[HN Gopher] Controversy continues over whether hot water freezes...
___________________________________________________________________
Controversy continues over whether hot water freezes faster than
cold
Author : sohkamyung
Score : 266 points
Date : 2022-06-30 01:12 UTC (21 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| rossdavidh wrote:
| I am wondering why, with all of the many experiments done to
| investigate this, none of them were done with ice cream:
|
| "Mpemba opted to skip waiting for his boiled-milk-and-sugar
| concoction to cool to room temperature like the other students
| had done. An hour and a half later, his mixture had frozen into
| ice cream, whereas those of his more patient classmates remained
| a thick liquid slurry..."
|
| Boiled milk and sugar is not going to freeze in the same way as
| pure water, there are effects from the mixing of anything, but
| some of the substances in milk are oily so you might even have
| colloidal effects. It seems like if that's where Mpemba first saw
| it, you should start there to attempt to reproduce it?
| fny wrote:
| Can someone explain why Newton's Law of Cooling isn't a
| sufficient explanation? The temperature delta is clearly larger
| with hot water, do one would expect it to cool faster, no?
| epgui wrote:
| Yes, it cools faster at t=0, but it will take longer to cool
| down.
| happyopossum wrote:
| Cooling faster and reaching sub zero temperatures faster are
| two different things. The hotter water will drop more degrees
| per minute/hour based on Newton, but at some point the two
| liquids will be the same temperature, and then you're assigning
| some sort of thermal momentum that Newton doesn't cover.
| ddingus wrote:
| Would the warmer water not have currents present from
| convection? Those are a source of inertia. There is a case
| for the warmer liquid never actually reaching the same state
| the cooler liquid is in.
| saeranv wrote:
| Other people have answered, but I think this is slightly more
| intuitive way to put it: Newton's law of cooling states that
| the rate of temperature reduction (heat loss) of a body is
| proportional to the temperature delta between it and its
| ambient surroundings. We can write this as:
|
| dT/dt = kT
|
| Where T is temperature delta, t is time, and k is some
| proportionality constant. So the rate of cooling is changing as
| the delta temperature reduces, specifically, it is getting
| exponentially reducing as it gets closer to thermodynamic
| equilibrium (where no heat is exchanged):
|
| T(t) = Ce^kt C = T(0)
|
| Which means there is a larger cool down with the hot water at
| first, but under this simple model, once the T(t) hits the same
| temperature of the cool water it's being compared to, it's
| cooling rate should be equivalent (and a lot slower).
| fny wrote:
| This should have been obvious.
| andrewchambers wrote:
| Hot water would need to first reach the temperature of the cold
| water (and overtake it) before freezing... Your explanation
| doesn't make sense.
| toss1 wrote:
| >>a lesson from the initial skepticism and dismissal that the
| schoolboy's counterintuitive claim had faced: >>"It points to the
| danger of an authoritarian physics."
|
| Key point there, at the very end, worth taking on board (which is
| different from listening to every willfully ignorant nutter).
| akeck wrote:
| Don't let the clickbait headline fool you. This article is a
| wonderful account of when the effect got its name after being
| noticed in a secondary school in Tanzania and all the fascinating
| science that's happened since then.
| ZiiS wrote:
| I would expect most freezers to detect the heat and run their
| chillers and fans more?
| fuzzfactor wrote:
| >researchers often have trouble determining if they have
| accounted for all confounding variables.
|
| You can't account for all confounding variables.
|
| That's what makes them confounding to begin with.
| prerok wrote:
| What I don't get is why there is no mention of evaporating effect
| on the energy level of the system.
|
| Evaporating water takes quite a lot of energy so the rest of the
| water is cooled:
|
| > During evaporation, energetic molecules leave the liquid phase,
| which lowers the average energy of the remaining liquid
| molecules. The remaining liquid molecules can then absorb energy
| from their surroundings. This process can take place at any
| temperature because some of the molecules in a liquid will always
| have enough energy to enter the gas phase.
|
| Source for the quote:
| https://highschoolenergy.acs.org/content/hsef/en/how-can-ene...
|
| I always thought this was the reason when observing the effect
| when trying to pour hot instead of cold water to defrost your
| windscreen during cold winter months :)
| BrandoElFollito wrote:
| > pour hot instead of cold water to defrost your windscreen
|
| This sounds like a risky experiment, the windshield is likely
| to shatter with such thermal differences and the associated
| tensions.
|
| If I am not mistaken, a windshield is created already with
| internal pressure to ensure it will shatter to pieces when the
| local pressure changes abruptly (via punching e windshield with
| a hard pointy object for instance)
| SpEd3Y wrote:
| When I was a kid we used hot water to defrost the windscreen
| regularly. A couple of hundred times over multiple winters.
| Temperatures around -20. No cracks or anything like that ever
| occurred.
| BrandoElFollito wrote:
| When was that, out of curiosity? I seem to remember that
| the structure of the windshields (at least in Europe)
| change in the 70's from ones that would break with a crack,
| to the ones that would shatter in plenty of small pieces
| (due to the internal tension, and at least two layers)
| johnatwork wrote:
| Maybe they meant hot from the tap and not boiling, which
| will certainly crack the windshield.
| prerok wrote:
| I saw that in the 90s but people were definitely doing it
| in the 70s as well. I am not aware of any stories of
| cracks or shattered windscreens because of it, though I
| am sure that if you do that with boiling water and not-
| so-frozen windscreen often enough, you would get that
| result :)
| prerok wrote:
| It was definitely not boiling but hot from tap, so could be
| about 60degC, and, yeah, somehow the windshield did survive
| it. It's probably not that bad because you actually pour the
| water on the frosted windscreen so most of the heat would
| dissipate already before actually reaching the material.
| DantesKite wrote:
| >The abstract findings suggested that the components of a hotter
| system, by virtue of having more energy, are able to explore more
| possible configurations and therefore discover states that act as
| a sort of bypass, allowing the hot system to overtake a cool one
| as both dropped toward a colder final state.
|
| > "We all have this naive picture that says temperature should
| change monotonically," said Raz. "You start at a high
| temperature, then a medium temperature, and go to a low
| temperature." But for something driven out of equilibrium, "it's
| not really true to say that the system has a temperature," and
| "since that's the case you can have strange shortcuts."
|
| I gotta admit, that's pretty cool and unintuitive.
| tshaddox wrote:
| What still isn't clear is what the the initially-hot system has
| "learned" at the moment it reaches the starting temperature of
| the initially-cool system. There must be some difference
| between the two states at that temperature if the initially-hot
| one is going to catch up with and surpass the initially-cool
| system.
| Noughmad wrote:
| The simple answer is that the cool system has a very uniform
| temperature (because it's been at that temperature for a long
| time), while the hot system that's cooling down is doing so
| with large spatial variations of temperature. In other words,
| all the water in the initially cool system is at 10degC, but
| the initially hot system has pockets of 5degC and pockets of
| 15degC.
|
| It doesn't have to "learn" anything in order for there to be
| a substantial difference.
| ephbit wrote:
| These temperature gradients obviously lead to convection.
|
| Stronger convection means greater heat transfer, thus
| greater rate of cooling down.
|
| But it'd be surprising if the inertia of the convection of
| the initially hot system didn't just gradually decline
| (because of friction) to almost exactly (little bit
| greater) the same level of convection (which the initially
| cool system had in the beginning) when it reaches the same
| average temperature.
| londons_explore wrote:
| Water currents can last a surprisingly long time. Pour
| one liquid into another of a slightly different color and
| the swirling mixing process can go on for minutes at
| least. Perhaps hours in the right circumstances.
| infogulch wrote:
| Interesting theory for a specific mechanism: "local
| temperature difference-induced convective cooling". Could
| be falsified by giving both initially-hot and initially-
| cold samples a stir rod, expecting the cooling performance
| of the initially-cold sample to improve to match the
| initially-hot sample.
| waterhouse wrote:
| Ho ho, guess what?
|
| > The Scottish scientist Joseph Black investigated a
| special case of this phenomenon comparing previously-
| boiled with unboiled water; the previously-boiled water
| froze more quickly. Evaporation was controlled for. _He
| discussed the influence of stirring on the results of the
| experiment, noting that stirring the unboiled water led
| to it freezing at the same time as the previously-boiled
| water, and also noted that stirring the very-cold
| unboiled water led to immediate freezing._
| https://en.wikipedia.org/wiki/Mpemba_effect
|
| I'm surprised the article didn't mention this.
| infogulch wrote:
| Wow! I guess that's the final answer then. Maybe to seal
| it, devise the inverse experiment where you somehow
| inhibit normal convection in boiled water and expect to
| see it take as long as the cold water.
|
| Very interesting!
| xelxebar wrote:
| From my reading, the point is that the glass of water doesn't
| really "have a temperature" any more, since the cooling
| transition means it's no longer in thermal equilibrium.
|
| That is, you could perhaps model the bulk of water as having
| a temperature _field_ , and clearly every point in that field
| passes through the starting temperature of the initially-cool
| system, but the gradient landscape is vastly different.
| andbberger wrote:
| no, temperature is simply average kinetic energy and is
| always defined, even for systems far from equilibrium
| eternauta3k wrote:
| The point is that the state of the system consists of
| more than its average temperature. It's not just
| "catching up" to the state of the initially-cold water,
| it's going through other configurations.
| AndrewThrowaway wrote:
| But we are still dealing with energy? Every point should
| still transfer energy which should take time?
| ephbit wrote:
| Would be interesting to know what the outcome of fluid
| dynamics and physics simulations of this setup is.
|
| Heat transfer, convection, conduction, evaporation and so
| on should be available in useful implementations in state
| of the art simulation software.
| Ericson2314 wrote:
| Think annealing.
|
| It's found it's way into the "minimal valley" whereas an
| arbitrary lukewarm state might be closer to a "ridge".
|
| Clearly, yes, if you could start at the magic state that
| would be ideal. And there should be experiments that use a
| bunch of thermometers too compare the time from the same
| average temp (the initially warmer one just gets the "running
| start").
|
| Mathematically it is not hard to _abstractly_ characterize
| what is going on. (This is not saying the actual physics is
| easy!!) Temperature is an equivalence class on fluid states,
| but the average time to transition between those states does
| _not_ form a metric space. The failure of the triangle
| property shows that the composition of transitions induces
| non-uniform distributions within the temperature equivalence
| classes that subvert the expected transition time by which we
| had attempted to build a metric space to begin with.
|
| Just like non-euclidian space in the 19th century, this is
| the sort of thing where the mathematics can say "yeah sure
| seems legit" before the physics stops saying "wait wtaf",
| lisper wrote:
| Problem with that theory:
|
| 1. Temperature is monotonically related to energy content.
| A warmer system has more energy than a colder one, all else
| being equal.
|
| 2. To cool, a system must release energy. The rate at which
| a system can release energy is monotonically related to the
| difference in temperature between the cooling system and
| the cold sink to which its energy is being released. The
| bigger the temperature difference, the higher the rate of
| energy release (all else being equal).
|
| For the Mbemba effect to be real, one of those two premises
| must be false. Which one is wrong?
| extr wrote:
| I don't think either one is false. To me, 2 looks false
| if you consider the "system" in question to be the entire
| glass of water. But my interpretation is that in this
| case the glass of water is really many very small systems
| (pockets of unequal amounts of energy) all interacting
| with each other.
|
| Let's say you're given a few hot potatoes and have to
| cool them down as fast as possible. You have a
| refrigerator but you can only keep one of them in it at a
| time. How do you decide which one to put in at any given
| time so they all reach temp the fastest? When the
| "system" involves many possible pairings of temperature
| differentials it feels very intuitive that some
| configurations would be better than others. So it removes
| the mind-bending thermodynamics law breaking aspect of
| it.
|
| Admittedly, this isn't really what the article says! I
| honestly don't get how my above intuition squares with
| the whole energy minima thing, and so while useful as a
| thought exercise to at least help me entertain the idea,
| I'm not really sure it's correct?
| eternauta3k wrote:
| The second one is false.
|
| https://en.wikipedia.org/wiki/Leidenfrost_effect
| eternauta3k wrote:
| Basically, heat transfer rate in a liquid goes down when
| a certain temperature is reached, because an insulating
| layer of vapor is formed.
| cycomanic wrote:
| IIRC (my thermodynamics is a long time ago), but 2
| applies only when the two (cooling system and sink) are
| near equilibrium, which is not the case here.
| teraflop wrote:
| > Temperature is monotonically related to energy content.
| A warmer system has more energy than a colder one, all
| else being equal.
|
| _In equilibrium_ , the temperature of the water is
| directly related to the energy content, with the heat
| capacity being the conversion factor.
|
| But under the thermodynamic definition, the temperature
| of the system depends not only on its energy but also on
| its entropy. And one of the points the article is making
| is that even when we know the total amount of energy
| entering or leaving the system, its entropy may not be
| nearly so easy to measure or calculate when its state is
| far from equilibrium.
| lisper wrote:
| Sorry, but that is not true. The _definition_ of the SI
| unit of temperature, the Kelvin, is given in terms of
| Boltzmann 's constant, which has units of Joules (i.e.
| energy) per Kelvin.
| RandomLensman wrote:
| That would imply that water once heated and then cooled is
| different from water never heated - at least for a time. I
| wonder if there is any evidence for that.
| speed_spread wrote:
| One difference could be that hot water is not as good a
| solvent for gas (think soda bubbles). So heated water
| would have shed most of its gas content which would lead
| to a different (faster?) freezing process, if freezing
| occurs faster than gas can dissolve back in.
| bordercases wrote:
| Should make for interesting computational effects
| asojfdowgh wrote:
| https://en.wikipedia.org/wiki/Simulated_annealing
| andbberger wrote:
| jeremy england's lab has done interesting work in this area
| k__ wrote:
| So, it's a bit like Chromium throttling cache requests?
| sgtnoodle wrote:
| How about this theory. The hot water causes the freezer to run
| its compressor sooner. The system is nonlinear due to intentional
| hysteresis in thermostats, and the excess energy stored in the
| hot water is trivial compared to the capability of the heat pump.
|
| Introducing hot water into a freezer rapidly raises the ambient
| temperature of the freezer. This causes the thermostat to click
| on, and the heat pump therefore runs until the ambient air drops
| down below the thermostat's shut off point. A lot of heat is
| removed very early on, and the ambient freezer temp is at its
| lowest going into the liquid to solid phase change.
|
| With the cold water, the ambient air of the freezer doesn't rise
| nearly as much. The thermostat does n't click on until a long
| while later, once enough heat leaks out of the freezer. The
| ambient air is warmer going into the phase change.
|
| It doesn't work if you put the two cups in the same freezer, but
| at least it could explain some anecdotes.
| incanus77 wrote:
| I observed the phenomenon growing up when I filled buckets of
| hot and cold water and left them outside in freezing
| temperatures.
| Jensson wrote:
| Hot water melts snow, and the ground is much colder than the
| snow which isolates heat. So in your case the simple
| explanation is just that, the hot buckets melted the snow/ice
| and therefore stopped being isolated against the cold ground
| so froze faster.
| incanus77 wrote:
| They were set on concrete.
| ascar wrote:
| The assumption being they were placed on snow, which I
| would argue is an unlikely assumption to hold up. We have
| much more days with freezing temperatures than days with
| snow lying around and even then with lots of snow there are
| enough places that don't have any (like a balcony).
| sgtnoodle wrote:
| Some more ideas. Cold water holds more dissolved air in it
| than hot water, and the hot water heater would have recently
| rendered most of those gases out. That dissolved air could
| negatively impact the thermal conductivity of the water.
|
| Near boiling water is about 4% less dense than near freezing
| water. If you filled the buckets right to the top, there
| would be less water in the hot bucket.
| cowtools wrote:
| I think it might have something to do with the convection
| current of the hot water. Perhaps the higher initial velocity
| allows the ice crystals to nucleate faster?
| stdbrouw wrote:
| > the excess energy stored in the hot water is trivial compared
| to the capability of the heat pump
|
| I don't know if I'd call it trivial, bringing 1 kg of water
| from 85C to 0C involves about 100W for a duration of one hour
| whereas a typical freezer removes heat at, what, 200W or 300W?
| chmod775 wrote:
| >bringing 1 kg of water from 85C to 0C involves about 100W
|
| That looks like the amount of energy needed to _heat_ that
| water.
|
| But cooling things doesn't work the same way. Instead you
| want to extract that energy/move it somewhere else. The
| efficiency of some approaches, unlike when heating something,
| can hugely depend on ambient temperature. It's possible
| you'll be able to extract more useful energy cooling
| something than you'll have to expend!
| [deleted]
| danieljacksonno wrote:
| But a 100ml beaker would only require 10W
| comboy wrote:
| I love that theory. I would assume they use big enough freezer
| or with enough ice inside for that not to matter, but that
| would be a hilarious solution.
| bbarnett wrote:
| _How about this theory. The hot water causes the freezer to run
| its compressor sooner._
|
| This may happen but, in a normal freezer, the action of the
| compressor is meaningless.
|
| My feezer is 70% full, with probably 30kg of stuff in it. This
| stuff is mostly at -22C (I have a thermometer), and so, this
| mass is what cools new things added to the freezer, fast.
|
| (Why -22C instead of -18C? Salt water fish is in there, and I
| also want a little headroom.)
| titzer wrote:
| Unless the new things are physically touching the other mass,
| then my guess is that the primary cooling effect is due to
| 1.) convection through the air in the freezer, followed by
| the 2.) conduction through the surface the new things are
| sitting on, followed by the 3.) IR radiation of the new
| thing. The existing thermal mass in the freezer interacts
| with the new thing via 1 (indirectly) and 3 (by absorption),
| but I suspect that the compressor cooling the air is a bigger
| effect.
| bbarnett wrote:
| All documentation I've read, and personal experience, says
| otherwise.
|
| For example, most manuals urge one to not have an empty
| freezer. That cold mass is one reason why.
|
| When I was a kid, we had a fridge from the 50s in a
| cottage. When I started drinking, we'd put 5 cases of beer
| (24*5) in glass bottles in there, and 2 hours, yes hours
| later they'd be at 1C.
|
| Now, modern fridges/freezers literally do not have that
| degree of cooling power. This is on purpose, for it is more
| efficient to power a small compressor all the time, than a
| massive compressor for 10 minutes.
|
| You may want to argue this point, and that's fine, but I am
| merely providing info both from the manuals of modern
| freezers and fridges, which I have read, and from online
| when looking at why they are so bloody slow to cool things.
|
| My LG fridge manual actually says not to put warm meat in
| my freezer, unless the thing has loads of frozen stuff in
| it.
|
| Otherwise the meat could go bad before freezing.
|
| Oh, another wonder of modern fridges. If you buy one and
| put it outside, or in an unheated garage, the freezer
| becomes useless.
|
| This is because many fridges have no thermometer in the
| freezer part, and only get a reading when the compressor
| comes on, to cool the fridge.
|
| As the fridge is always cool when it is 3C outside, or
| cooler, the freezer never keeps stuff frozen.
| sgtnoodle wrote:
| You're putting a lot of faith into manuals for mass
| produced household appliances. All that holds true to a
| first order, but the actual thermodynamics going on is
| certainly way more complex than what's captured in a
| consumer facing manual. When discussing the relative
| freeze times of a modest amount of water for the purposes
| of a science experiment, the transient higher order
| effects are going to be an important factor.
| titzer wrote:
| Oh, I totally get the argument to keep it full. Both the
| colder thermal mass and less cold air to escape make it
| considerably more efficient. I just doubt that the
| thermal transfer from the mass to the newer items is as
| significant as the compressor kicking in. After all, the
| compressor is going to flood the internal radiator with
| liquid far below the ambient temperature of the freezer.
|
| > My LG fridge manual actually says not to put warm meat
| in my freezer, unless the thing has loads of frozen stuff
| in it.
|
| That sounds silly to me. In my experience ice cubes
| freeze within an hour and meat within a couple hours.
| atwood22 wrote:
| The compressor is moving the air around in the freezer, which
| can have an impact on how much heat is transferred. You can
| sit comfortably in a sauna, but your arms will heat up
| quickly if you wave them around. Similarly, a convection oven
| heats things faster than a conventional oven.
| SulphurSmell wrote:
| The compressor doesn't move any air inside the
| refrigerator. There is a separate fan in the freezer
| compartment that does that.
| atwood22 wrote:
| True, but I meant that fan runs when the compressor is
| running to blow air over the coils. That movement of air
| will allow more transfer of heat from the stuff in the
| freezer.
| aaron695 wrote:
| > Controversy Continues
|
| It's never been controversial. It doesn't.
|
| It's cute a high school kid in Africa did an experiment that had
| a wrong answer and people are not sure exactly why.
|
| Position in freezer, different sizes etc. And exploring why is
| worthy of science at all levels.
|
| But this nut jobbery shows why science is broken at a structural
| level.
|
| It's on _them_ to make this replicable . Which they have not for
| _60 years_
|
| Yet science does not denounce the result after 60 years. Think
| about that.
|
| There's little point talking about something as broken as
| "Science" but I suspect this is the same issue as why we have
| only recently understood syphons. Science doesn't have an answer
| for when multiple things are happening.
|
| There are multiple genuine things screw up the result. But it's
| probably mainly one. But since the other things are legitimate
| science can't converge.
| thrusong wrote:
| Not scientific whatsoever but when I worked in my high school's
| canteen, we'd put hot water in the Slush Puppie machine to
| recover quickly during rushes and it sure seemed to work faster
| than cold water.
| NicoJuicy wrote:
| Wasn't it that hot water dampens and it dampens the warmth first.
|
| So it quickly reduces temperature.
| irrational wrote:
| > When the experimental parameters were tuned just right, the hot
| system's particles almost immediately found their final
| configuration, cooling exponentially faster than the warm system
|
| My daughter tried to replicate the effect in our home freezer for
| a science fair project. Apparently her experimental parameters
| were not tuned just right because she was not able to replicate
| the effect. The fair judges were very interested in her project,
| even though she wasn't able to make it work.
| duxup wrote:
| I can't say I followed it until the end of the article but it is
| a great story.
|
| The effect must be very small for it to be so difficult to
| measure.
| omoikane wrote:
| It might have been difficult to follow because the article was
| verbose, wikipedia's article on the same topic felt easier to
| digest:
|
| https://en.wikipedia.org/wiki/Mpemba_effect
| dontknowwhyihn wrote:
| Has anyone thought to run the experiment, and then examine the
| structure of the ice after it's frozen?
|
| Maybe the hot water is somehow more able to align its molecules
| in a crystal structure faster, and this somehow causes it to
| freeze first?
| bergerjac wrote:
| In 11th-grade 'Honors' Chemistry, we ran the 'Which water
| freezes faster' experiment outside in ~0.
|
| 2 glass beakers with same amount of water: 1 hot, 1 cold,
| stirring both at the same rate. Teacher told us the experiment
| was over once ice starting forming on the surface...
|
| Cold water developed ice first.
| winReInstall wrote:
| Hot water has potentially a much larger surface, as the molecules
| all have the brownian energy to escape the fluid bonds and cool
| the remainder down doing so.
|
| So given a big enough surface (turn it into a foam) should allow
| it to dissipate the heat much faster, then a liquid, were the
| energy first has to do a slightly chaotic thermodynamic walk to
| the surface (away from gravity), aggregate with other local
| energy spikes into one spike big enough to eject a molecule from
| the bonds and allow energy to escape.
|
| Sending soundwaves through the liquid that intersect with each
| other creating cavitys, aka foam would also help.
| Lammy wrote:
| Anecdote: a plumber in my family swore by this effect and always
| connected ice makers to the hot water line in new houses. Their
| justification was that even if their observation was wrong it was
| still one less thing that was likely to freeze in winter.
| eru wrote:
| That sounds like a waste of electricity.
| 1970-01-01 wrote:
| Hot water lines connected to hot water tanks will contain more
| impurities. Those impurities will slow the melting rate of the
| ice cubes.
| GrantZvolsky wrote:
| Hot water pipes don't have the same potability guarantees as
| the cold pipes, so this practice would unnecessarily increase
| your risk of poisoning.
| cyounkins wrote:
| Hot water heaters frequently have sacrificial anode rods to
| prevent corrosion. The rods corrode over time and dissolve
| into the water. Lower cost aluminum rods have health
| concerns, not as much with magnesium rods.
|
| See also Tom Scott's video on Britain's history with separate
| hot and cold taps and why hot taps sometimes had unsanitary
| cold water tanks to supply them:
| https://www.youtube.com/watch?v=HfHgUu_8KgA
| BrandoElFollito wrote:
| Recommendations for Belgium: do not drink the hot water
|
| For Geneva: you can drink the hot water
|
| For France: you can drink the hot water but it is disgusting
|
| All of them talk about the same effects (fisdolved oxygen,
| piping ,...) and come to different conclusions.
|
| My take on that: nobody questions the potability of cold tap
| water so I will use that one, there is nothing to gain
| (possibly a shorter time to boil? that trumps the extra
| cost?)
| brutusborn wrote:
| I think that depends where you live. Regulations in my
| location changed so that hot had to have the same
| configuration as cold (and supposedly the same potability?).
| Old houses in the area have separate taps for hot / cold
| whereas newer houses built after the change have combined
| hot/cold.
| notjtrig wrote:
| Hot water tanks and heating elements corrode over time.
| rootusrootus wrote:
| The tank is lined with glass, it shouldn't corrode unless
| that surface gets broken.
| sgerenser wrote:
| The sacrificial anode rod is designed to corrode.
| Depending on its composition that means higher
| concentration of aluminum or magnesium in the water.
| mbreese wrote:
| I don't know why I think this, but I have it in my head
| that hot water heaters can have more mineral build up. This
| is why I always thought you should use cold water. Even
| though the hot was still drinkable, it might be harder
| water.
|
| I have no idea if this is accurate.
| EamonnMR wrote:
| Anecdotally, sometimes your hot water heater or holding
| tank will rust off a piece and turn your water iron-rich.
| bombcar wrote:
| Sometimes houses with water softeners will run non-
| softened water to the _cold tap_ on the kitchen sink for
| cooking purposes, as some people don 't want the salted
| water.
| bpodgursky wrote:
| Yeah hot water can leach metals from the pipes that you
| really don't want to be drinking. This could either just
| taste bad, or add unhealthy amounts of lead and copper.
|
| I would not use this plumber.
| rootusrootus wrote:
| We are fast approaching a time when all houses have pex.
| At this point we've been using it in new construction for
| decades, and most houses with original galvanized pipes
| are either already replaced with pex, or getting really
| close.
| throwaway742 wrote:
| I think you are vastly underestimating the age of the
| average house.
| bpodgursky wrote:
| I'll be honest, I also am not enthusiastic about drinking
| the volatile organics that hot water leaches out of
| polyethylene piping.
| checkyoursudo wrote:
| Give me copper or give me death.
| [deleted]
| jahewson wrote:
| Correct. Water heaters fill with sediment and should be
| flushed annually. Of course, nobody does this.
| GrantZvolsky wrote:
| If your water provider says it's potable I'm not going to
| question it, but I have yet to see a provider that makes
| such a claim about their hot water. It is difficult to make
| a statement that would apply worldwide, but where I live,
| it is easier to keep legionella out of cold water even if
| the infrastructure for hot and cold is the same.[1]
|
| [1]: [Czech language] https://www.moni.cz/aktuality/tepla-
| voda-jako-pitna
| brutusborn wrote:
| We are supplied cold water which then goes through a hot
| water system. My understanding was that the new
| configuration required the use of check-valves to prevent
| backflow in such a way as to reduce the risk of cross-
| contamination between hot/cold. I'm guessing this is now
| irrelevant because hot water system standards have
| changed to reduce risk of corrosion / infections
| occurring, but there is also a chance I was getting mixed
| up with UK regulations [0].
|
| [0] https://www.bbc.com/news/uk-england-42948046
| analog31 wrote:
| In my part of the country, the hot water is softened, so it
| has an elevated salt content.
| IshKebab wrote:
| Depends on your heating system surely?
| unlikelymordant wrote:
| wouldn't the refrigerator have to work a lot harder in that
| case to remove the extra heat? doesn't seem beneficial from an
| energy use perspective, considering you have to pay to heat the
| water, then cool it again.
| klyrs wrote:
| That was my initial reaction too, but upon consideration, how
| long does it take for hot water to get down the line?
| Typically, both the hot and cold water lines are full of
| room-temperature water until you run them long enough to heat
| up all of the piping between source and sink. I'm having
| trouble picturing an in-home ice maker having enough
| throughput for it to actually matter.
| bombcar wrote:
| This is the reality of the matter, though there may be some
| minor benefit to "using up" hot water a bit more.
| Practically it's unlikely to matter either way.
| d--b wrote:
| Yes, I'd be pretty pissed off if my plumber did that.
| jazzyjackson wrote:
| heat isn't real, it's just a measurement of the entropy in a
| system
|
| if hot water takes shortcuts and freezes faster than cold,
| then it requires less energy to do so - that is the crux of
| the debate
| eru wrote:
| > heat isn't real, it's just a measurement of the entropy
| in a system
|
| You say that as if entropy ain't realy.
|
| > if hot water takes shortcuts and freezes faster than
| cold, then it requires less energy to do so - that is the
| crux of the debate
|
| You can't escape the first and second laws of
| thermodynamics.
| checkyoursudo wrote:
| It took energy both ways though, first to heat the water
| and then freeze it. Pretty sure the final bill will not
| show less energy used.
| philco wrote:
| Whaaaattt, whoa. Wouldn't that cause the temperature in the
| food in the freezer to heat cycle, causing very no bueno
| bacterial consequences?
| boomboomsubban wrote:
| No? The freezer is cold without any water connected to the
| ice maker.
| Groxx wrote:
| Unless they were inches from the water heater, or had a hot
| water recirculation system in place:
|
| lol no. Those pipes never even got warm. Ice makers don't use
| anywhere near enough water to flush even a short line.
|
| ---
|
| For doing the math! Many ice makers use about _four fluid
| ounces_ of water per cycle, slightly over 100ml:
| https://products.geappliances.com/appliance/gea-support-sear...
|
| Per _hourly_ cycle. Or nearly.
|
| When was the last time you turned on the hot water an hour
| after anyone else used it, and had even the most sightly warmed
| water within _one half to one third of a glass_ of water?
|
| Never. Even the under-sink heaters hardly work that fast, and
| their water only has to travel like a meter at most.
| jcampbell1 wrote:
| The hot water has less dissolved gasses. In some cases it has
| less chlorine. I put water in a glass pitcher in the fridge
| because it tastes better after sitting at a lower pressure
| and off gassing. I agree that the water enters the ice maker
| at room temperature, but it may not be a bad idea regardless.
| HPsquared wrote:
| If it's heated within a closed system (i.e. mains pressure
| hot water), the gases are still there - they can only leave
| the water if you have an open tank, which is a bit old-
| fashioned. You can observe this by running hot water into a
| glass from the faucet: it appears cloudy, which is the
| dissolved gases boiling off.
|
| This means, once the hot water within the pipe has cooled
| (within the pipe) it'll still have the same gas content as
| cold water.
| luckystarr wrote:
| My hypothesis: I'd say the hot water will experience faster
| convection which will continue even when it's cooled down because
| of inertia. That way the transport of heat to the outside is
| faster than with the cold water, which will experience slower
| convection.
| smegsicle wrote:
| that sounds reasonable, but consider this complimentary
| explanation: temperature itself has inertia, so the (formerly)
| hot water's temperature momentum is greater as it smashes
| against the 32F buffer zone, forming crystals instantly
|
| not to mention static vs rolling friction (as in rolling boil)
| etothepii wrote:
| Is this an example of GPT-3 generated fake science. I really
| can't tell.
| bergerjac wrote:
| In 11th-grade 'Honors' Chemistry, we ran the 'Which water freezes
| faster' experiment outside in ~0.
|
| 2 glass beakers with same amount of water: 1 hot, 1 cold,
| stirring both at the same rate. Teacher told us the experiment
| was over once ice starting forming on the surface...
|
| Cold water developed ice first. Outside in that weather, no one
| waited to see how long hot takes to become ice.
| FunnyBadger wrote:
| The answer is BOTH. And sometimes neither.
|
| Because this is literally about the time-domain solution to heat
| conduction and fluid dynamics (convention) with different initial
| conditions and boundary conditions (shape of vessel, temperatures
| at boundaries, etc.).
|
| For example if there isn't uniformity of initial temperature, if
| there isn't uniformed of the applied cold sink, if there isn't
| uniformity of heat flux capacity due to conductivity of the
| interfacing boundary conditions, etc. There's even questions
| about the water purity or contaminants which definitely would
| change the answer.
|
| These are not and often can not be controlled. And in practice at
| home, you will never duplicate even what the answer is in a
| control lab experiment.
|
| It is the norm to have a range solutions that entirely depend
| upon initial conditions and boundary conditions. And there's also
| uncertainty of even knowing the values of these conditions.
|
| Expecting a simplistic yes/no answer to many problems ESPECIALLY
| those involving heat transfer and fluid dynamics is like asking
| for a specific date for your date from your date of birth or your
| childhood medical history.
|
| See also climate change - we DO NOT KNOW there is a specific date
| when things "tip" and ANYONE claiming there is a date let alone
| timeframe is a liar or so ignorant they can't possibly be right!
| The error of any model is larger than the estimate value itself.
|
| You'd think people would have learned something from the 20th
| century about the FACT that we do not live in a 19th clock-work
| universe with complete predictability. We know how it can never
| be predictable especially if fluid dynamics is involved - you
| know, mathematical chaos and all.
| dmurray wrote:
| > The answer is BOTH.
|
| Both means yes. It's pretty clear that in some circumstances
| cold water freezes faster. The interesting thing is whether hot
| water EVER freezes faster.
| LeifCarrotson wrote:
| Having empirically tested this, I can answer that yes, in my
| circumstances, hot water can freeze faster.
|
| Some years ago, I kept chickens here in Michigan through the
| winter. I wanted to leave them with access to water while I
| was gone at work, but after filling the tank in the morning,
| I'd often come back to a block of ice and thirsty chickens in
| the evening. My experimental procedure was as follows: On a
| Saturday when I'd be home all day, I filled two identical 5
| quart poultry waters to the same level, one with 115F hot tap
| water and one with normal 47F cold tap water. Both came from
| our well, from a sink with an aerator, and both passed
| through a softener. The hot water had been heated in a tank-
| style residential water heater, not over the stove. The
| troughs were left outside of the coop in a shed (several feet
| apart) to prevent unpredictable chicken activity from messing
| with the results. I put thermocouples in both troughs and
| checked in every half hour to take readings.
|
| The hot water froze first by several hours!
|
| The cold one reached 32F first, and after hitting 47F the
| rate of change of the one that had been hot was similar to
| the cold one (maybe slightly faster, but my measurements were
| too coarse to be confident of that), but both spent quite a
| bit of time at exactly 32F. The hot one just spent so much
| less time at 32F that the couple of hours to drop to 47F
| didn't matter. Both still had a considerable volume of liquid
| water in the bulk tank, the part exposed to air froze first,
| but that's still a failure because the chickens couldn't
| drink. I didn't have any equipment to measure dissolved
| gasses in the water, or measure convection or stacking in the
| tank, measure the rate of evaporative cooling, measure the
| rate of cold air being pulled over the surface by convection
| driven by the hot water temperature differential, or
| otherwise test any other factors that might be different from
| two apparently identical 32F tanks of water, all I knew is
| that the hot water froze faster.
|
| The result of the experiment was just to put a small electric
| heating adapter under the base. After that it didn't matter.
| Another tragic example of pragmatism over curiosity...
| laserlight wrote:
| > The interesting thing is whether hot water EVER freezes
| faster.
|
| Here's an example [0].
|
| [0] https://www.youtube.com/watch?v=jKMNSvpB9dY
| LarsAlereon wrote:
| Jesus Christ. This is super simple, we all learned this math in
| highschool: time taken to freeze is a function of the mass that
| needs to freeze and the temperature difference. If you cheat by
| allowing some of the hot mass to evaporate in order to lower the
| temperature more quickly you can potentially get the remaining
| mass to freezing temperature faster, but that isn't relevant to
| overall freezing time.
| beanlog wrote:
| Why am I unable to find a single good video on the internet
| demonstrating the mpemba effect occuring? If people were really
| heating their water before freezing it in Aristotle's time, then
| this shouldn't require any precise technology to reproduce. The
| article addresses the fact that this seems very easy to test, but
| then doesn't explain why no one's reproduced it on camera.
| NavinF wrote:
| Because it's bullshit. People who observed this supposed effect
| don't even bother to place both containers in the same fridge
| at the same time (so it's not affected by the thermostat's
| timing) nor do they weigh the ice afterwards.
| haunter wrote:
| Youtube is full of videos showing how boiling water freezing
| mid air in very cold temps
|
| https://youtu.be/pTf7X_COAvM
|
| https://youtu.be/7syogXi5LIY
| inexcf wrote:
| But there is no frozen water. You can see a lot of steam
| being carried away by the wind and water droplets leaving
| trails but no ice or snow.
| HPsquared wrote:
| What happens if you do that with cold water? Also, there's no
| indication of the water freezing. How would you tell if the
| water is freezing or not once it's landed in the snow?
| beanlog wrote:
| Yes, the only videos demonstrations involve throwing the
| water into the air, but nothing in this article or the
| Wikipedia page about the effect suggest that throwing the
| water is necessary for the effect to occur. I don't even know
| where these people got the idea to do the experiment that
| way. Their result is way less interesting because it can be
| explained by surface area.
| aPoCoMiLogin wrote:
| https://www.youtube.com/watch?v=SkH2iX0rx8U
| beanlog wrote:
| I see, so the answer is that the effect doesn't exist but
| it's easy to think it does by not controlling for a variable
| very well.
| strathmeyer wrote:
| The only controversy is whether you should pretend to believe the
| plumber's dumb story or make him hook up your ice maker
| correctly.
| MobileVet wrote:
| Shortly after completing my second class in thermo during
| undergrad, this topic came up between with a friend. The
| particular situation was wether to put the creamer in the coffee
| and let it cool or wait until it cooled and then put the creamer
| in. The wait time being the same, which would produce the hottest
| coffee.
|
| I got a little hyper focused and spent well into the night
| scrawling equations on a huge whiteboard and I don't think I ever
| felt that I proved it one way or the other.
|
| I remember being pretty defeated because I thought I had all the
| knowledge I needed to solve the problem. I guess it wasn't so
| simple after all :)
| vaidhy wrote:
| Did you actually do an experiment? This seems simple enough to
| run a quick experiment and see which one works better.
|
| You had all the knowledge needed to solve the problem, you just
| chose the wrong approach :)
| MobileVet wrote:
| For sure, lol. I was just trying to convince myself I was a
| genius because I could model the real world.
| vaidhy wrote:
| What did you find? You left that interesting part out :)
| marlor wrote:
| That's precisely the approach taken in studying the Mpemba
| effect, with wildly differing results.
|
| The article goes into the difficulties in analysing out-of-
| equilibrium systems (both experimentally and theoretically).
| vaidhy wrote:
| I did read the article and interestingly, I learnt we do
| not know a lot about phase transitions. Even more
| interesting, why would hot and cold water reach 0'c almost
| at the same time (according to one of the experiments)? If
| the rate of cooling were same, we would expect hot water to
| be tepid when cold water is 0'c.
|
| Btw, I have seen similar experiments in really cold
| climates, where a cup of hot water thrown freezes
| immediately, but cold water does not.
|
| Anyways, I am always one for running the experiments :)
| semi-extrinsic wrote:
| > Btw, I have seen similar experiments in really cold
| climates, where a cup of hot water thrown freezes
| immediately, but cold water does not.
|
| IIRC this is because the surface tension of hot water is
| significantly lower, so the thrown hot water
| disintegrates into much smaller droplets, which freeze
| more quickly.
| EamonnMR wrote:
| We actually ran this experiment in high school physics,
| reasoning that the color change should decrease the rate of
| energy loss.
| andbberger wrote:
| you did noneq stat mech in undergrad?
| LegitShady wrote:
| an experimentalist would have tried both and figured it out in
| a half hour.
| unlikelymordant wrote:
| if it were that easy to solve, the question wouldn't be still
| be uncertain (i'm talking about the original "does hot water
| or cold cool faster", to which this question seems related.)
| thrwyoilarticle wrote:
| If it's not that easy to solve, the answer won't matter.
| vkou wrote:
| A simple application of newton's law of cooling will tell me
| that for most normal ratios of coffee and creamer, you want to
| mix them first. Because the bigger the temperature delta
| between the coffee and the environment is, the more heat the
| coffee will lose per second. Mixing first will lower the
| coffee's temperature, and will cause it to lose less heat per
| second.
|
| Is there something I'm missing?
| eru wrote:
| Details depend on how the surface to volume ratio changes
| when you add them together.
| amluto wrote:
| Please elaborate. Newton's law of cooling would seem to
| suggest that the cooling rate is higher if the liquid is
| hotter. Of course, depending on the exact scenario, the
| creamer might also be warming up at the same time...
| vkou wrote:
| I expect the rate of heating for the creamer to be lower
| than the rate of cooling for the coffee, unless your coffee
| is 90% creamer.
| rowanG077 wrote:
| I disagree. But I'm no physicist. My reasoning goes like
| this, with the assumption that the coffee milk has room
| temperature:
|
| The coffee cools faster if it's hot. So putting in the
| creamer immediately steals the most efficient cooling period
| for the coffee. There are some caveats that I don't consider
| that could make a difference like that the surface area of
| the combined liquid is larger so it transfers heat more
| efficiently.
| plank wrote:
| This was actually a controversy some twenty years+ ago on a
| science show in the Netherlands ('nationale wetenschaps
| kwis'). The wanted answer was that putting creamer in first
| was better as the resulting fluid had lower temperature and
| thus cooled slower. But a given answer was that the creamer
| made a top layer that made the coffee evaporate slower,
| hence keeping more of the warmth as well. During the show
| this answer was counted wrong. But (theoretically, I don't
| think an actual experiment was made) this is a factor
| working towards the same answer (first creamer keeps the
| resulting fluid warmer compared to adding the creamer
| later) that may actually have a larger contribution. So
| afterwards the consensus was that this may actually have
| been a better answer.
| vkou wrote:
| > So putting in the creamer immediately steals the most
| efficient cooling period for the coffee.
|
| Correct, and we're trying to optimize for hot coffee after
| 30 minutes. We want cooling to be less efficient.
| grog454 wrote:
| Why would you not consider that caveat? The cooling rate
| should be generally proportional to both the surface area
| and the current temperature delta between the liquid and
| whatever it contacts. Taken to the extreme, if you added
| the creamer and then poured the coffee out onto the floor,
| the creamer's mass and temperature would have relatively
| little impact compared to the surface area change.
| HALtheWise wrote:
| For a non-sealed cup of hot liquid, the significant
| majority of cooling happens at the exposed surface with
| the air, through a mixture of convection and evaporation.
| Modeling the cooling as being proportional to total
| surface area would be pretty inaccurate, although there
| is definitely still some conduction through the cup/mug
| worth considering as well.
| MobileVet wrote:
| ... my work here is done.
| im3w1l wrote:
| Assume for simplicity that we mix half coffee and half
| creamer. This halves delta t. However the area will not
| double, since the top and bottom areas are the same.
| Furthermore, as another commenter pointed out, the top
| area is where most of the action happen.
|
| _However_ , if we really want to overcomplicate things
| we could consider the possibility of an insulating air
| pocket in a half-empty cup, leading to less convective
| losses. Consider a vacuum flask half full of hot coffee
| outside in a strong wind. If you fill it to the brim with
| creamer it might cool faster. Evaporation might become
| important too.
| somebodynew wrote:
| If you don't fill the thermos to the top, we can also add
| the small temperature impact of Helmholtz resonance from
| the wind blowing across the lip to make this more
| complicated. (Now I wonder how loud a sound needs to be
| to boil water...)
| blindmute wrote:
| My reasoning is that the coffee will cool in a curve having
| high rate of temperature change at first, and slower later in
| a long tail until it reaches room temp. Adding the cream will
| "remove" a fixed amount of "heat units". The heat of the cup
| can be graphed as heat/time and it will look like exponential
| decay.
|
| If you remove those units at the start, you've reduced the
| starting temp a bit, but you haven't much changed the long
| tail of the cooling. You essentially just started the coffee
| at a slightly cooler temperature, but this doesn't affect the
| curve much. Or to think of it another way, the change in
| temperature at the start corresponds to a small amount of X
| axis (time) on the curve.
|
| If you add the cream later, the temperature reduction
| corresponds to a larger amount of time on the curve. This
| means the temperature will be lower than the above.
|
| So to my intuition, cream first should yield hotter coffee
| [deleted]
| waterhouse wrote:
| I searched for "mass" and "dens" and didn't find anyone
| mentioning this, so: Note that hot water is less dense than cold
| water. Therefore, an "equal volume" of hot water contains less
| mass than the same volume of cold water. The article mentions
| 35degC vs 100degC; going by https://www.usgs.gov/special-
| topics/water-science-school/sci... and interpolating, it looks
| like the density difference would be about 3.6%. That seems worth
| mentioning, even if it's not a large effect.
|
| Also, if some of the water starts at 100degC, then presumably
| some of it will evaporate, and one should measure how much
| remains... The article does mention _this_ possibility, though it
| doesn 't evaluate or quantify it.
| Supermancho wrote:
| > if some of the water starts at 100degC, then presumably some
| of it will evaporate,
|
| Under controlled conditions, it would not. Ideally it would be
| in a pressure regulated space.
| autoexec wrote:
| > Ideally it would be in a pressure regulated space.
|
| That probably wouldn't do much to help us answer the question
| of what will happen in our own freezers which aren't pressure
| regulated. While I'm glad there are and will continue to be
| all kinds of investigation and exploration around the Mpemba
| effect what I (and many others) really want to know is if
| heating water first would help with real world situations
| like having ice cubes ready in time for an event.
|
| I can accept that it might not _always_ be the case, but
| considering that at this point we can 't say for sure if the
| effect is real at all it'd be nice if we could get it settled
| that it worked often enough and potentially decreases
| freezing time enough to be worth the time and trouble of
| microwaving our ice cube trays even if all the details and
| underlying mechanisms aren't well understood.
|
| I should probably just start trying it myself and see what I
| come up with on my own, but even imperfect lab experiments
| would be much more accurate and trustworthy. I regularly do
| things like heat a mug full of water in the microwave, get
| distracted, and then forget all about it for hours.
| Supermancho wrote:
| > That probably wouldn't do much to help us answer the
| question of what will happen in our own freezers which
| aren't pressure regulated
|
| It would do a great deal to answer the question. Knowing
| the base physics (or at least the outcomes) then allows
| further experimentation and calculation to answer more
| complicated (less controlled) conditions.
|
| > I regularly do things like heat a mug full of water in
| the microwave, get distracted, and then forget all about it
| for hours.
|
| AFAIK, everyone does that ^^
| jbay808 wrote:
| > if heating water first would help with real world
| situations like having ice cubes ready in time for an
| event.
|
| If the mechanism is that 10% of the water evaporates away,
| allowing the remaining 90% to freeze smaller by virtue of
| its reduced mass, the take-away for making ice in a hurry
| is not to use hot water. It is to use cold water, but a
| little less of it.
| autoexec wrote:
| A difference that large should show up in the resulting
| cubes though right? You wouldn't even need to figure out
| exactly how long they took to freeze or which tray froze
| first, just put a heated water tray and a cold water tray
| in together and compare them after waiting long enough
| for both to freeze. That'd give you an idea of how much
| more/less water you'd need to compensate.
|
| Honestly, smaller ice cubes is probably your best bet if
| you need ice in hurry either way.
| legitster wrote:
| When we did this as an experiment in high school, we started
| with the same volume at room temperature. 500mL was brought
| near boiling, and another 500mL while the control was left out.
| bryanrasmussen wrote:
| Was the 500ml still really 500ml when near boiling?
| https://www.cooksillustrated.com/how_tos/5684-when-to-
| measur...
| dhzhzjsbevs wrote:
| Density and freezing outside in could have a pressurising
| effect too.
|
| I'd be curious to know if the defect remains while stirring
| until frozen.
| waterhouse wrote:
| Good instinct to try stirring. Search the comment thread (or
| the Wiki page for "Mpemba effect") for "Joseph Black", and
| you'll find something intriguing.
| comboy wrote:
| I got into this rabbit hole once and remember having the same
| thought, I checked some paper and they used the same mass as
| measured when putting water into the freezer. AFAIR it was also
| tight, closed container so evaporation shouldn't have a big
| impact (checking mass after sure seems like a good idea).
|
| Intuitively hot water should be much slower to freeze. It takes
| quite a bit of time for it to get to the same state that the
| cold water is at the start. After that time, the way we look at
| it, it's at the same point in experiment as if we have just put
| the colder water in. So we seem to still be missing something.
| Tuna-Fish wrote:
| > After that time, the way we look at it, it's at the same
| point in experiment as if we have just put the colder water
| in. So we seem to still be missing something.
|
| Unless there is very substantial mixing, the container of hot
| water will never be at the same state as the container of
| cooler water.
|
| We did this in high school, with 3 open-topped 10L buckets,
| one with 25C water, one with 95C water and no mixing, and one
| with 95C water sitting on a magnetic mixer. They were not in
| a freezer, but outside at ~-25C.
|
| The freezing order was 95C unmixed, 25C, 95C mixed. We also
| had a few temperature probes in the 95C unmixed bucket, and
| found that about halfway through, the temperature of the
| water in the middle about 1/3rd of the way from bottom was
| sitting at 0C, while the temperature of the water at middle
| at the top was still >70C.
|
| Our final findings were that:
|
| 1. The buckets cool mainly by evaporation, which is only
| happening at the top, and which is much faster for hotter
| water
|
| 2. Water of different temperature stratas can be surprisingly
| stable even in a small container
|
| 3. In the unmixed bucket a stable, slow flow of water down
| the sides and up the middle forms. New hot water emerges from
| the middle, gets cooled and pulled to the sides, and then
| pulled down to the bottom. This circular flow is probably
| caused by the fact that water at the sides get initially
| cooled a bit and so it sort of biases the flow.
|
| But now there's people saying it still works with closed-top
| containers, so I dunno. My physics teacher clearly did this
| every year and knew what was going to happen. The class came
| up with those findings after some very specific questions by
| him.
| comboy wrote:
| Very cool finding with the flow. It explains why when you
| take some round container and freeze water in it, the ice
| will be crystal clear at the edges, but it will not be
| crystalized so cleanly in the middle (turbulence once edges
| are frozen)
|
| Who knows, perhaps there's even some laminar flow happening
| at the edges :)
| Zenst wrote:
| ANother aspect would be that the hotter an atom is the faster
| it will vibrate. So would it be that a vibrating atom is able
| to transfer thermal energy quicker akin to comparing a warm
| start to a cold start in a way that see's the thermal transfer
| inertia at the aromic level has better initial acceleration
| over the colder atom. So even when the atom reaches the
| vibrational speed of the colder atoms, the momentum of thermal
| transfer is already accelerated. Sorry if that sounds fluffy
| but a bit of a theory. One which trying other liquids and
| testing would give more insight.
|
| Or is this effect unique to just water?
| waterhouse wrote:
| I don't think there's such a thing as thermal transfer
| inertia, at least at the level of individual atoms (or
| molecules) within a fluid. In the inertial reference frame of
| the fluid, each molecule just has its own kinetic energy (and
| maybe excitation of electrons or whatever) representing its
| current temperature; I'm not sure what physical state at the
| molecular level could capture "how rapidly the temperature
| had recently dropped".
|
| At the macro level, one can come up with ideas. Convection
| currents within the fluid, and in the air touching the fluid,
| maybe; though, say, once the hot water has cooled from
| 100degC to 35degC over a period of--what, ten minutes?--I'm
| pretty skeptical that there would be so much inertia in those
| currents that they'd persist, and persist _strongly enough_
| to accelerate the 35deg to -1deg cooling, enough to beat the
| head start of the water that _started_ at 35deg.
|
| Maybe the initially cool water forms some uniform layer of
| very-cool water at the top (which, being very cool, doesn't
| exchange much heat with the very-cool air above it; an ice
| layer would be an example of this), which is held together by
| surface tension or something; whereas with initially hot
| water, that layer is not uniform and there's more mixing (and
| constantly-created convection as a result)? I have no idea if
| any of that is realistic. If so, it would suggest that
| shaking a cup of cold water (perhaps after a few minutes in
| the freezer) would work as well as having it start hot.
|
| ... After writing the above, I saw that (a) infogulch below
| has the same idea, (b) Wikipedia has an anecdote appearing to
| confirm it[1], and (c) the article doesn't seem to mention
| shaking, stirring, or otherwise agitating the cup.
|
| [1] _The Scottish scientist Joseph Black investigated a
| special case of this phenomenon comparing previously-boiled
| with unboiled water;[9] the previously-boiled water froze
| more quickly. Evaporation was controlled for. He discussed
| the influence of stirring on the results of the experiment,
| noting that stirring the unboiled water led to it freezing at
| the same time as the previously-boiled water, and also noted
| that stirring the very-cold unboiled water led to immediate
| freezing._ https://en.wikipedia.org/wiki/Mpemba_effect
| teknopaul wrote:
| I was surprised that convection didn't get a mention. Seems
| so obvious that _not_ mentioning it, if it has been
| discounted, seems strange.
| cubancigar11 wrote:
| What you are describing is Newton's law of cooling and won't
| have an effect here.
| Zenst wrote:
| aha - thank you.
| pdpi wrote:
| Density is going to factor in somewhere, sure, but when you
| have 100 -> 0 vs 35 -> 0, that's almost 3x more energy (given
| constant specific heat) you need to sink somewhere. a single-
| digit percentage is a drop in the ocean here.
| achow wrote:
| It says in the article..
|
| _A more mundane explanation is that hot water evaporates
| faster than cold, decreasing its volume and thus the time it
| takes to freeze._
|
| [Edit - Cannot delete my comment now. As 'ketzo' points out
| below my point is not same as OP's. OP points out that hot
| water is less dense. Maybe less dense liquid cools faster]
| ketzo wrote:
| That's not the same point, actually.
|
| That quoted sentence says that hot water _loses volume_
| compared to cold water.
|
| The person you're replying to is saying that hot water simply
| _takes up more space_ as well.
| atoav wrote:
| So that means two things:
|
| - 1 m3 of hot water is less water (in terms of mass) than 1
| m3 of cold water
|
| - 1 kg of hot water has a bigger surface area than 1 kg of
| cold water
|
| I am still not convinced that this alone would explain such
| a phenomena. I'd rather believe there is some weird fluid
| dynamics and layering involved.
| stonemetal12 wrote:
| The first seems compelling to me, less mass means less
| over all energy to freeze.
|
| The second less so. Hot water has a bigger surface area,
| which makes heat loss fast at first. When it gets to cold
| water's temp it should have the same surface area as the
| cold water making the advantage disappear.
| wildmanx wrote:
| > such a phenomena
|
| The singular is "phenomenon".
| atoav wrote:
| Thanks, English is not my first language.
| hprotagonist wrote:
| it's all greek to me!
| MarcellusDrum wrote:
| Actually, it is mentioned in the article:
|
| > Water is a strange substance, less dense when solid than
| liquid, and with solid and liquid phases that can coexist at
| the same temperature.
| IshKebab wrote:
| Isn't that last point true for all substances? That's just
| how phase changes work.
| ascar wrote:
| It's true for first-order phase transitions but not for
| second-order phase transitions. The article actually
| talks about it further down:
|
| > the Mpemba effect could happen through a related
| mechanism that Raz has previously described with Lu in
| systems that undergo a second-order phase transition,
| meaning that their solid and liquid forms can't coexist
| at the same temperature. Water is not such a system (it
| has first-order phase transitions),
| OJFord wrote:
| It would be pretty wild if they were immediate! But.. if
| you keep it moving at say -1, will it still eventually
| freeze? I struggle to imagine but don't know why really.
| Maybe it means something like that. Not that I know why
| that would be unique to water either.
| themitigating wrote:
| I always thought they use volume as a measurement for
| experiments. Like "we took 100ml of water..."
| eru wrote:
| Depends on what experiment you are doing. Either mass
| (approximated via weight) or volume can be the right thing to
| do.
|
| In theory you could also go by number of particles (measured
| in mol).
| OJFord wrote:
| Agreed, if it's about 'is this the case' especially in home
| freezer context then it's volume that's interesting: ice
| cube tray sized volumes. (But also are the resultant cubes
| the same volume? Presumably not.)
|
| If it's 'yes this does happen, why?' then it seems right to
| try same mass, to test that theory.
| walnutclosefarm wrote:
| It would be very interesting to do this experiment in a freefall
| environment, where you could release identical quantities of
| water at different temperatures as spherical bubbles into chilled
| air. This would eliminate the effects of the container, and of
| convection within the fluid on the effect. Interestingly, the
| article doesn't mention convection as a confounding effect, but
| it is one of the ways in which a "hot" system could explore more
| non-equilibrium states in a given time than a "cold" system.
| fer wrote:
| I'm also convinced convection is the reason why it can happen
| under completely equal conditions (which are arguably missing
| from most experiments).
|
| 1. The larger the temperature differential, the faster the heat
| transfer.
|
| 2. The outer layer of hotter water will lose temperature very
| quickly and generate a convection flow inside the container,
| which would replace outer layers of now colder water with hot
| water relatively quickly, accelerating the heat transfer.
|
| 3. The outer layer of the colder water will lose temperature
| much more slowly, thus the convection will be less pronounced,
| not significantly accelerating an already slow heat transfer.
|
| Of course all this will depend, non linearly, on:
|
| 1. The temperatures involved (1C water will certainly freeze
| faster than 99C water when put in a -25C freezer).
|
| 2. Shape of the containers (as it affects the convective flows
| and hence the heat transfer).
|
| 3. Total mass of water: I suspect for small (in range of up to
| decilitres) and large (beyond decalitres), cold water tends to
| freeze faster. The first, because convection effects are very
| limited for both waters, and second, because convection effects
| on cold water become significant enough to improve heat
| transfer.
|
| 4. Environmental conditions: relative humidity of air in the
| freezer, temperature of the freezer, is freezer empty, are the
| walls close or far, the actual freezer performance, etc. All of
| that affects the heat transfer coefficient and make it
| potentially _non constant_ and _not equal_ between the two
| waters.
| HPsquared wrote:
| You mean, in space? This would be a fun thing to try on the
| space station.
| SpEd3Y wrote:
| How cool would that be? You don't need containers for the
| water as they will just float, outer space is very cold.
| Perfect medium for this experiment.
| t_mann wrote:
| How would this eliminate convection between the outer and inner
| layers?
| Const-me wrote:
| Convection works because density of warm fluid is less than
| density of cold one. The gravity causes hot fluid to rise,
| and cold fluid to sink.
|
| For this reason, switching off the gravity stops the
| convection.
| ezconnect wrote:
| Hot water molecules move more than cold water so the area exposed
| to cooling get more molecules than a cold water since their
| molecules are almost sleeping because it's cold.
| munchler wrote:
| That can't be the whole story. If the hot one freezes first,
| the two containers have to be at the same temperature at some
| point in time before freezing. And yet the one that started hot
| somehow continues to cool faster beyond that point.
| walnutclosefarm wrote:
| Not really. It makes no physical sense to say the container
| "is at a temperature" when it's not in an equilibrium state.
| Neither of the two containers in the experiment will be at
| equilibrium until they are entirely frozen and uniformly
| cooled to the temperature of the freezer they're in, and
| there is no guarantee that they will at any point be in
| identical non-equilibrium states at any time.
| ezconnect wrote:
| Probably momentum plays a role.
| ycccc wrote:
| Zenst wrote:
| Has this effect been observed in other materials or liquids in
| there cooling?
| rossdavidh wrote:
| The article lists several substances other than water where the
| effect is apparently easier to demonstrate.
| NavinF wrote:
| It hasn't even been reproduced for water under controlled
| conditions. Not much of a point testing more materials.
| kuu wrote:
| Have you read the article? Because the specifically mention
| that is been proved in other materials, then the doubt if
| water would behave in the same way...
| mrb wrote:
| There are so many potential explanations for that phenomenon:
|
| - hot tap water has more mineral contents than cold water which
| helps form the initial ice crystals
|
| - hot water in a container creates a convection effect on the air
| surrounding it, thus creating an influx of cold air around the
| container, helping cool more quickly
|
| - hot water in a container melts the ice/snow under it (if the
| experiment is conducted outside), reducing the insulation with
| potential colder ground
|
| - thermal motion of hotter H2O molecules may help create
| accidental collisions that trigger the formation of ice crystals
| 1970-01-01 wrote:
| The inverse may also be true:
|
| https://www.pnas.org/doi/10.1073/pnas.2118484119
|
| A cold system normally takes longer to warm up than a cool
| system. Yet recent theoretical studies have suggested that the
| reverse may sometimes be possible. Here, using a colloidal
| particle in a heat bath, we present experimental evidence for
| this inverse Mpemba effect. By carefully choosing the energy
| landscape, we can make the cold system heat up exponentially
| faster than the heating rate of cool systems. While similar
| behavior has been seen in systems that cool down--the Mpemba
| effect--we find that entropic effects generally make anomalous
| heating harder to observe than anomalous cooling.
| simonjgreen wrote:
| There was a video by Derek/Veritasium as few years ago explaining
| this and also walking through all the experiments that
| demonstrate the effect does not in fact exist.
|
| https://youtu.be/SkH2iX0rx8U
| gilgoomesh wrote:
| For anyone interested... Derek quotes 5 theorised causes but
| rules all but 2 insignificant, leaving the primary confounding
| problems as:
|
| 1. Warm liquid changing the nature of the freezer (Derek
| mentions melting frost into a conducting layer but there's also
| warm water triggering the thermostat of a freezer to work
| harder)
|
| 2. Supercooling can prevent ice forming in calm water, even
| when the water is below freezing (the effect is wildly
| unpredictable leading to experimental problems)
|
| Derek then quotes the following study and meta analysis from
| 2016 that carefully accounts for these and other problems,
| which finds no evidence of the effect and concludes the other
| studies' claimed effects are within their own margins of error
| (including issues like placing thermometers at slightly
| different locations in a vessel giving dramatically different
| timing results):
|
| https://www.nature.com/articles/srep37665
| Jensson wrote:
| The hot water could also increase the humidity, making the
| air more effective at cooling.
| 22c wrote:
| Seems like a simple way to control this would be have 4
| containers of water.
|
| Container 1A - Control temperature
|
| Container 1B - Control temperature
|
| Both go into freezer 1
|
| Container 2A - Control temperature
|
| Container 2B - Near boiling
|
| Both go into freezer 2
|
| Now see if there's a difference in freezing time for
| Container 1A compared to 2A.
| fallingfrog wrote:
| Ok..but hot water must _become_ cold water before it can freeze,
| no? Are you saying that certain glasses of cold water have a
| memory of having been hot, but others do not? How long ago must
| it have been hot for that to count?
|
| This all just feels like sloppy measurement of some kind.
| margalabargala wrote:
| Calling it "memory" is a misleading way to think about the
| effect.
|
| It could be better written as: exposing hot water to a large
| temperature gradient allows the water to change energy states
| in a way that it cannot do when that temperature gradient is
| small. Those fun new energy state changes allow the hot water
| to give up more energy faster and freeze sooner.
|
| So no, the hot water in this case is not becoming cold water in
| the sense that you mean.
| fallingfrog wrote:
| Do you have a background in physics or is this just stuff you
| read?
|
| I have an undergrad degree in physics and nothing you just
| said sounds right at all. It all just sounds like gibberish.
| What kind of "energy states"? Molecules bounce off each other
| and vibrate around. The measurement of that kinetic energy is
| called "temperature".
| blarg1 wrote:
| I wonder if the difference is some kind of momentum, with the
| hot water's being higher.
| IshKebab wrote:
| Temperature doesn't have momentum though. More likely the
| temperature isn't uniform.
| science_tsoui wrote:
| How about this theory. the greater delta H heat flow induces
| structural changes in water molecule matrix that causes
| rearrangements that more quickly attain to the optimal ice
| lattice structure more quickly, and this 'enhanced
| crystallisation through increased delta heat' causes it to cool
| quicker. That this occurs in water and not other substances is
| related somehow to its 'paradoxical' expansion on cooling*. That
| the effect is flakey is related to the outsize effects of
| impurities and defects in the crystal lattice, and how these
| flaws can propagate their crystallisation inefficiencies.
|
| Maybe a more thermodynamic way to state this is:
|
| - temperature is an aggregate measure, so because of the nature
| of the ensemble, you can have some states at -22 C that still
| have some molecules of water (or groups of molecules of water
| connected by intermolecular bonds), at a higher temperature.
|
| - when you start higher, you are going to end up with more of
| these higher temperature pockets in your ensemble, as your
| aggregate temperature drops through the degrees toward freezing
|
| - these higher energy / "higher temperature micro regions",
| provide more possible microstates for the water crystals to
| rearrange, than if these intermolecular clumps of water were
| lower temp, (or were less numerous, as would be the case starting
| lower), and the higher temp clumps (or pockets, in the honeycomb
| of partially crystallized water) will probe the space of
| configurations of molecular arrangements more effectively (than
| if they were lower temperature, or less numerous, as would be the
| case starting lower).
|
| - By being able to search more of the configuration space, they
| naturally are able to more quickly find optimal water crystal
| arrangements of molecules, where some H20 from the higher temp
| clumps, can slot into the existing lattice, or accrete onto the
| existing surfaces of forming ice crystals. So these ice crystals
| in a solution of liquid that has these higher temp pockets can be
| more effective at generating the movement necessary to find
| configurations where molecules, and clumps of molecules
| effectively fit together in the lattice, letting it find more
| effective crystal structures, leading to more efficient (and
| faster) crystallisation.
|
| These other commenters are talking about the same thing:
|
| - https://news.ycombinator.com/item?id=31929552
|
| - https://news.ycombinator.com/item?id=31928942
|
| - https://news.ycombinator.com/item?id=31929779
| elevaet wrote:
| My grade 6 "science" teacher gave me a C- when she sent us home
| to test if cold or hot water froze faster in ice cube trays. I
| reported accurately that the cold one froze faster, knowing that
| she was expecting us to report back "hot". I will never forgive!
| Barrera wrote:
| There's way too much theory and not nearly enough experiment in
| this writeup.
|
| I still don't know what happens if I put two glasses of water,
| one at room temperature and the other at 100 C, into a freezer
| and determine which one freezes first. If I don't see the hot
| water freezing first all (most?) of the time, there's nothing
| else to consider.
|
| Maybe the controversy has to do with the lack of solid
| experimental data.
| bumblebritches5 wrote:
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