  An explanation for why hot water will sometime freeze more rapidly than cold
water is offered. Two specimens of water from the same source will often have
different spontaneous freezing temperatures; that is, the temperature at which
freezing begins. When both specimens supercool and the spontaneous freezing
temperature of the hot water is higher than that of the cold water, then the
hot water will usually freeze first, if all other conditions are equal and
remain so during cooling. The probability that the hot water will freeze first
if it has the higher spontaneous freezing temperature will be larger for a
larger difference in spontaneous freezing temperature. Heating the water may
lower, raise or not change the spontaneous freezing temperature. The keys to
observing hot water freezing before cold water are supercooling the water and
having a significant difference in the spontaneous freezing temperature of the
two water specimens. We observed hot water freezing before cold water 28 times
in 28 attempts under the conditions described here.
