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Hot water freezes faster than cold water


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#1 HydrogenBond

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Posted 30 May 2008 - 10:40 AM

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.

#2 modest

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Posted 03 June 2008 - 06:10 AM

The perfect science fair project :esmoking:

#3 Tormod

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Posted 03 June 2008 - 06:15 AM

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

#4 Tormod

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Posted 03 June 2008 - 06:18 AM

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. :esmoking:

#5 modest

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Posted 03 June 2008 - 06:37 AM

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.

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#6 alexander

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Posted 03 June 2008 - 07:47 AM

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

YouTube - INSTANT ICE!! Awesome!!

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

mmm sodium acetate

#7 modest

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Posted 03 June 2008 - 08:01 AM

supersaturated sodium acetate perhaps

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#8 REASON

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Posted 03 June 2008 - 08:23 AM

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?

#9 alexander

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Posted 03 June 2008 - 08:56 AM

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 :D

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?

#10 Moontanman

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Posted 03 June 2008 - 09:10 AM

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.

#11 alexander

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Posted 03 June 2008 - 10:32 AM

well, it depends on how you setup the experiment, if you boil both waters first and then reheat them, and use distilled water, then the chance of those being the reasons this happens, are zero to null :D

#12 Tolouse

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Posted 08 June 2008 - 10:59 AM

The perfect science fair project ;)


i remember someone in middle school (back in the early 90's) someone did do this as a science experiment

i never quite got the concept on how it worked though, and still find myself wondering how the molecules bond when they are just moving at such a fast speed whereas the cooler molecules are slower thus should be able to bond a lot quicker

maybe i'm just thinking too much about it, but since middle school, i have used hot water in my ice trays more than cold water

and everyone just thinks i am weird even after i tell them that hot water freezes faster than cold water

#13 modest

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Posted 08 June 2008 - 11:25 AM

i remember someone in middle school (back in the early 90's) someone did do this as a science experiment

i never quite got the concept on how it worked though, and still find myself wondering how the molecules bond when they are just moving at such a fast speed whereas the cooler molecules are slower thus should be able to bond a lot quicker

maybe i'm just thinking too much about it, but since middle school, i have used hot water in my ice trays more than cold water

and everyone just thinks i am weird even after i tell them that hot water freezes faster than cold water


Yeah, if you just think about it in terms of temperature then it doesn't make any sense. I think that's why people have trouble believing it. But, there is a lot more than just temperature as a single property of the water. There is how much gas is dissolved in the water. There are convection currents distributing the water differently between the two. The colder one may form a thin layer of ice on its surface preventing further evaporation while the hotter one continues to evaporate until it gets far colder. There are so many variables. That's why I think it's a good science fair project. All those graphs and controls - not to mention; it's unbelievable and therefore interesting.

-modest

#14 HydrogenBond

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Posted 08 June 2008 - 04:45 PM

The mechanism hasn't been settled. Evaporation, dissolved gases and thermal momentum have all been proposed. One of the latest ones has to do with the structure within water. At colder temperature, the water is more hydrogen bonded but in shapes that are somewhat different than in ice. To form ice there needs to some rearrangement. In the warmer water, the structure is more open so the molecules can rearrange faster into ice.

A loose analogy is two people building a card bridge. One is starting with a deck of card and the other with a card house. The house looks closer but we got to carefully remove some of the cards to make this into the bridge. By then the person with the deck is catching up and finishes first.

Plato proposed water was an icosahedral. It turns out he was right in the sense of hydrogen bonded structure, which turns out to be more common in the cold water. The figure show a transition between two states.

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#15 alexander

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Posted 08 June 2008 - 05:00 PM

Modest, let me, and this is not to be taken as a hostile action or anything of that matter, reflect back on your post.

You say that it depends on the amount of gases dissolved in the water, however, as i have said before, the cool water can have the same level of gas absorbed in it as the hot water, the fact that the hot water is not boiling, keeps the levels about equal throughout the experiment. You could also used freshly-made distilled water, one that will not have any gas dissolved in it, and the experiment would still turn in favor of the warm water :wink:

If you think of it temperature-wise, it really does not make sense, but, if you think of it in terms of energy, and steam, the picture starts to become clearer. Then take into acount the convection currents, that distribute the chilled water on top, to the bottom, perhaps if the top of the water is infact brought down to a lower temperature, but the rest of the mass of water is still fairly warm, you will have convection currents going, even though, the water leaving the top is colder then the water on top of the cold-water comparison. Mind you the water is only said to freeze at 0 degrees C, in reality, much of the body of water gets super-cooled prior to freezing, storing enough energy to turn to ice (and that takes a lot of energy, less, though, then turning the water to steam).

Then you think about the colder water, as a body that will cool fairly uniformly, and when it reaches 4 degrees C, the convection flows will stop, and the colder water will stay at the top, such are the density properties of this weird liquid.

The other thing is heat, may change the surrounding "microclimate" of the area around the glass, releasing more heat, will in term cool the water more. Also there may be consideration of water vapor forming on the outside of the glass, that may aid the thermal bond of the glass to the shelf it stands on, thus once again, drawing out more energy, leading to shorter freeze times.

But this is in line of questions like "can you boil water, using a burner and a single piece of paper, or a plastic bottle" and so forth, i'll try to come up with more :eek2:

#16 modest

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Posted 08 June 2008 - 09:24 PM

Modest, let me, and this is not to be taken as a hostile action or anything of that matter, reflect back on your post.


Least hostile post ever :(

You say that it depends on the amount of gases dissolved in the water, however, as i have said before...


The point of my post was that there are many variables besides just temperature. Dissolved gas is necessarily one of those. You correctly point out a good control to eliminate that variable. We could also agitate the water and eliminate convection or distribution as a control. We could seal the container and eliminate evaporation in a control. I was pointing out that there are many variables to test in this manner. It looks like you agree, and in fact, I agree with the rest of your post.

-modest

#17 alexander

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Posted 09 June 2008 - 10:13 AM

sweet, well i've been looking at some more experiments you can do with water for science fair projects :confused:

1:
floating paper clip (using the surface tension to make people wonder). take one paperclip and unfold it to use as the lowering tool, put another paper clip on top, lower the paperclip into the water, slowly, once the paperclip on top floats off, you can remove the tool clip :)
1.5:
now drop a drop of dish soap into the water, see what happens :)

2:
using water as a prism, in a clear round glass, filled with water, you put a mirror angled upwards. turn the lights off, and use the flashlight to acheive a rainbow :)

3:
Unmixable water, using 2 jars (baby food jars work well), some water, and a playing card, and 2 color dyes, you can do the following. First, fill the bottom jar with cold water, add some food coloring, like blue, mix it, finish filling the jar, all the way, to the rim, until water looks like it's almost about to run out.
take the second jar, put a little hot water into it, add some food coloring, different color from the first jar, and also put in enough warm water to where it looks like it's just about to float over.
take the playing card, and put that on top, tap gently to remove air, and make a "seal" in a sense, where the card is in flushwith the top, and there is no air in the jar. now, put your hand over the card, and flip the card and the jar over. now position the contraption over the first jar (make sure the lines meet, to prevent leaking). now, carefully remove the playing card from the middle. (what do you think is going to happen?)

off to find more fun things to do with water :)