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Re: "Mpemba effect", online refs

Thanks, William, for those refs! This topic comes up every year in my
class, and when I don't have a "ready" answer for my students, they seem
somewhat disappointed.

Now I have some places to send them to see for themselves.


Ruth Ann Wolfson
College of Applied Sciences and Arts
Southern Illinois University at Carbondale
Carbondale, IL 62901


----- Original Message -----
From: "William Beaty" <billb@ESKIMO.COM>
To: <>
Sent: Friday, February 16, 2001 1:36 PM
Subject: "Mpemba effect", online refs

On Fri, 16 Feb 2001, Robert A Cohen wrote:

When you say "same amounts of water", do you mean same amounts initially
or same amounts after evaporation has taken place (I'm guessing more of
the warm water will evaporate)?

That's one possible artifact. Jearle Walker mentions a few others:

From the AMATEUR SCIENTIST article (Jearle Walker Sept 1977)

One of the first explanations was that the hotter container melted the
ice under it and so made better thermal contact with the freezer shelf.
The effect is the same, however, when the container is thermally
insulated from the freezer shelf, as Mpemba and Osborne's was. Moreover,
the explanation would not serve for the examples of washing cars and
filling birdbaths or for frost-free freezers where no ice forms on the
freezer shelf.

Several factors appear to be involved in the effect. First, in the
hotter container the liquid may circulate better, so that the hot water
in the central region moves rapidly to the walls of the container or to
the top surface of the water. Second, more of the gas dissolved in the
water may be released if the water is warmer. Dissolved gas delays
cooling, and its elimination before cooling allows the water to reach
the freezing point sooner. The hot-water pipes in your home are more
likely to freeze than the cold-water pipes because the dissolved gas was
eliminated when the water was heated. Third, the warmer water may lose
more of its mass and heat to evaporation than the cooler water does.
Thus there would be less mass to cool, and the water would reach the
freezing point sooner. If there is a significant loss of mass, then
(once the freezing point is reached) the initially hot water will
certainly freeze faster because there is less mass from which heat must
be removed to achieve the transition from liquid water to ice.

The SCI.PHYSICS FAQ has some info:

Hot Water Freezes Faster than Cold! (new: 11/4/98)

The FAQ gives lots of refs. Here are two recent ones:

"Supercooling and the Mpemba effect", David Auerbach, in American
Journal of Physics, Vol. 63, No. 10, pp 882-885; Oct, 1995.

"The Mpemba effect: The freezing times of hot and cold water", Charles
A. Knight, in American Journal of Physics, Vol. 64, No. 5, p 524; May,

This site shows that the controversy is still alive, in that the supplied
answers are contradictory, and one of them clearly wrong (about the
fast-changing temperature "outrunning" that of the cold water!)


SciAm "ask the experts"

New Scientist THE LAST WORD

Hyperphysics: thermo: mpemba

Here's a good site for an overview of water:

Water: its structure and importance
"Because of its clear importance, water is the most studied material on
Earth. It comes as a surprise, therefore, to find that it is so poorly
understood, not only by people in general, but also by scientists
working with it everyday. "

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