Wednesday, March 3, 2010

The Unbelievable Mpemba Effect

by Richard Crews
From a letter to New Scientist, March 3, 2010: "If identical vessels of hot and cold water are placed in a freezer at the same time, the hot water will freeze first. This was first alluded to by Aristotle and then reported in the late 1960s as a result of observations by a young Tanzanian student. The findings were eventually published (Physics Education, vol 4, p 172), but the reason is still unexplained."

But my cousin, John LeGates, writes--

"I discovered it myself in the early seventies. I took it to a very senior professor of physics at MIT who happened to also be a neighbor and good friend. He said that the explanation is simple. Heating the water drives out the air dissolved in it.

"There’s a test for this explanation, which I devised and carried out. Heat two vessels of water, driving out the dissolved air. Keep one hot and let the other cool off. Then put them simultaneously in the freezer. Sure enough, the cool one freezes first. Incidentally preheating water before you freeze it also makes for clear ice with no bubbles."