SUPERCOOLING AND THE MPEMBA EFFECT - WHEN HOT-WATER FREEZES QUICKER THAN COLD

Temperature measurements taken near vessel walls show that initially hot water may well begin to freeze quicker than cold. This is not, as previously surmised, due to the cooling history of the water (e.g., air expulsion during heating). Rather, supercooling virtually always takes place. On those occasions where the cold water supercools sufficiently more than the hot the Mpemba scenario is the following: The hot water supercools, but only slightly, before spontaneously freezing. Superficially it looks completely frozen. The cold water (in larger volume than that of the hot sample) supercools to a lower local temperature than the-hot before it spontaneously freezes. This scenario can occur more often for ambient cooling temperatures between -6 degrees C and -12 degrees C. (C) 1995 American Association of Physics Teachers.