What is the Mpemba Effect?

In this edition I would like to explain why hot water freezes faster than cold water, when put into the freezer. It is a very counter-intuitive observation, it's a paradox. This is called the Mpemba Effect.

An episode of the TOKTalk.net podcast, hosted by ok, titled "What is the Mpemba Effect?" was published on December 24, 2007.

December 24, 2007 · TOKTalk.net

Summary

http://www.toktalk.net/wordpress/wp-content/uploads/MP3/001-toktalk-mpemba-effect.mp3 In this edition I would like to explain why hot water freezes faster than cold water, when put into the freezer. It is a very counter-intuitive observation, it’s a paradox. This is called the Mpemba Effect. The effect is named according to Tanzanian high-school student Erasto B. Mpemba who re-discovered the effect while making ice-cream, back in 1963. The Mpemba Effect is a nice example how the change of one variable, the temperature, can have unexpected side effects. Most people assume that the difference between a hot glass of water and a cold glass of water is only the temperature. But this is not the case. Just by heating the water we are introducing a range of other variables that have an unexpected effect on the outcome. In this edition of TOK-Talk I would like to explain why hot water freezes faster than cold water, when put into the freezer. How is this possible? Listen to find out! The Mpemba Effect is a very, how shall I say, counter-intuitive observation, it’s a paradox. It is the observation that under certain circumstances hot water freezes faster than cold water. The effect is named according to Tanzanian high-school student Erasto B. Mpemba who re-discovered the effect while making ice-cream, back in 1963. Aristoteles already described the effect 300BC. So it really is not a new discovery. The effect occurs only under certain circumstances. You put a glass of hot water into the freezer, next to a glass of cold water. If the conditions are right the hot water will have turned to ice before the cold water. The actual causes of this effect are not fully understood. There seem to be a number of factors responsible, however. This effect will only work for certain temperatures. If you place water with 99C next to a glass of water with 1C then the 1C water will of course freeze faster. But at other temperatures the issues are different. Let’s assume that 30C water takes, lets say, 30 minutes to freeze. And let’s also assume that a glass containing 70C water takes 20 minutes to cool to 30C, which is the starting temperature of the fist glass. We would conclude that it takes the 70C a total of 50 minutes (20min+30min) to freeze. This is not what we observe, however. And yes, the volume of the water is initially the same for both. The shape and material of the container are also the same. So why does the hot water freeze faster? We may not even need experiments to determine possible causes for this observation. Most people assume that the difference in temperature of the water is the only variable. But if the observations don’t fit and there was no error in the method, then we may need to question our assumptions. And in out case we have to question the assumption that the only difference in the two water containers is the temperature. We have to look for other differences that may be responsible for the faster cooling of the initially hot water. Now close your eyes and imagine two glasses of water standing in front of you – one hot and one cold. The hot water is steaming. The hot water loses volume due to faster evaporation. Water with less volume will cool down faster. And as the water starts to freeze, the volume will increase again. Ice is less dense than liquid water. We would actually see a volume increase in the glass, and would not notice the loss. There are, however, several other reasons involved as well. The loss of volume of the water actually changes the concentration of minerals in the water. The glass with some evaporated water will have a slightly higher mineral concentration than the glass with the original volume. The salts in the water will not evaporate out. Now, I know that a higher concentration of salt actually lowers the freezing temperature, that’s why we put salt on the roads during winter to prevent ice. So in my view the higher mineral concentration would actually have the opposite [...]

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