Mpemba Effect

This morning Kateri and I were exchanging holiday stories in the kitchen. While she was in the cold Midwest playing in snow and blowing soap bubbles (to watch them freeze mid-air), I was drinking bad tequila in the tropics and scratching at a dozen mosquito bites. Not sure who had the better time.. But she reminded me about a unique behavior in freezing water -  boiling water actually freezes faster than cold water. Remember that from Physics class? Neither did we. But a brief look on the Internets brought up interesting facts about the so called Mpemba effect (which btw, I cannot pronounce).

[if you hate science, do not read on]

This behavior was so contrary to thermodynamics, that it wasn't actually discovered and recognized by the science community until the late 1960s. Many standard physical theory effects contribute to the phenomenon, but no single explanation is conclusive nor does it happen all the time. Variables such as evaporation, convection, time of freezing and the presence of solutes and dissolved gases all affect whether the Mpemba effect will take place. Scientist still do not know which of these factors is the most important because the actual time of freezing will fluctuate based on the initial temperature of the water, the size, shape and material of the container, and the temperature of the fridge. And additionally there is also disagreement on what the definition of "frozen" is. Is it when the water reaches 0°C or when the volume of water becomes a block of ice? 

[OY is right!]

From the articles that I skimmed, it seems that these variables need to be in place for consistent results when comparing one ice tray to another:

• Evaporation: As hot water cools, it may lose a significant volume of water. The reduced mass of water will make it cool faster than a similar volume of cold water. 
• Convection: In short, accelerated heat transfers. Density generally decreases with increasing temperature, so the surface of the hot water will be warmer than the bottom -- this has been called a "hot top."And because water loses heat primarily through the surface, the water with a "hot top" will lose heat faster than the volume of water without a "hot top".
• Dissolved Gasses: Large amounts of gas can escape water upon boiling.  This changes the properties of the water, making it easier to develop convection currents or decreasing the amount of heat required to freeze the water, or changing the boiling point. These are speculations and are not proven theories.
• Supercooling: This occurs when the water freezes not at 0°C, but at some lower temperature. Some experiments found that hot water supercools less than cold water. Which means that hot water may freeze at a higher temperature than cold water.