Mpemba Effect

Our hot water was frozen this morning, but not the cold.  I've noticed this happening before, but when I mentioned it to my daughter, Liz, she doubted this was based on any scientific knowledge.  I believe our conversation went something like, "Mom, isn't that completely contrary to principles of thermodynamics?" She didn't really say that, but wouldn't you be impressed if she did. I think she thinks I have been cursed and there isn't a scientific principal to prove this, but let me educate you and her:

Hot liquids sometimes freeze faster than cold.  I think plumbers and ice cream makers all over the world have known this for as long as there have been plumbers and ice cream makers. This phenomena gained attention when a 15 year old Tanzanian student, Erasto B. Mpemba, observed this while making ice cream as a science experiment in his class. Erasto, from the continent of Africa (we are fans of Africa) didn't follow the directions, which stated he should cool his ice cream before putting it in the refrigerator. His classmates did follow the directions and pre-cooled there ice cream.  Whose do you think set faster? That rule beaker Mpemba!

There are  a few theories, but I have not found a definitive answer because it doesn't always happen except at my house!  The theories are: 

1. Faster evaporation of hot water, which reduces the volume left to freeze.

2.  formation of a frost layer on cold water, insulating it.

3.  different concentrations of solutes such as carbon dioxide, which is driven off when the water is heated.

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