
The physics of why hot water sometimes freezes faster than cold water - nate
https://www.smithsonianmag.com/smart-news/new-experiment-shows-why-sometimes-hot-water-cools-faster-cold-water-180975543/
======
crdrost
Arxiv link to the actual paper:
[https://arxiv.org/abs/2008.02373](https://arxiv.org/abs/2008.02373)

Looking at the actual graphs, what is presented appears at first blush to be a
bistable system prepared in its higher-energy stable state. (Or at least,
prepared in a way that several of its constituent subsystems "fall into" that
state.) Transitions to the lower-energy stable state require overcoming a
barrier, and jumping over that barrier is of course faster at higher
temperatures. At lower temperatures you essentially have a slow internal self-
heating from the periodical jumps of subsystems through this forbidden region
by thermal excitations, not unlike phosphorescence having to tunnel through an
energetically forbidden region; this self-heating prevents the thing from
cooling down fast enough.

Actual applicability to water is kind of harder to evaluate. I've long thought
that you could indeed have an Mpemba effect caused by persistent induced
convective currents in a fluid: so a hotter thing placed into a fridge might
create a more dramatic internal flow as its outer boundary layer cools and
changes in density, the convective current would certainly increase the slope
of the hotter cooling system beyond the naive temperature scaling, and might
then persist as a physical difference even after the two systems arrive at the
same temperature, leading to a faster cooling speed of the convective system
that "started out hotter". But I mean I have never really done experiments to
confirm that sort of thing.

~~~
thewileyone
You've put into scientific terms what I can't cause it's been too long since
I've done any physics.

In layman terms, the molecules in hot water are bouncing around more and thus
easier to form crystal structures because of the higher-energy state.
Molecules at room temperature have to be forced into crystal structures which
takes longer.

~~~
baddox
A little more explanation is required, since the boiling pot still needs to
pass through room temperature on its way to freezing, and it’s not clear how
at that point it has a head start on the other room temperature pot.

~~~
jhrmnn
Especially since there is no such thing as water memory.

~~~
iso947
I wonder how many kids watching frozen 2 will subconsciously accept “water has
memory” as a fact and believe it in 20 years time, being unable to source why
they think it, just something “they know”

~~~
thaumasiotes
That's something people have believed basically forever. The most modern
example would be homeopathy, which is explicitly based on the idea that water
has memory and significantly predates Frozen 2.

So, the answer to your question is "a really large number", but it's not
obvious that Frozen 2 would have influenced them.

~~~
stan_rogers
It would be more correct to say that homeopathy _became_ explicitly based on
water memory after atomic theory and the size of atoms achieved mainstream
acceptance. And while Dalton and Avogadro had certainly made convincing
arguments based on chemistry in the early 19th century, and thermodynamicists
like Boltzman had models of heat and heat transfer that seemed to depend on
particles, it wasn't until after Einstein's paper on Brownian motion and
Rutherford's interpretation of the Geiger-Marsden experiment that homeopathic
dilution just being dilution became an unsustainable notion.

It was always a silly notion, especially when numbers like 30C - a dilution of
one part in 10^60, or one molecule of the target substance in 3 × 10^34kg of
water, _after_ you understand what molecules are - were being thrown around,
but there was at least a plausible explanation before, say, WW1.

------
inetknght
Once upon a generation ago, I worked at an ice rink. "Cleaning the ice" using
an ice resurfacer involves cutting the top 1/16" of ice off and laying down
new water. Hot water is preferred because it freezes faster and produces a
denser sheet of ice; a denser sheet of ice is going to be better for skating.

It's nice to see more papers on it.

~~~
lmilcin
Hot water and is preferred because it has less gasses dissolved and this way
it produces clear ice. Also cold water could freeze too quickly, if the rink
is very cold, before it has time to spread evenly.

I am pretty sure any tiny time difference, if it exists, would not be worth
the hassle of supplying hot water.

~~~
sasaf5
> Hot water is preferred because it has less gasses dissolved

true, but while the temperature decreases the air dissolves back in.

What makes ice clear is directional freezing, the sheet freezes from below,
pushing the gases up. You can try that in your freezer by putting in it a
styrofoam box full of water. Just in this case it will freeze from the top and
push the gases downwards. The upper layer will be crystal clear.

~~~
lmilcin
I did this experiment many times, myself. If you start with boiled water you
will get much thinner layer with bubbles.

Air actually takes a bit of time to diffuse into water if it isn't getting
mixed with air (that's why your fish tank needs air to be actively mixed with
water). Starting with thin layer of water that was recently hot gives not
enough time for any significant amount of air to dissolve.

I work with electronics. I use electronic water for parts cleaning. Electronic
water is pure water, distilled, deionized, degassed (no oxygen in it) so it
does not oxidise copper traces. It does go bad (bad meaning gets oxygen
dissolved) when opened, but think more like hours or even days if proper
precautions are taken (don't shake or stir once the container is opened).

~~~
saagarjha
Does it taste any different than normal tap water?

~~~
lmilcin
It has awful taste close to regular distilled water. Mineral profile greatly
influences water taste.

------
jrochkind1
Somehow this article provides no explanation at all of how this actually
happens. Perhaps the research being reported doesn't really have an
explanation either, it's just more demonstration under more controlled
conditions?

Or the explanation as it were is just "abstract" and "geometrical", basically
just mathematical?

Very odd phenomenon.

~~~
Florin_Andrei
The fact is, you do need to get rid of more heat when using hot water.

Ergo, some sort of self-reinforcing feedback loop must appear when you use hot
water, that sucks heat out of it faster. If you use cold water, the
"turbocharged" mechanism does not operate for some reason, and heat is lost at
a lower rate.

I've no idea what that mechanism is. It's nonlinear behavior for sure.

~~~
jakear
A hypothesis I’ve heard is that the hot water is more likely to produce
currents inside the water, as there’s more of a temperature difference between
the inner water and the water right at the edge. These currents then cause the
water to cool more uniformly as opposed to outside-in. I believe the person I
heard this from tested this by adding baffling to the container to prevent
currents, but I’m not sure what the results of that experiment were.

~~~
wombatmobile
That's baffling, jakear.

~~~
Florin_Andrei
Cell convection is relatively common.

For example, this is why large amateur telescopes perform worse for about an
hour after they're brought outside: the mirror is "hot" compared to the air,
so a network of toroidal convection cells (spinning doughnuts of air) forms on
its surface, ever so slightly messing with the rays of light.

There's actually an attractor for some of these networks (speaking in
general), where they form hexagonal cells. Of course, in most cases, it's more
disorderly than that.

I'm not sure how a turbocharged network could form, but I'm looking forward to
learn more on this topic.

~~~
jakear
I think they were making fun of my spelling, but I'm not sure because I don't
know if I was wrong.

~~~
Florin_Andrei
Oh, I see. I think it was just a simple play on words.

~~~
wombatmobile
Yes! Just word play. There's nothing wrong with Jakear's spelling or his
comment, which was informative.

Next time I will smile :-)

------
jointpdf
I have a vivid memory of being taught this in 2nd grade, probably because it
seemed (and still seems) extremely counterintuitive. I think the intuition
given was that hot water is less dense than cold water, so the coldness can
more easily permeate through the liquid (I imagine a vein-like pattern).
Something like that.

At least according to Wikipedia, it seems that it’s a bit more complex:
[https://en.m.wikipedia.org/wiki/Mpemba_effect](https://en.m.wikipedia.org/wiki/Mpemba_effect)

Anyways, I’ve been repeating this fact for 2.5 decades, so shout out to my
teacher for blowing my young mind.

~~~
gvjddbnvdrbv
Cold is not something that can penetrate.

Cold is merely a relatively low amount of heat energy per unit of mass.

When something cools heat energy is conducted/convected/radiated away from an
object causing it to have a lower amount of heat energy per unit mass.

~~~
jointpdf
Admittedly, my understanding of thermodynamics has not progressed much beyond
the 2nd grade level (aside from a class on PDEs giving me at least some
enhanced intuition). My takeaway from the PDE class was that anything beyond a
simple textbook problem needs to be simulated and/or observed experimentally,
rather than solved for analytically.

But other things being equal, wouldn’t cold air permeate a less dense barrier
(say a layer of insulation in a home) more easily than a dense barrier? Just
as water (or cold air) would permeate soil more readily than rock.

~~~
gvjddbnvdrbv
Cold AIR permeates. That is correct. But there is no physical concept of COLD.
The physical concept that exists is heat energy. Temperature is defined as the
amount of heat energy per unit of matter. When something gets colder the
physical process happening is that heat energy is leaving the thing and being
transferred somewhere else/being converted to another form of energy.

So when cold air comes into a house heat energy is transferred from e.g. the
material that makes up the wall into the air warming the air and cooling the
walls. The cold air is replacing warm air which moves out of the building.

------
pdm55
All the talk in the comments about dissolved oxygen brings back a few memories
of student lab work that went wrong. I was given a freeze-pump-thaw protocol
to follow in order to remove oxygen from a copolymer solution. Basically we
used liquid nitrogen to freeze the mixture, then used a vacuum pump to remove
the gases above the mixture. When we thawed the mixture, the dissolved oxygen
would escape, in order to equilibrate with the amount above the surface. I
repeated this for a few cycles. It was all done with sealed glassware.

Unfortunately, the glassware kept cracking during the thaw stage. This went on
for a couple of months, till finally my supervisor reviewed the procedure, and
realised that, whether cracking occurred, depended upon the relative
concentrations of the two polymers. If I recall correctly, the problem
occurred when the polymer with the lower freezing point (which, thereby,
thawed first) was present at a lower concentration. It was basically trapped
inside the other still-frozen polymer, and expanding, as it thawed ... till
crack!

The solution was surprisingly simple. Change the method. Bubble nitrogen
through the mixture to drive out (sparge) the oxygen. Replacing the oxygen
with nitrogen was beneficial, as nitrogen didn't interfere with the
copolymerisation reaction.

The main problem was that I had lost two months of experimental time, and this
was just the start of a series of experiments and instrumental analyses. It
was now Easter and the deadline to hand in my student thesis was early
December.

Please don't believe anyone who says students have an easy life.

------
fersho311
I read the whole article and couldn’t understand how or why this happens. Is
the reason behind this unknown?

~~~
mmerlin
My high school chemistry teacher simplified it down to the greater "rate of
change" of temperature dropping, for the hot water, causes it to end up
freezing faster than the cold water.

~~~
ntsplnkv2
The rate of change would slow down as it approached the colder temperature
though, would it not?

------
tzs
Just a few days short of 35 years ago, this came up in “The Straight Dope”
[1].

[1] [https://www.straightdope.com/columns/read/422/which-
freezes-...](https://www.straightdope.com/columns/read/422/which-freezes-
faster-hot-water-or-cold-water/)

~~~
umanwizard
Random tangent: TIL that The Straight Dope predates the web and was originally
a newspaper column.

~~~
nemo44x
Well I’m officially old because I:

1) didn’t know Cecil Adams still existed.

2) still remember looking forward to the Straight Dope in my weekly alt-paper.

------
m-i-l
This was known by many Antarctic scientists before 1963. I remember my dad
telling me you could throw boiling water into the air and it would freeze into
millions of tiny ice crystals before reaching the ground, but that the same
wasn't true of cold water. I think he said it was due to thermal inertia.

~~~
TylerE
That sounds like a totally DIFFERENT effect, almost certainly caused by
droplet size.

~~~
brennanpeterson
It could be the same.

I always learned the explanation as: 1\. Cool to freezing via evaporation.
Because this goes as T^4, the time difference is negligible to reach 0°C.
But...hot water loses more volume. 2\. Freeze, bit now you need less energy to
freeze less material.

I did this at home, and the simple check worked: volume was lower in the
frozen hot water.

This explanation might be wrong! But it works out in simple tests and
mathematically.

------
handmodel
I'm not 100% sure it is the same phenomenon but growing up in a very cold
climate where I had to park my car outside in high school I remember many
times getting into the car where a half full Poland Spring plastic water
bottle would sit with water. If I tapped the bottle - the entire thing would
freeze in under a second. It looked amazing.

(I always thought it was a pressure thing - but not positive if that is what
the article is describing or something more nuanced)

~~~
senectus1
there is a similar inverse process for hot water as well... if you take a very
very clean and perfectly smooth surfaced (on the inside) cup/glass and
microwave it to just before the point of boiling, then drop anything into it
(sugar, coffee, a teaspoon... pretty much anything) it will _very_
energetically, instantly boil.. often vertically... be careful performing this
experiment.

Edit: here this will help explain it
[https://www.animations.physics.unsw.edu.au/jw/superheating.h...](https://www.animations.physics.unsw.edu.au/jw/superheating.htm)

~~~
kosievdmerwe
Why I never boil anything in a microwave.

Because as you say it explodes when you interact with it and are therefore are
close to it rather than when it's sitting safely inside the microwave.

------
mensetmanusman
If you put warm water in your ice try, you can get clearer ice near the
surface, because the ice will freeze from the top surface instead of from all
sides at once trapping bubbles of air with no where to go :D

~~~
bxparks
That's interesting. But don't drink the hot water from the hot water tank. It
can be dissolved with unhealthy minerals and chemicals. A few years ago, I had
to replace a couple of hot water tanks. When I drained them, the last few
gallons at the bottom of the tanks looked like yellow vomit, it was seriously
disgusting.

~~~
quickthrower2
Which makes me think ... always ask for "no ice" when you are at a bar or
restaurant. Unless you know how they make it.

~~~
Ductapemaster
I think that's a pretty un-based fear. Almost certainly any restaurant, and
especially a bar, will use a professional ice making machine. At the volumes
if ice they go through, it wouldn't make sense to make it in any other way.

------
montalbano
There has been some controversy about this for years, see for example:

[https://www.chemistryworld.com/news/mpemba-effect-in-hot-
wat...](https://www.chemistryworld.com/news/mpemba-effect-in-hot-water-after-
doubt-cast-on-its-existence/2500087.article)

[https://www.repository.cam.ac.uk/handle/1810/263847](https://www.repository.cam.ac.uk/handle/1810/263847)

This new paper may prove to be conclusive in supporting its existence, but as
I'm not an expert I'll give it some time in peer review before coming to any
conclusion.

------
ssivark
This might be a better link: [https://www.sciencenews.org/article/physics-new-
experiment-h...](https://www.sciencenews.org/article/physics-new-experiment-
hot-water-freeze-faster-cold-mpemba-effect)

Punchline seems to be that “hot” but out-of-equilibrium material cannot be
summarized by a “temperature”, providing many paths towards freezing (on the
landscape of microstates), some of which could (efficiently) circumvent the
“low temp” state.

------
squarefoot
A marginally related question for the experts here: I love extremely cold
water, juices and drinks, so I often keep them in the freezer in plastic
bottles. Now, if I take out a bottle of something that is just about to
freeze, as soon as I open the bottle, it almost immediately freezes although I
am certainly not removing heat from it but actually doing the opposite by
keeping it with my hands, not to mention the warmer environment in which it is
now. I couldn't explain it in any other way than with the different pressure,
that is, the water expands when it freezes, so the almost freezing liquid is
already under higher internal pressure compared to the moment it was closed,
but as soon as I open it the pressure drops abruptly, which would be enough to
compensate for the higher temperature, de facto freezing the water.

Is this correct?

------
air7
This is one my favorite science stories. A tale of how truth eventually
surfaces and holds in front of ridicule.

[https://en.m.wikipedia.org/wiki/Mpemba_effect](https://en.m.wikipedia.org/wiki/Mpemba_effect)
See the section called "Mpemba's Observation"

------
saeranv
Perhaps it has to do with the increased turbulence in the hotter fluid? This
would be a result of the higher saturated vapor pressure of the liquid leading
to greater evaporation/boiling. Even as it cools down the higher temperature
difference between the hot fluid and room temperature should produce a more
turbulent boundary layer along the surface of the water.

The turbulence will reduce the thermal resistance associated with laminar
surface air films on colder surfaces, which would normally be insulating
surface, so you'd get a faster heat loss then with a more stagnant layer of
cold water.

------
RcouF1uZ4gsC
Sometimes when we are so familiar with something, we forget how special it is.

Water is really amazing. It expands when it freezes. Hot water freezes faster
than cold water. It dissolves almost anything, but not too fast.

------
surround
Suggested explanations

[https://en.wikipedia.org/wiki/Mpemba_effect#Suggested_explan...](https://en.wikipedia.org/wiki/Mpemba_effect#Suggested_explanations)

------
irrational
We tried to replicate this for one of the kid's elementary school science fair
project. We were unsuccessful, but the judges really liked it anyway.

------
maydemir
yes faster. but not earlier.

~~~
devin
came here to say this. the headline for this finding is almost always
misleading.

~~~
maydemir
Yes, exactly.

------
seventytwo
Emphasis on “sometimes”.

~~~
dumbfoundded
Yeah but a reproducible sometimes is pretty amazing. Sounds like some
interesting applications await.

------
adamnemecek
I wonder if this is a right analogy. Imagine two cars one going fast, one
slow. If driver of the fast car steps on the break harder than the driver of
the slow car, he will stop faster. The temperature difference is similar to
how fast you step on the break.

~~~
muti
But cooling rate is proportional to the temperature difference (see Newton's
law of cooling). Thus the hotter water may start cooling faster, once it
reaches the initial temperature of the cooler water, it would cool at the same
rate as the cooler water did initially. It then couldn't possibly reach
freezing point earlier.

But in some conditions it does, which is the surprising result.

~~~
sliken
Hot water freezing faster in an ice cube tray is commonly reported, a bunch of
reasons could explain it. Evaporation can reduce the volume of water, and the
smaller volume freezes faster. A hot tray will settle into any ice build up in
the freezer (which is common) increasing the rate of heat exchange. Hot water
will create currents in the air and water (driven by the higher water
temperature) which help the exchange. On the flip side lower temperature
differences will often cause a stagnant layer that will reduce the temp
exchange, much like sitting in a cold pool which gets better if you want a few
seconds... until you move again.

The reported experiment reduces the variables by using glass instead of water,
so there's no internal flow, but that doesn't stop the airflows generated in
the air.

------
aaron695
> Hot Water Sometimes Freezes Faster Than Cold Water

No it doesn't.

Unless you want to word play between English and really specific small
technical processes, perhaps.

What's interesting is it's an old urban legend the predates Usenet.

Usenet did kill the glass is a liquid legend though.

~~~
aaron695
We all get the Mpemba effect is totally fake right?

You all own freezers so you can test it for yourself, it was found by a child
making icecream so it will be easy!

And if you're the !first! to put it on Youtube that'd be cool.

Sciencium talks about it here if you want an ok run down and how it's not real
-
[https://www.youtube.com/watch?v=SkH2iX0rx8U](https://www.youtube.com/watch?v=SkH2iX0rx8U)

