14 ms·

Controversy continues over whether hot water freezes faster than cold

275 points by sohkamyung 4y ago

fny 4y ago
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 4y ago
  Yes, it cools faster at t=0, but it will take longer to cool down.
- happyopossum 4y ago
  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 4y ago
    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 4y ago
  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 4y ago
    This should have been obvious.
- andrewchambers 4y ago
  Hot water would need to first reach the temperature of the cold water (and overtake it) before freezing... Your explanation doesn't make sense.
avastmick 4y ago
Anecdotal and the opposite effect. When I was a kid I used to keep lizards as pets. I quickly noticed that if I filled up the water trays with cold water it would evaporate within a day from the hot heat lamps. Hot or very warm warm out of a kettle would last much much longer. I made a study as a science experiment for school and scored badly as the teacher said it was impossible.
u_tdubz 4y ago
Hot isn't a temperature. Neither is cold. Presumably hot is some figure above freezing and below or up to boiling. The scolding point of water is about 115. Just encase someone wants to know, this can be measured in BTUs British thermal units. It's the measure of energy used per heating water till it's boiling point . It's also measurable if cooling the water to it's freezing point. Fun fact is about state changing from 212 to 212 liquid to vapor is significantly more BTUs then the 1:1 (btu to one degree Fahrenheit) .
akeck 4y ago
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.
duxup 4y ago
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 4y ago
  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 https://en.wikipedia.org/wiki/Mpemba_effect
MobileVet 4y ago
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 :)
- andbberger 4y ago
  you did noneq stat mech in undergrad?
- vkou 4y ago
  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?
  - amluto 4y ago
    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 4y ago
      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 4y ago
    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.
bumblebritches5 4y ago
DantesKite 4y ago
>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.
- bordercases 4y ago
  Should make for interesting computational effects
  - asojfdowgh 4y ago
    https://en.wikipedia.org/wiki/Simulated_annealing https://en.wikipedia.org/wiki/Simulated_annealing
- tshaddox 4y ago
  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.
  - Ericson2314 4y ago
    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",
Lammy 4y ago
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.
- 1970-01-01 4y ago
  Hot water lines connected to hot water tanks will contain more impurities. Those impurities will slow the melting rate of the ice cubes.
- eru 4y ago
  That sounds like a waste of electricity.
- GrantZvolsky 4y ago
  Hot water pipes don't have the same potability guarantees as the cold pipes, so this practice would unnecessarily increase your risk of poisoning.
  - analog31 4y ago
    In my part of the country, the hot water is softened, so it has an elevated salt content.
  - GrantZvolsky 4y ago
    This thread has revealed an interesting cultural difference, with me being in the minority that gets hot water from a remote provider, as opposed to having a heating system and heating it myself.
  - brutusborn 4y ago
    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.
waterhouse 4y ago
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 35°C vs 100°C; going by https://www.usgs.gov/special-topics/water-science-school/science/water-density 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 100°C, 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.
- joseda-hg 4y ago
  Would using an sealed hidrophobic container isolate the water enough that it would condense into itself as temperature drops?
- themitigating 4y ago
  I always thought they use volume as a measurement for experiments. Like "we took 100ml of water..."
  - eru 4y ago
    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 4y ago
      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.
- Supermancho 4y ago
ezconnect 4y ago
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 4y ago
  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 4y ago
    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 4y ago
    Probably momentum plays a role.
    - ycccc 4y ago
strathmeyer 4y ago
The only controversy is whether you should pretend to believe the plumber's dumb story or make him hook up your ice maker correctly.
fallingfrog 4y ago
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 4y ago
  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 4y ago
    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".
    - margalabargala 4y ago
      I'm regurgitating what I understood from reading the article. Here are some relevant quotes: > 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.”
dontknowwhyihn 4y ago
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 4y ago
  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.
beanlog 4y ago
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.
- haunter 4y ago
  Youtube is full of videos showing how boiling water freezing mid air in very cold temps https://youtu.be/pTf7X_COAvM https://youtu.be/pTf7X_COAvM https://youtu.be/7syogXi5LIY https://youtu.be/7syogXi5LIY
  - HPsquared 4y ago
    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?
  - inexcf 4y ago
    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.
  - beanlog 4y ago
    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 4y ago
  https://www.youtube.com/watch?v=SkH2iX0rx8U https://www.youtube.com/watch?v=SkH2iX0rx8U
walnutclosefarm 4y ago
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.
- t_mann 4y ago
  How would this eliminate convection between the outer and inner layers?
  - Const-me 4y ago
    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.
- fer 4y ago
  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.
prerok 4y ago
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-energy-change/energy-of-evaporation.html 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 :)
- saberdancer 4y ago
  This makes sense to me. Surface layer partly evaporates, it immediately cools down remaining surface layer and then convection spreads it around. It would enable quick "forced" cooling similar to AC and due to convection, it can continue for a long time as warm water and cold water rotate. Quite counter intuitive. You would expect to have water cooling slow down when it reaches temperature of colder water.
- BrandoElFollito 4y ago
  > 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 4y ago
    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.
simonjgreen 4y ago
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 https://youtu.be/SkH2iX0rx8U
- gilgoomesh 4y ago
  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 https://www.nature.com/articles/srep37665
  - Jensson 4y ago
    The hot water could also increase the humidity, making the air more effective at cooling.
    - 22c 4y ago
      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.
sgtnoodle 4y ago
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 4y ago
  I observed the phenomenon growing up when I filled buckets of hot and cold water and left them outside in freezing temperatures.
  - cowtools 4y ago
    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?
  - Jensson 4y ago
    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.
    - ascar 4y ago
      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).
ZiiS 4y ago
I would expect most freezers to detect the heat and run their chillers and fans more?
luckystarr 4y ago
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 4y ago
  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 4y ago
Is this an example of GPT-3 generated fake science. I really can't tell.
LarsAlereon 4y ago
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.
irrational 4y ago
> 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.
Zenst 4y ago
Has this effect been observed in other materials or liquids in there cooling?
- NavinF 4y ago
  It hasn’t even been reproduced for water under controlled conditions. Not much of a point testing more materials.
  - kuu 4y ago
    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...
- rossdavidh 4y ago
  The article lists several substances other than water where the effect is apparently easier to demonstrate.
winReInstall 4y ago
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.
mrb 4y ago
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
aaron695 4y ago
> 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.
bergerjac 4y ago
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.
- dfdz 4y ago
  In case you missed this in the article, the effect you described is a possible explanation of why the effect happens … > Or perhaps external factors come into play: A layer of frost in a freezer can act as an insulator, keeping heat from leaking out of a cold cup, whereas a hot cup will melt the frost and cool faster.
elevaet 4y ago
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!
fuzzfactor 4y ago
>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.
NicoJuicy 4y ago
Wasn't it that hot water dampens and it dampens the warmth first. So it quickly reduces temperature.
Barrera 4y ago
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.
FunnyBadger 4y ago
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 4y ago
  > 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 4y ago
    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...
1970-01-01 4y ago
The inverse may also be true: https://www.pnas.org/doi/10.1073/pnas.2118484119 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.
science_tsoui 4y ago
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=31929552 - https://news.ycombinator.com/item?id=31928942 https://news.ycombinator.com/item?id=31928942 - https://news.ycombinator.com/item?id=31929779 https://news.ycombinator.com/item?id=31929779
toss1 4y ago
>>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).
thrusong 4y ago
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.
rossdavidh 4y ago
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?

275 points by sohkamyung 4y ago

fny 4y ago

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 4y ago
Yes, it cools faster at t=0, but it will take longer to cool down.
happyopossum 4y ago
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 4y ago
  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 4y ago
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 4y ago
  This should have been obvious.
andrewchambers 4y ago
Hot water would need to first reach the temperature of the cold water (and overtake it) before freezing... Your explanation doesn't make sense.

avastmick 4y ago

Anecdotal and the opposite effect. When I was a kid I used to keep lizards as pets. I quickly noticed that if I filled up the water trays with cold water it would evaporate within a day from the hot heat lamps. Hot or very warm warm out of a kettle would last much much longer. I made a study as a science experiment for school and scored badly as the teacher said it was impossible.

u_tdubz 4y ago

Hot isn't a temperature. Neither is cold. Presumably hot is some figure above freezing and below or up to boiling. The scolding point of water is about 115. Just encase someone wants to know, this can be measured in BTUs British thermal units. It's the measure of energy used per heating water till it's boiling point . It's also measurable if cooling the water to it's freezing point. Fun fact is about state changing from 212 to 212 liquid to vapor is significantly more BTUs then the 1:1 (btu to one degree Fahrenheit) .

akeck 4y ago

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.

duxup 4y ago

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 4y ago
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 https://en.wikipedia.org/wiki/Mpemba_effect

MobileVet 4y ago

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 :)

andbberger 4y ago
you did noneq stat mech in undergrad?
vkou 4y ago
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?
- amluto 4y ago
  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 4y ago
    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 4y ago
  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.

bumblebritches5 4y ago

DantesKite 4y ago

>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.

bordercases 4y ago
Should make for interesting computational effects
- asojfdowgh 4y ago
  https://en.wikipedia.org/wiki/Simulated_annealing https://en.wikipedia.org/wiki/Simulated_annealing
tshaddox 4y ago
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.
- Ericson2314 4y ago
  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",

Lammy 4y ago

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.

1970-01-01 4y ago
Hot water lines connected to hot water tanks will contain more impurities. Those impurities will slow the melting rate of the ice cubes.
eru 4y ago
That sounds like a waste of electricity.
GrantZvolsky 4y ago
Hot water pipes don't have the same potability guarantees as the cold pipes, so this practice would unnecessarily increase your risk of poisoning.
- analog31 4y ago
  In my part of the country, the hot water is softened, so it has an elevated salt content.
- GrantZvolsky 4y ago
  This thread has revealed an interesting cultural difference, with me being in the minority that gets hot water from a remote provider, as opposed to having a heating system and heating it myself.
- brutusborn 4y ago
  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.

waterhouse 4y ago

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 35°C vs 100°C; going by https://www.usgs.gov/special-topics/water-science-school/science/water-density 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 100°C, 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.

joseda-hg 4y ago
Would using an sealed hidrophobic container isolate the water enough that it would condense into itself as temperature drops?
themitigating 4y ago
I always thought they use volume as a measurement for experiments. Like "we took 100ml of water..."
- eru 4y ago
  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 4y ago
    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.
Supermancho 4y ago

ezconnect 4y ago

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 4y ago
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 4y ago
  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 4y ago
  Probably momentum plays a role.
  - ycccc 4y ago

strathmeyer 4y ago

The only controversy is whether you should pretend to believe the plumber's dumb story or make him hook up your ice maker correctly.

fallingfrog 4y ago

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 4y ago
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 4y ago
  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".
  - margalabargala 4y ago
    I'm regurgitating what I understood from reading the article. Here are some relevant quotes: > 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.”

dontknowwhyihn 4y ago

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 4y ago
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.

beanlog 4y ago

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.

haunter 4y ago
Youtube is full of videos showing how boiling water freezing mid air in very cold temps https://youtu.be/pTf7X_COAvM https://youtu.be/pTf7X_COAvM https://youtu.be/7syogXi5LIY https://youtu.be/7syogXi5LIY
- HPsquared 4y ago
  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?
- inexcf 4y ago
  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.
- beanlog 4y ago
  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 4y ago
https://www.youtube.com/watch?v=SkH2iX0rx8U https://www.youtube.com/watch?v=SkH2iX0rx8U

walnutclosefarm 4y ago

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.

t_mann 4y ago
How would this eliminate convection between the outer and inner layers?
- Const-me 4y ago
  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.
fer 4y ago
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.

prerok 4y ago

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-energy-change/energy-of-evaporation.html 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 :)

saberdancer 4y ago
This makes sense to me. Surface layer partly evaporates, it immediately cools down remaining surface layer and then convection spreads it around. It would enable quick "forced" cooling similar to AC and due to convection, it can continue for a long time as warm water and cold water rotate. Quite counter intuitive. You would expect to have water cooling slow down when it reaches temperature of colder water.
BrandoElFollito 4y ago
> 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 4y ago
  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.

simonjgreen 4y ago

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 https://youtu.be/SkH2iX0rx8U

gilgoomesh 4y ago
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 https://www.nature.com/articles/srep37665
- Jensson 4y ago
  The hot water could also increase the humidity, making the air more effective at cooling.
  - 22c 4y ago
    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.

sgtnoodle 4y ago

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 4y ago
I observed the phenomenon growing up when I filled buckets of hot and cold water and left them outside in freezing temperatures.
- cowtools 4y ago
  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?
- Jensson 4y ago
  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.
  - ascar 4y ago
    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).

ZiiS 4y ago

I would expect most freezers to detect the heat and run their chillers and fans more?

luckystarr 4y ago

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 4y ago
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 4y ago

Is this an example of GPT-3 generated fake science. I really can't tell.

LarsAlereon 4y ago

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.

irrational 4y ago

> 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.

Zenst 4y ago

Has this effect been observed in other materials or liquids in there cooling?

NavinF 4y ago
It hasn’t even been reproduced for water under controlled conditions. Not much of a point testing more materials.
- kuu 4y ago
  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...
rossdavidh 4y ago
The article lists several substances other than water where the effect is apparently easier to demonstrate.

winReInstall 4y ago

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.

mrb 4y ago

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

aaron695 4y ago

> 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.

bergerjac 4y ago

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.

dfdz 4y ago
In case you missed this in the article, the effect you described is a possible explanation of why the effect happens … > Or perhaps external factors come into play: A layer of frost in a freezer can act as an insulator, keeping heat from leaking out of a cold cup, whereas a hot cup will melt the frost and cool faster.

elevaet 4y ago

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!

fuzzfactor 4y ago

>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.

NicoJuicy 4y ago

Wasn't it that hot water dampens and it dampens the warmth first. So it quickly reduces temperature.

Barrera 4y ago

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.

FunnyBadger 4y ago

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 4y ago
> 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 4y ago
  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...

1970-01-01 4y ago

The inverse may also be true: https://www.pnas.org/doi/10.1073/pnas.2118484119 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.

science_tsoui 4y ago

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=31929552 - https://news.ycombinator.com/item?id=31928942 https://news.ycombinator.com/item?id=31928942 - https://news.ycombinator.com/item?id=31929779 https://news.ycombinator.com/item?id=31929779

toss1 4y ago

>>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).

thrusong 4y ago

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.

rossdavidh 4y ago

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?