What is the name of the phenomenon whereby hot water freezes faster than cold water?

It's called the Mpemba Effect

Minnesota ices this one.

Is it really true? And if so, under what conditions?

"The effect can be achieved when the container holding the water is placed on a surface of frost or ice. The higher temperature slightly melts the icy surface on which the container rests, greatly improving the thermal contact between the container and the cold surface. The increased rate of heat transfer from the container and contents more than offsets the greater amount of heat that has to be removed."

Got that? And I always thought it was a myth!

http://www.sciencenet.org.uk/database/Physics/Original/p00201d.html

Actually, I thought it was true. However, my understanding is that it occurs when the water is at a relatively high temperature (about 180°F) where it will give off vapor which aids in the heat transfer. Much hotter or cooler than that and it does not work.

Now, if that 'fuse guy will just check his question and tell us his version of the truth...

Here's a good reference from Monwhea Jeng of the
Department of Physics, University of California:

http://www.weburbia.com/physics/hot_water.html

As a side note, I used to work with a gentleman who co-authored the "Mechanical Refrigeration" section of a major reference work used by Mechanical Engineers. He had no doubts about the Mpemba Effect. Though the precise reason for the effect is not known, only observed and speculated about, my former colleague believed that the momentum gained by the rapid initial cooling of the hot water was a major contributor.

Don't mess with the 'Fuse on maters of cold!

God ! some people will believe anything....

NO, IT IS NOT TRUE...TOTAL YOU KNOW WHAT...

I always thought that it was due to the facts that the water's molecules were farther apart (thus more air space surrounding each molecule), allowing the heat to dissapate more quickly.

Donrent, I generally admire your posts. With due respect to your position above, and in spite of its defiance of common logic, this phenomenon has been demonstrated over and over again. The linked article, from a physicist at a respected institution, provides many references. My own personal reference comes from a gentleman who is one of the top people in his field, who worked for DuPont for most of his life, who answered my question several years ago after an observation of the event had me befuddled. No, it may not work in every circumstance - but it does work!

Seems to me that if something will only work in certain insstances, then that is not the norm...So,then I say, under normal cercumstances, cold WILL freeze quicker than hot....
I can see where if the temp is something like 50 below, then those lil molecurs will run their little butt around so fast that it could take place....
You stated, that "No, it may not work in every circumstance- bbut it does work.."
Thats like saying a hopped up car can run 200 mph, therefore all cars can....
Sorry, I STILL TOTALLY disagree with you.....

This discussion has been going on for eons. I always thought that hot would freeze faster than cold. There is something that you all can do that at the very least is fun. (What are we going to do today, Mr. Wizard?) If you live in a part of the world where the outside temperature gets into the teens or lower, take a cup of boiling water and a cup of cold water, go outside and throw the water (not the cup) into the air. The hot water will never come down; it turns into a vapor and drifts away. And makes a really cool ripping noise in the process. Much of the cold water will hit the ground, so watch out. And this proves......... Oh, probably nothing, but it's a lark.

This phenomenon was first observed in the days when buckets were made of wood. A bucket of hot water versus a similar bucket of cold water, the hot water is cooled by both the reduced total mass due to evaporation, and the cooling effect due to the hottest molecules being evaporated most rapidly. Duplicating the experiment in a refrigerator freezer with metal trays and conduction effects may be misleading and probably produce all sorts of conflicting results. But water at or near boiling and water at 50 deg C in insulated and uncovered containers such as wooden pails will behave as Dogspit promised.