The Mpemba effect, describing the phenomenon of initially hot water freezing before cooler water, occurs only when the water supercools and the cooler water has a lower nucleation temperature than the warmer water. The Mpemba effect is not observed if the hot water freezes first but all conditions during cooling are not “identical,” this is the result of increased thermal conductivity. In this case, the hot water container affected the experimental conditions.
Under normal conditions, ice that is warmed from less than 0 o C will always begin melting when its temperature reaches 0o C. However, when liquid water is cooled from above 0o C, it often will not begin freezing until it has supercooled to several degrees below 0o C. This is why hot water can freeze before cooler water when all experimental conditions are identical except for the initial temperatures of the water. Hot water will freeze before cooler water only when the cooler water supercools, and then, only if the nucleation temperature of the cooler water is several degrees lower than that of the hot water. Heating water may lower, raise or not change the spontaneous freezing temperature.
The key is creating identical conditions; if the hot water causes better thermal conductivity between the hot water container and the cooling environment, then the conditions are no longer identical. An observed example of this was the melting of freezer frost, shown in Figs. 9 and 10. When conditions for the two samples are identical, the hot water always takes longer to reach 0o C, as shown in Fig. 21.