

Why Hot Water Freezes Faster Than Cold – Physicists Solve the Mpemba Effect - jonbaer
https://medium.com/editors-picks/d8a2f611e853

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mtdewcmu
> There have been reports of similar phenomena since ancient times, although
> with insufficient > detail for the claims to be replicated. As indicated by
> Aristotle [2]: "The fact that the water has previously > been warmed
> contributes to its freezing quickly: for so it cools sooner”. Hence many
> people, when they > want to cool water quickly, begin by putting it in the
> sun. Although there is anecdotal support for this > paradox [3], there is no
> agreement on exactly what the effect is and under what circumstances it
> occurs.

?? You can reproduce this in a kitchen, but not in a physics lab? That's where
I started to wonder if this was written by crackpots and stopped reading.

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ninkendo
What I've never understood about this phenomenon, and what this article fails
to clear up is: the phrase "freezes faster" is ambiguous. It could be
interpreted as hot water "drops in temperature at a faster rate than cold
water", or "reaches 0°C before cold water, if both placed in a freezer at the
same time."

I would almost _certainly_ think only the former interpretation makes sense.
Because hot water _becomes_ cold water on its way to becoming ice. So if I
have two glasses of water, one at 20°C and one at 30°C, and I put them both in
a freezer, the hot one may _cool down_ faster, but how does it _overtake_ the
cooler one? Because once the 30° glass cools down to 20°, it should now be
cooling at the same rate as the 20° one did when it was first put in the
freezer... Right?

~~~
zem
no, hot water can indeed freeze (i.e. turn into ice) faster than cold water,
in the same environment.

[http://math.ucr.edu/home/baez/physics/General/hot_water.html](http://math.ucr.edu/home/baez/physics/General/hot_water.html)
explains it well. he addresses your point directiy:

[The "impossibility proof"] implicitly assumes that the water is characterized
solely by a single number — its average temperature. But if other factors
besides the average temperature are important, then when the initially warmer
water has cooled to an average temperature of 30°C, it may look very different
than the initially cooler water (at a uniform 30°C) did at the start. Why?
Because the water may have changed when it cooled down from a uniform 70°C to
an average 30°C. It could have less mass, less dissolved gas, or convection
currents producing a non-uniform temperature distribution. Or it could have
changed the environment around the container in the refrigerator. All four of
these changes are conceivably important, and each will be considered
separately below. So the impossibility proof given above doesn't work.

