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Does hot water freeze faster than cold water?

up vote 14 down vote favorite

Is it true that hot water freezes faster than cold water and if so, what practical applications have there been found for this phenomenon?

2 Answers

up vote 4 down vote accept

In certain settings, cold water freezers slower than hot water. This is called the Mpemba effect.

The Mpemba effect is the observation that warmer water sometimes freezes faster than colder water. Although the observation has been verified, there is no single scientific explanation for the effect.

Can hot water freeze faster than cold water?, Monwhea Jeng, University of California, 1998

Hot water can in fact freeze faster than cold water for a wide range of experimental conditions. This phenomenon is extremely counterintuitive, and surprising even to most scientists, but it is in fact real. It has been seen and studied in numerous experiments. While this phenomenon has been known for centuries, and was described by Aristotle, Bacon, and Descartes [1—3], it was not introduced to the modern scientific community until 1969, by a Tanzanian high school student named Mpemba.

Some suggested reasons given in the paper:

  1. Evaporation — As the initially warmer water cools to the initial temperature of the initially cooler water, it may lose significant amounts of water to evaporation. The reduced mass will make it easier for the water to cool and freeze. Then the initially warmer water can freeze before the initially cooler water, but will make less ice. [...]

  2. Dissolved Gasses — Hot water can hold less dissolved gas than cold water, and large amounts of gas escape upon boiling. So the initially warmer water may have less dissolved gas than the initially cooler water. [...]

up vote 3 down vote

This was, actually, my 6th 5th grade Science Fair experiment. :)

And I'd never heard of this effect before; it was a random experiment I thought of and tried.

My answer: it depends on what you mean by "freeze".

Cold water starts freezing sooner (entering 0 degrees C), but hot water finishes freezing sooner (leaving 0 degrees C). I measured this with a digital thermometer.

No idea why, but I'm darn sure my experiment was accurate.


Edit:

I found the data!

Page 1

Page 2

I blurred out the years to avoid carbon dating myself. ;)


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Does hot water freeze faster than cold water?

up vote 14 down vote

Is it true that hot water freezes faster than cold water and if so, what practical applications have there been found for this phenomenon?


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up vote 9 down vote

In certain settings, cold water freezers slower than hot water. This is called the Mpemba effect.

The Mpemba effect is the observation that warmer water sometimes freezes faster than colder water. Although the observation has been verified, there is no single scientific explanation for the effect.

Can hot water freeze faster than cold water?, Monwhea Jeng, University of California, 1998

Hot water can in fact freeze faster than cold water for a wide range of experimental conditions. This phenomenon is extremely counterintuitive, and surprising even to most scientists, but it is in fact real. It has been seen and studied in numerous experiments. While this phenomenon has been known for centuries, and was described by Aristotle, Bacon, and Descartes [1—3], it was not introduced to the modern scientific community until 1969, by a Tanzanian high school student named Mpemba.

Some suggested reasons given in the paper:

  1. Evaporation — As the initially warmer water cools to the initial temperature of the initially cooler water, it may lose significant amounts of water to evaporation. The reduced mass will make it easier for the water to cool and freeze. Then the initially warmer water can freeze before the initially cooler water, but will make less ice. [...]

  2. Dissolved Gasses — Hot water can hold less dissolved gas than cold water, and large amounts of gas escape upon boiling. So the initially warmer water may have less dissolved gas than the initially cooler water. [...]

edit

o.O Consider my mind blown. I very nearly voted down the question thinking it's just plain absurd. - Kit Sunde May 19 '11 at 7:22

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