As promised last week, here is the first of two old wives tales that are factual: Hot water can and frequently does freeze faster than colder water.  This phenomenon has been discussed for centuries; although, it wasn’t until 1969 that a Tanzanian high school student introduced it to the scientific community.  For his troubles, the phenomenon was named the “Mpemba Effect” after the student.

Since then, physicists have verified that hot water often freezes more quickly than cold water.  Scientists have not been able to definitively explain why this happens, but a few phenomena are thought to contribute to the effect:

Evaporation—Hot water evaporates faster than cold water and brings away steam (water), thereby reducing the amount of liquid to be cooled.  Less ice is created, but it is made at a faster rate.  This can only be a partial explanation, however, because studies of completely enclosed containers have also exhibited the Mpemba effect.

Convection—As the water cools, its temperature does not remain uniform throughout.  The swirling around of the hotter and cooler sections may contribute to faster cooling.

Dissolved Gases—Hot water holds fewer amounts of dissolved gases than cold water.  These gases may prevent convection or change the temperature necessary to freeze the liquid.

Container and Surroundings—The container in which the water is held, as well as the surface or the surroundings in which it is placed, may also affect the cooling process differently at different temperatures.

Interesting fact: Many ice cream makers use warm milk to take advantage of the Mpemba effect.

Another interesting fact: Cecil Adams of the Straight Dope basically got this one wrong!

 

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