
The Mpemba Effect - splat
http://en.wikipedia.org/wiki/Mpemba_effect
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lpgauth
Was this ever reproduced properly?

I remember in my first year of physics some people had taken this experiment
has class project. No one was able to reproduce that "effect".

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KiwiNige
When I was growing up it was a known phenomenon in my family that hot water
pipes would freeze before cold pipes in winter. And that using hot water to
de-ice your car would cause it to freeze up again more than cold water.

It's something I've always been meaning to test out properly to see if it was
real.

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baddox
Hot water pipes have water running through then less often. Hot water might
cause your car windows to fog up, but I doubt the Mpemba effect, even if it's
real, could be noticed in that situation.

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raintrees
Maybe this is also why it is recommended by food persona Alton Brown (among
others) to thaw a turkey in the sink with running cold water, not warm, to get
the fastest results.

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robobenjie
I'm pretty sure that this is to reduce the chance of bacterial growth on the
warm raw meat (<http://www.fsis.usda.gov/FactSheets/Big_Thaw/index.asp>)

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jurjenh
And pre-cooking the meat. Using water that is too hot can tend to start
cooking the meat, leading to a tougher, drier end product.

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robobenjie
I'm pretty sure that this Mpemba Effect is folklore. The hot water must
necessarily pass through the temperature that the cooler water starts at. In
fact if we are to believe this than at some point the initially hot water and
the initially cold water must, at some point, be the same (average)
temperature. Why then would it start to cool faster?

Options that I can think of 1) initially hot water cools very asymmetrically
so some hot portion gives of heat (to the air!) faster 2) more rapid
temperature change sets up strong convective currents that, because of
momentum, stay around, causing the initially hot water to give up heat faster.

These both seem very unlikely. Fluid water is a very good heat conductor (and
air a good insulator) making it unlikely that there would be significant
temperature gradients and intuition tells us that freezing water takes much
longer than it takes any sort of current to die down. (also note that these
two effects would act to cancel each other out)

As for the reasons mentioned in the wikipedia article:

1)Definition of frozen: I suppose that this could account for the effect if
you measured the time for cold water to completely freeze and hot water to
reach 0 degrees C! But in general as long as you use the same metric this
won't matter

2) Evaporation (reducing the volume to be frozen): So an initially hot fluid
will evaporate more and I suppose result in a slightly smaller volume to
freeze. Since the thermal mass is proportional volume and the heat transfer is
proportional to area this is a real effect. However the % change in volume of
any reasonable shape of water is so insignificant compared to the difference
in energy of hot water and cold water that this effect is almost certainly
negligible. (perhaps if the claim was hot water freezes faster than cold water
if it is laid out in very very very thin sheets I would buy it because of
this.)

3) Convection: see above.

4) Frost: Since the water must pass through the same temperature (0 C) for
frost to start to build the frost must grow differently on initially hot water
than initially cold. I really see no reason to believe this is the case

5)Super cooling: again, if the convective forces persist for a mush longer
time than seems reasonable...

6) Solutes and dissolved gases: I mean if one fluid is a different chemical
composition than the other then I suppose all bets are off, but I feel the
claim of "warmer water freezes faster than colder water" implies they are both
the same substance

However the main reason I believe this to be an Urban Myth is that it makes
such a good story. It fits our belief as a culture that there are weird things
about science that we don't understand and it challenges our initial intuition
in a memorable way. In addition it requires more effort to test than most of
us are willing to put forth (including me: I would be very interested if
someone would actually do the test recommended in the wikipedia article.)
Lastly; hot water freezing faster is a much better story than the other way
around. Telling people that hot water freezes faster makes you seem to have
some kind of secret knowledge; a spell you can cast over ice cube trays to
make them into ice faster.

I bet a reasonable rule for judging truth is that, all else being equal, the
better story is less likely to be the true one, because if the truth was also
the better story then no one would tell the boring one.

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Confusion
_However the % change in volume of any reasonable shape of water is so
insignificant compared to the difference in energy of hot water and cold water
that this effect is almost certainly negligible._

You underestimate the energy the evaporating water takes away. The evaporation
energy is much higher than the specific energy. The amount of water that
evaporates is not only significant: it also causes the temperature to drop
more sharply than you would expect based on only the larger temperature
difference. When they reach the same temperature, there is less of the water
that was hotter initially. As such, that body of water will be frozen earlier.

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CamperBob
With no other substance involved besides the water, I don't see how this could
happen. Say you put a vessel of hot water and a vessel of colder water in the
freezer at the same time. If the hot water does indeed reach 32F before the
cold water does, then there has to be an earlier point where their temperature
versus time slopes cross, at which point their temperatures are equal. Once
that point is reached, there's no longer any difference between the "hot" and
"cold" water.

Since the point of temperature equality can't be reached in negative time, the
hot water can't possibly freeze sooner than the cold water that was placed in
the freezer at the same time.

So: given that the effect does, in fact, happen, there's some factor at work
other than the water temperature. Some property of the container, perhaps? If
the hot container loses its heat at a faster rate than the cold container,
perhaps it ends up cooler than the surrounding water, and can continue to
accelerate the freezing process beyond the point where the temperature slopes
cross each other. That'd be my guess.

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btilly
The requirement for reproducing the result is that the bucket containing the
water be a good insulator. A wooden bucket works well, a metal bucket does
not.

If you do that then the cooling mostly happens through evaporation. If you
start with full buckets of water at different temperatures, at the point where
the temperatures cross the bucket that started with hot water has less water
in it and therefore is able to freeze faster after that point.

