that this is because heating expels impurity particles that acted as nucleation sites for ice. It has been claimed that this effect leads to hot-water pipes bursting more readily than cold, since deeper supercooling leads to ice fingers that advance right across the pipe and block the flow, while freezing nearer to 0 °C just produces a sheath of ice on the pipe surfaces with an open channel in the centre.
Absolutely the opposite, mate!
To greg laden: In other words, it is not true that hot freezes before cold … what is true is that people sometimes make poor observations and draw incorrect conclusions.
This experiment had been done carefully by physicists, who discovered the existence of the phenomenon but has yet to understand its cause.
To Doctor Fedora: For what it’s worth, water that’s been heated up AND ALLOWED TO COOL BACK DOWN to a comparable temperature will freeze faster than water of that temperature that was never heated, but that’s only because heating the water removes dissolved gases.
From my understanding, this experiment (conducted by Logan McCarty, a chemist at Harvard University) still used hot water and cold water - not hot water that's allowed to cool back down. His conclusion was that gases were responsible for the effect (i.e. degassed cold water freeze first).
To saen: Read the article and the reference articles. This headline is inaccurate sensationalism, but the point is that VERY hot water (90c-195f) does freeze faster than hot water (80c-175f).
That may be so, but there were a lot of experiments that used hot water (90° C) and cold water (18° C). See: D. Auerbach, Supercooling and the Mpemba effect; when hot water freezes quicker than cold, Am. J. Phys. 63 (1995) 882-885. Link
One with hot, the other with cold.
Put them in your freezer.
The cold one will turn to ice first.
We had a bet on this at work. The cold cup turned to ice first.
phenomenon my ass!
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.
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.
3.Evaporation
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.