
Mpemba effect: warmer water can freeze faster than colder water - mike_esspe
https://en.wikipedia.org/wiki/Mpemba_effect
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mattdeboard
I have no idea what this has to do with HN, but, one of the best "life hacks"
I've ever learned was from my Iraq tours: evaporative cooling.

If you're outside on a hot day and your drink is hot (in our case, it was
palettes and palettes full of bottled water), get a wash rag or a skivvy shirt
wet, wrap it around your drink container, and let it sit out in the sun. In a
little while, voila, cold drink.

Apropos of nothing.

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SiVal
It works better in the shade, and best of all in the shade exposed to a
breeze. You don't need to warm the water in the rag to evaporate it. It will
evaporate in the dry air, even if cool. Putting it in the sun raises its
temperature as the evaporation lowers it. If there's a nice breeze,
evaporation will still win, but it will get cooler if you don't add energy at
the same time you are removing it.

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mattdeboard
That's it, it's been awhile (6 years... wow). Thanks for the correction. Heat
+ breeze (even if the only breeze is warm).

~~~
SiVal
+1 thanks for your service

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InclinedPlane
And nobody understands exactly why.

The British Royal Society offered a 1,000 pound prize recently for an
explanation, so far this is the winner, though it's not complete:

<http://www.rsc.org/mpemba-competition/mpemba-winner.asp>

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sk5t
I can perceive this as nothing but a horrible, horrible failure to account for
unseen variables.

~~~
gijjk
AKA, every unsolved problem in science.

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ck2
Didn't they prove this happens only if the water is impure?

If the water is purified, ie. steam distilled, the warm vs cold effect
disappears?

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slm_HN
Back in my high school chemistry class this effect was noted and explained in
the following way:

Ice needs to form a particular crystalline structure when it freezes. This is
what makes ice float in water. The molecules in warm water are moving faster
and therefore have a greater chance to wander into this correct formation. The
cold water molecules are moving more slowly and simply take more time to find
the correct alignment.

There wasn't really any proof to this theory, but it was followed with a bunch
of anecdotal evidence, like Canadian's washing their cars with cold water,
etc.

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Jach
Newton's cooling law can be written as t = -ln(T(t) - Te)/k + ln(To - Te)/k.
T(t) is the object's temperature at t, Te is the surrounding environmental
temperature that's assumed constant, To is the object's starting temperature,
and k is a constant scaling factor related to the material of the object in
question. Since this effect is contrary to that law, the law is wrong, i.e.
too simplistic. It's not hard to imagine that the surrounding Te is changed by
the object or that k of water+container can change depending on its
temperature or purity...

~~~
manglav
I don't quite follow your reasoning of why the law is wrong. You say yourself
"Te is assumed constant", yet you imagine that converting the Te from a
constant to a variable could affect the result. However, if you change Te from
a constant to a variable, Newton's Law of Cooling is null and void, because a
simplification is that Te is constant, and that allows a lot of /dt's to
cancel.

The law is correct for when you can assume a constant Te, for example an
object within a fluid flow (like an airplane wing during flight). The problem
is, for stagnant situations, convection plays a _huge_ role in heat transfer
within the air. Controlling convection is really really hard, and even harder
in a scientific setting that needs to be reproducible. Also, when dealing with
transition temperatures, various things need to be taken care of, such as
supercooling. This can literally stop working if your beaker is slightly
dirty. So the problem is there isn't a way to universally control the
conditions for this experiment. However, I did find that the winner of the RSC
paper had a suitable explanation that involves the presence of supercooling
for the colder sample of water. Essentially, if the colder sample supercools,
it will take slightly more time to freeze, because while it should have
frozen, the ice does not have a nucleation site. On the other hand, the warmer
sample does _not_ supercool, and freezes upon reaching 0 C. This experiment
only works if the temperature difference is slight, probably within 15-30 C.
Quite ingenious really!

Edit: After reading the rest of the comments, air bubble/filtered water/de-
ionized water all have great effects on ice nucleation and supercooling, which
for me supports the winner's paper.

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jnellis
The controller/thermostat on a stabilized freezer turns on and off in a steady
state as it reaches setpoint and then drifts away from it when the motor is
off. Filling your ice cube trays with hot water forces the thermostat on
sooner, makes it stay on longer because of hysteresis in the controller
settings and so freezes faster. That is the only demonstrable reason I have
ever seen.

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drakaal
Warm water has less disolved gas, and provides less bouyancy so particulates
that would lower the freezing point settle to the bottom. These two factors
make the water freeze faster.

This isn't "unexplained" unless you never took a college level physics course,
which is pretty typical of the people who edit WikiPedia pages.

~~~
derleth
Then why did the Royal Society create a contest to explain it?

~~~
drakaal
RSC 1000 GBP contests are to encourage youth to develop science experiments.
Did you think that amount was enough to finance a study? The 1000 GBP contests
are all things for which RSC already has answers.

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themgt
A friend of mine swears that if you're making say a gin & tonic, filling the
glass with ice will result in less ice melting and therefore your drink will
be colder but less watery.

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nohat
That makes sense - the ice cubes are below freezing, so it will take less
melting to reach melting point equilibrium temp. In this case you are simply
using the heat storage of ice rather than the phase transition energy required
to melt the ice.

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phamilton
If I recall correctly, ice frozen after being heated has fewer air bubbles. I
wonder if there is a correlation.

~~~
marshray
The explanations I heard were that heating the water drove off trapped gases
or the warmer water took _longer_ to freeze giving them longer to escape.

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dsjoerg
This is clearly madness.

~~~
etfb
All the best science is.

Sadly, most of the worst pseudo-science is too, which makes it largely useless
as an indicator.

Ah well.

