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Thread: Hot ice cream freezes faster than cold ice cream.

  1. #1

    Hot ice cream freezes faster than cold ice cream.

    In 1969, Easto Mpemba, a Tanzanian schoolboy claimed that ice cream mixture froze faster when it started out hot than when it started out cold. He was right.

    Forget what you know about H2O
    By Robert C. Cowen
    We take the properties of water for granted. Yet scientists assure us that we have a lot to learn about our biologically essential old friend. Research supporting the counterintuitive claim that hot water freezes faster than cold illustrates this point.

    The claim made news in 1969 when Erasto Mpemba, a Tanzanian schoolboy, said that his ice cream mixture froze faster when it started out hot than when it started out cold. Never mind that others have reported this strange behavior of water for centuries. Skeptics scoffed. The boy's teacher spoke derisively of "Mpemba's physics."

    It's time to rethink the derision. Jonathan Katz at Washington University has studied the "Mpemba effect" and finds the claim valid. He explains that it has to do with the hardness of water. Dissolved bicarbonates of calcium and magnesium, which make water hard, also depress the water's freezing point and slow its rate of cooling compared to softer water that has less of the bicarbonates. Heating water causes the bicarbonates to precipitate out. This forms the deposits in hot water kettles, for example. Thus, preheating water (or Mpemba's ice cream mix) softens the water, allowing it to freeze faster and at a higher temperature than cold, hard water.

    Dr. Katz speculates that the Mpemba effect doesn't always show up because experimenters are using soft water to begin with, according to a report in New Scientist. In that case, there would be no advantage to preheating the water.

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  2. #2
    I remember reading about this a long time ago; possibly it was in the New Scientist. I thought his observation was just with water, but then my memory was from 20 or more years ago, so is likely off. I wonder what happened to Mpemba, where he is now.
    - Richard

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