"Mpemba effect", online refs

[William Beaty <billb@ESKIMO.COM>]

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)
  http://www.desy.de/user/projects/Physics/hot_water.html
  http://hermes.astro.washington.edu/faq/physics/hot_water.html

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.
  http://www.cwr.uwa.edu.au/cwr/publications/papers/748.html

  "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,
  1996.


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!)

   http://sciencenet.oit.net/database/Chemistry/Original/c00024d.html

More:


  SciAm "ask the experts"
  http://www.sciam.com/askexpert/physics/physics21.html

  New Scientist THE LAST WORD
  http://www.last-word.com/lastword/answers/lwa197bubbles.html

  Hyperphysics: thermo: mpemba
  http://230nsc1.phy-astr.gsu.edu/hbase/thermo/hques2.html
  http://230nsc1.phy-astr.gsu.edu/hbase/hph.html


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

  Water: its structure and importance
  http://www.sbu.ac.uk/~water/
  "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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