Hot Water Freezing?

The rumor persists that hot water will freeze faster than cold water. It sounds implausible because the hot water has more internal energy which must be removed before it can start the phase change. Perhaps this tale had its origin in the days of non-frost-free refrigerators in which a considerable buildup of ice on the walls of the freezing compartment was a common occurrence. Ice acts as an insulator, inhibiting the cooling process.

Another scenarioThe Mpemba effect
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What about snap-freezing?

Another scenario suggested for the "hot water freezing" story is the possibility of supercooling and snap-freezing by the hot water.

The Mpemba effect
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The Mpemba Effect

There is in fact a large body of literature about initially hot water freezing faster than cold water and this phenomenon has been labeled the "Mpemba effect" (see references). The literature cited here takes the position that the phenomenon of supercooling is the main mechanism involved and that the history (dissolved gases, etc.) are not crucial to the phenomenon. Supercooling to -5 C or lower is cited and it is suggested that both the initially hot and the initially cold water supercool. If the cold water supercools more, then the the hot water may in fact freeze quicker with nothing more than the degree of supercooling to distinguish the two samples. It was also pointed out that the degree of supercooling is not easily predictable, since it probably depends upon tiny particles or bubbles which nucleate the crystal formation. Experiments showed different amounts of supercooling for the same sample when it was repeatedly liquified and refrozen.

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Cooling Coffee

Leave it black for a while, then add cream.

The intent is to get the coffee cool enough to drink in the shortest possible time. The heat loss rate by conduction, convection, and radiation all depend upon the difference in temperature between the hot object and its surroundings. Therefore the cooling rate is greater if you don't immediately add cream.

An often cited but bogus reasoning in this example is to say that black objects radiate better than white ones. This is true for visible light, but this radiation is in the infrared where the visible color is not a factor.
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Cooling on very hot day?

The standard heat transfer mechanisms act against you, transferring heat into your body on a very hot day. The main cooling mechanism you have available is the evaporation of perspiration. Fortunately, it is a very effective mechanism because of the large heat of vaporization of water. Note that you must continue to have a net outflow of about 90 watts, even on a hot day.

What if ambient is below body temperature?Cooling mechanisms for the body
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Heat Flow to Hotter Region

Although internal energy will not spontaneously flow from a hot region to a cold region, it can be forced to do so by doing work on the system. Refrigerators and heat pumps are examples of heat engines which cause energy to be transferred from a cold area to a hot area. Usually this is done with the aid of a phase change, i.e., a refrigerant liquid is forced to evaporate and extract energy from the cold area. Then it is compressed and forced to condense in the hot area, dumping its heat of vaporization into the hot area.

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