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

Temperature measurements taken near vessel walls show that initially h ot water may well begin to freeze quicker than cold. This is not, as p reviously surmised, due to the cooling history of the water (e.g., air expulsion during heating). Rather, supercooling virtually always take s place. On those occasions where the cold water supercools sufficient ly more than the hot the Mpemba scenario is the following: The hot wat er supercools, but only slightly, before spontaneously freezing. Super ficially it looks completely frozen. The cold water (in larger volume than that of the hot sample) supercools to a lower local temperature t han the-hot before it spontaneously freezes. This scenario can occur m ore often for ambient cooling temperatures between -6 degrees C and -1 2 degrees C. (C) 1995 American Association of Physics Teachers.