Hot water freezes faster than cold water

[HydrogenBond]

http://arxiv.org/pdf/physics/0512262?

We review the Mpemba effect, where intially hot water freezes faster than initially cold water. While the effect appears impossible at first sight, it has been seen in numerous experiments, was reported on by Aristotle, Francis Bacon, and Descartes, and has been well-known as folklore around the world. It has a rich and fascinating history, which culminates in the dramatic story of the secondary school student, Erasto Mpemba, who reintroduced the effect to the twentieth century scientific community. The phenomenon, while simple to describe, is deceptively complex, and illustrates numerous important issues about the scientific method.

This was one of those water anomalies that was around for centuries but defied scientific common sense. It should be impossible for the same mass at high energy to freeze first. The student Mpemba found he could freeze ice cream faster by heating it first. He was met with skepticism. But he persisted until there was an investigation. After hundreds of years of thinking this was nothing but superstitious folklore, it turned out to be real.

[modest]

The perfect science fair project

[Tormod]

Wasn't this used to create ice in ancient Egypt? I thought it was.

[Tormod]

Found this:

Finally, one could use a little ingenuity. The Romans used to make ice in the deserts of North Africa or Palestine by taking advantage of the low humidity (and therefore the low temperatures at night). They would put what they wanted to freeze in a pit well-insulated with straw. The pit would be covered with highly-polished shields or other objects during the day, to reflect the heat of the sun; at night, the pit would be uncovered so that it could lose heat to the desert air.

Here:
How did they make ice before electricity? - Yahoo! Answers

Not sure if it relates the the topic but I found it interesting.

[modest]

I'd heard that too. And I think it does relate to HB's OP because evaporation is a key ingredient in what's happening in both cases.

Evaporation is better at cooling than people give it credit for. That's how they make a Bose–Einstein condensate (which I believe is the coldest substance ever made) They evaporate off the molecules with greater kinetic energy.

-modest

[alexander]

they didn't just stick the finger in, like, here

YouTube - INSTANT ICE!! Awesome!!

i mean what could be simpler then that...?

mmm sodium acetate

[modest]

supersaturated sodium acetate perhaps

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[REASON]

I admit, this sounds very counterintuitive, but science isn't really about intuition is it?

I havn't read the entire article linked in HB's OP. Can anyone give a brief synopsis of what is happening with the water that allows this phenomenon to occur?

Does this work in our refrigerator freezers if we still use ice trays? Do you actually get ice quicker if you put hot tap water in the ice trays?

[alexander]

Ok, i dunno if you guys are perplexed about this for real, so i will just post.

this is the effect similar to "can you boil water in a single piece of paper", and the answer is yes....

First and foremost, it's not hot water, i think the best results are achieved through the use of warm water (70 degree-ish vs 30 degrees or room temp). Then it's fairly simple thermodynamics, warm water will loose more heat, faster then cold water, loosing heat (that you can see as steam), will cool the warm water a lot faster then cold water, thus bringing down the temperature dramatically faster, and thus freezing it faster then the water that was initially at room temperature.

Vapor is the goal here, it takes lots of energy to convert water to its gas form, so then you have much energy released, colder water forms a convection current, exposing more hot water to the surface, which releases more steam, which uses up a lot of energy, and you have this process going on, the water will cool at an astonishingly faster rate, just not quite as uniformly as the room temperature water. end result is reaching a lower temperature over a fairly short period of time, and thus freezing faster

i know it's not a very scientific explanation, but that is as close as i can describe it, with my knowledge and without further investigation, but i hope it does well enough. linear math here would not work either, i would imagine that in order to describe this, you would need a fairly complex math model, water is one of those weird liquids, and liquids are hard to describe in math anyways... then it gets complicated by convection flows, vapor, change in density and the fact that below 4 degrees C, water will have a cold crust on top, which complicates everything, oh and did i mention supercooling?

[Moontanman]

Evaporation is important but you have left out a big part of the puzzle. Heating water drives out all the dissolved gasses. Dissolved gasses act like antifreeze driving the freezing point down making it harder to freeze water that contains the gasses. Heated water not only cools fast it also freezes closer to the freezing point of pure water.