12 ms·
The physics of why hot water sometimes freezes faster than cold water
- seventytwo 6y agoEmphasis on “sometimes”.
- dumbfoundded 6y agoYeah but a reproducible sometimes is pretty amazing. Sounds like some interesting applications await.
- ssivark 6y agoThis might be a better link: https://www.sciencenews.org/article/physics-new-experiment-hot-water-freeze-faster-cold-mpemba-effect https://www.sciencenews.org/article/physics-new-experiment-h... Punchline seems to be that “hot” but out-of-equilibrium material cannot be summarized by a “temperature”, providing many paths towards freezing (on the landscape of microstates), some of which could (efficiently) circumvent the “low temp” state.
- m-i-l 6y agoThis was known by many Antarctic scientists before 1963. I remember my dad telling me you could throw boiling water into the air and it would freeze into millions of tiny ice crystals before reaching the ground, but that the same wasn't true of cold water. I think he said it was due to thermal inertia.
- TylerE 6y agoThat sounds like a totally DIFFERENT effect, almost certainly caused by droplet size.
- brennanpeterson 6y agoIt could be the same. I always learned the explanation as: 1. Cool to freezing via evaporation. Because this goes as T^4, the time difference is negligible to reach 0°C. But...hot water loses more volume. 2. Freeze, bit now you need less energy to freeze less material. I did this at home, and the simple check worked: volume was lower in the frozen hot water. This explanation might be wrong! But it works out in simple tests and mathematically.
- LeifCarrotson 6y ago"Thermal inertia" is more formally known as specific heat capacity or volumetric heat capacity, which is a substance's resistance to change in temperature. It's similar to (but different from) mechanical inertia, which is an object's resistance to change in velocity. The latter includes the concept of momentum, that an object with nonzero velocity resists changing it's velocity. There is no such thing as thermal momentum, remove the temperature differential and a thermally uniform substance that's changing temperature at 10 degrees per second will stop getting colder instantly. For related reasons, there's no overshoot when you reach equilibrium, either.
- dhruval 6y agoMany Canadians have known about this for a while, here is what it looks like... https://m.youtube.com/watch?v=I_K6wAARLio https://m.youtube.com/watch?v=I_K6wAARLio
- refurb 6y agoI’ve done it before in -40C weather and the liquid water doesn’t freeze, just the vapor coming off. Same effect as seeing your breath in cold weather.
- bacon_waffle 6y agoIt definitely can all freeze, -40 (conveniently the same in F and C) is about the point where the trick starts working well, but the effect gets more pronounced as the temperature goes lower.
- refurb 6y agoI guess it depends on what you mean by “all”? I did it with a cup of boiling water and most of it fell back down as water.
- bacon_waffle 6y agoIf it's cold enough, and you throw a cup of boiling hot water in to the air, every bit of the thrown water will turn in to something like snow, and no liquid comes back down. It seems like the size of the flung water droplets is important, like if the water is thrown more vigorously the effect is more pronounced. I wintered at the South Pole in 2014, the year the ALS Ice Bucket Challenge was a big thing. One of our crew did it, outside. We discovered a similar effect: when it's even colder out, maybe -80s F: ice water makes what looks like a cloud of steam - in the video (sorry, can't find it quickly) it looked a bit like he was cheating.
- refurb 6y agoMust depend on technique. I could never get it to completely freeze, but the coldest was -40C. I could see if you can disperse it into small enough droplets it would work.
- crdrost 6y agoArxiv link to the actual paper: https://arxiv.org/abs/2008.02373 https://arxiv.org/abs/2008.02373 Looking at the actual graphs, what is presented appears at first blush to be a bistable system prepared in its higher-energy stable state. (Or at least, prepared in a way that several of its constituent subsystems "fall into" that state.) Transitions to the lower-energy stable state require overcoming a barrier, and jumping over that barrier is of course faster at higher temperatures. At lower temperatures you essentially have a slow internal self-heating from the periodical jumps of subsystems through this forbidden region by thermal excitations, not unlike phosphorescence having to tunnel through an energetically forbidden region; this self-heating prevents the thing from cooling down fast enough. Actual applicability to water is kind of harder to evaluate. I've long thought that you could indeed have an Mpemba effect caused by persistent induced convective currents in a fluid: so a hotter thing placed into a fridge might create a more dramatic internal flow as its outer boundary layer cools and changes in density, the convective current would certainly increase the slope of the hotter cooling system beyond the naive temperature scaling, and might then persist as a physical difference even after the two systems arrive at the same temperature, leading to a faster cooling speed of the convective system that "started out hotter". But I mean I have never really done experiments to confirm that sort of thing.
- saeranv 6y ago> Actual applicability to water is kind of harder to evaluate. I've long thought that you could indeed have an Mpemba effect caused by persistent induced convective currents in a fluid: so a hotter thing placed into a fridge might create a more dramatic internal flow as its outer boundary layer cools and changes in density, the convective current would certainly increase the slope of the hotter cooling system beyond the naive temperature scaling, and might then persist as a physical difference even after the two systems arrive at the same temperature, leading to a faster cooling speed of the convective system that "started out hotter". But I mean I have never really done experiments to confirm that sort of thing. This sounds like what I was thinking but I'm not sure. Do you mean the actual boundary layer in the hotter fluid would have a higher convective heat transfer, due to increased turbulence/thermal mixing at the surface of hot water (versus cold water)? Thus hot water will cool faster?
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- inetknght 6y agoOnce upon a generation ago, I worked at an ice rink. "Cleaning the ice" using an ice resurfacer involves cutting the top 1/16" of ice off and laying down new water. Hot water is preferred because it freezes faster and produces a denser sheet of ice; a denser sheet of ice is going to be better for skating. It's nice to see more papers on it.
- ISL 6y agoI've always imagined that hot water would also preferentially smooth the surface, melting any high spots, deep cuts, or granular ice that might have survived the zamboni.
- sosborn 6y agoThank you for getting the zamboni song stuck in my head. https://www.youtube.com/watch?v=vVkJbvv3pHg https://www.youtube.com/watch?v=vVkJbvv3pHg
- comboy 6y agoI don't think it's the effect described in the article. It probably wouldn't be noticeable. It may be that by melting more of the ice below it removes all roughness of the surface where there could be air which is working as an insulation. Additionally, depending on details, by melting more ice it could probably make exchange of heat faster between the surface down below.
- lmilcin 6y agoHot water and is preferred because it has less gasses dissolved and this way it produces clear ice. Also cold water could freeze too quickly, if the rink is very cold, before it has time to spread evenly. I am pretty sure any tiny time difference, if it exists, would not be worth the hassle of supplying hot water.
- sasaf5 6y ago> Hot water is preferred because it has less gasses dissolved true, but while the temperature decreases the air dissolves back in. What makes ice clear is directional freezing, the sheet freezes from below, pushing the gases up. You can try that in your freezer by putting in it a styrofoam box full of water. Just in this case it will freeze from the top and push the gases downwards. The upper layer will be crystal clear.
- fersho311 6y agoI read the whole article and couldn’t understand how or why this happens. Is the reason behind this unknown?
- GreeniFi 6y agoThe best explanation I got was that covalent bonds are weaker in hot water than cold.
- mensetmanusman 6y agoIt’s apparently not true for very pure water.
- mmerlin 6y agoMy high school chemistry teacher simplified it down to the greater "rate of change" of temperature dropping, for the hot water, causes it to end up freezing faster than the cold water.
- ntsplnkv2 6y agoThe rate of change would slow down as it approached the colder temperature though, would it not?
- deleted 6y ago[deleted]
- jointpdf 6y agoI have a vivid memory of being taught this in 2nd grade, probably because it seemed (and still seems) extremely counterintuitive. I think the intuition given was that hot water is less dense than cold water, so the coldness can more easily permeate through the liquid (I imagine a vein-like pattern). Something like that. At least according to Wikipedia, it seems that it’s a bit more complex: https://en.m.wikipedia.org/wiki/Mpemba_effect https://en.m.wikipedia.org/wiki/Mpemba_effect Anyways, I’ve been repeating this fact for 2.5 decades, so shout out to my teacher for blowing my young mind.
- copperx 6y agoIn a similar way, I find the idea of cold permeating a liquid counterintuitive, because I can picture heat going into cold, but not cold into heat. The energy has to go somewhere!
- roelschroeven 6y agoIt's probably better to think of it as heat leaving the liquid rather than cold entering it.
- gvjddbnvdrbv 6y agoThis is what is physically happening so it is a much better mental model.
- gvjddbnvdrbv 6y agoCold is not something that can penetrate. Cold is merely a relatively low amount of heat energy per unit of mass. When something cools heat energy is conducted/convected/radiated away from an object causing it to have a lower amount of heat energy per unit mass.
- jointpdf 6y agoAdmittedly, my understanding of thermodynamics has not progressed much beyond the 2nd grade level (aside from a class on PDEs giving me at least some enhanced intuition). My takeaway from the PDE class was that anything beyond a simple textbook problem needs to be simulated and/or observed experimentally, rather than solved for analytically. But other things being equal, wouldn’t cold air permeate a less dense barrier (say a layer of insulation in a home) more easily than a dense barrier? Just as water (or cold air) would permeate soil more readily than rock.
- adamnemecek 6y agoI wonder if this is a right analogy. Imagine two cars one going fast, one slow. If driver of the fast car steps on the break harder than the driver of the slow car, he will stop faster. The temperature difference is similar to how fast you step on the break.
- muti 6y agoBut cooling rate is proportional to the temperature difference (see Newton's law of cooling). Thus the hotter water may start cooling faster, once it reaches the initial temperature of the cooler water, it would cool at the same rate as the cooler water did initially. It then couldn't possibly reach freezing point earlier. But in some conditions it does, which is the surprising result.
- sliken 6y agoHot water freezing faster in an ice cube tray is commonly reported, a bunch of reasons could explain it. Evaporation can reduce the volume of water, and the smaller volume freezes faster. A hot tray will settle into any ice build up in the freezer (which is common) increasing the rate of heat exchange. Hot water will create currents in the air and water (driven by the higher water temperature) which help the exchange. On the flip side lower temperature differences will often cause a stagnant layer that will reduce the temp exchange, much like sitting in a cold pool which gets better if you want a few seconds... until you move again. The reported experiment reduces the variables by using glass instead of water, so there's no internal flow, but that doesn't stop the airflows generated in the air.
- maydemir 6y agoyes faster. but not earlier.
- devin 6y agocame here to say this. the headline for this finding is almost always misleading.
- maydemir 6y agoYes, exactly.
- hugh-avherald 6y agoNo. That's Newton's theory of cooling. The phenomenon is hot water freezing earlier than cool water.
- kellyjprice 6y agoThey do mean earlier, don't they? https://en.wikipedia.org/wiki/Mpemba_effect#Definition https://en.wikipedia.org/wiki/Mpemba_effect#Definition
- mensetmanusman 6y agoIf you put warm water in your ice try, you can get clearer ice near the surface, because the ice will freeze from the top surface instead of from all sides at once trapping bubbles of air with no where to go :D
- bxparks 6y agoThat's interesting. But don't drink the hot water from the hot water tank. It can be dissolved with unhealthy minerals and chemicals. A few years ago, I had to replace a couple of hot water tanks. When I drained them, the last few gallons at the bottom of the tanks looked like yellow vomit, it was seriously disgusting.
- quickthrower2 6y agoWhich makes me think ... always ask for "no ice" when you are at a bar or restaurant. Unless you know how they make it.
- Ductapemaster 6y agoI think that's a pretty un-based fear. Almost certainly any restaurant, and especially a bar, will use a professional ice making machine. At the volumes if ice they go through, it wouldn't make sense to make it in any other way.
- mensetmanusman 6y agoThis thinking would lead to ‘no everything’ It would actually be kind of funny to go into a restaurant and refuse to order anything because you don’t know how anything is made.
- ShakataGaNai 6y agoThat's why you're supposed to purge your hot water tank periodically. Also keep in mind that yellow sludge comes from somewhere... like the cold water coming into the hot water tank. So... it's probably in the "healthier" cold water no different.
- jrochkind1 6y agoSomehow this article provides no explanation at all of how this actually happens. Perhaps the research being reported doesn't really have an explanation either, it's just more demonstration under more controlled conditions? Or the explanation as it were is just "abstract" and "geometrical", basically just mathematical? Very odd phenomenon.
- Florin_Andrei 6y agoThe fact is, you do need to get rid of more heat when using hot water. Ergo, some sort of self-reinforcing feedback loop must appear when you use hot water, that sucks heat out of it faster. If you use cold water, the "turbocharged" mechanism does not operate for some reason, and heat is lost at a lower rate. I've no idea what that mechanism is. It's nonlinear behavior for sure.
- jakear 6y agoA hypothesis I’ve heard is that the hot water is more likely to produce currents inside the water, as there’s more of a temperature difference between the inner water and the water right at the edge. These currents then cause the water to cool more uniformly as opposed to outside-in. I believe the person I heard this from tested this by adding baffling to the container to prevent currents, but I’m not sure what the results of that experiment were.
- wombatmobile 6y agoThat's baffling, jakear.
- Florin_Andrei 6y agoCell convection is relatively common. For example, this is why large amateur telescopes perform worse for about an hour after they're brought outside: the mirror is "hot" compared to the air, so a network of toroidal convection cells (spinning doughnuts of air) forms on its surface, ever so slightly messing with the rays of light. There's actually an attractor for some of these networks (speaking in general), where they form hexagonal cells. Of course, in most cases, it's more disorderly than that. I'm not sure how a turbocharged network could form, but I'm looking forward to learn more on this topic.
- tzs 6y agoJust a few days short of 35 years ago, this came up in “The Straight Dope” [1]. [1] https://www.straightdope.com/columns/read/422/which-freezes-faster-hot-water-or-cold-water/ https://www.straightdope.com/columns/read/422/which-freezes-...
- recursive 6y ago> Jearl suggested that the most likely explanation for this was evaporation: when water cools down from near boiling to the freezing point, as much as 16 percent evaporates away, compared to 7 percent for water at 160 degrees.
- umanwizard 6y agoRandom tangent: TIL that The Straight Dope predates the web and was originally a newspaper column.
- nemo44x 6y agoWell I’m officially old because I: 1) didn’t know Cecil Adams still existed. 2) still remember looking forward to the Straight Dope in my weekly alt-paper.
- surround 6y agoSuggested explanations https://en.wikipedia.org/wiki/Mpemba_effect#Suggested_explanations https://en.wikipedia.org/wiki/Mpemba_effect#Suggested_explan...
- montalbano 6y agoThere has been some controversy about this for years, see for example: https://www.chemistryworld.com/news/mpemba-effect-in-hot-water-after-doubt-cast-on-its-existence/2500087.article https://www.chemistryworld.com/news/mpemba-effect-in-hot-wat... https://www.repository.cam.ac.uk/handle/1810/263847 https://www.repository.cam.ac.uk/handle/1810/263847 This new paper may prove to be conclusive in supporting its existence, but as I'm not an expert I'll give it some time in peer review before coming to any conclusion.
- handmodel 6y agoI'm not 100% sure it is the same phenomenon but growing up in a very cold climate where I had to park my car outside in high school I remember many times getting into the car where a half full Poland Spring plastic water bottle would sit with water. If I tapped the bottle - the entire thing would freeze in under a second. It looked amazing. (I always thought it was a pressure thing - but not positive if that is what the article is describing or something more nuanced)
- senectus1 6y agothere is a similar inverse process for hot water as well... if you take a very very clean and perfectly smooth surfaced (on the inside) cup/glass and microwave it to just before the point of boiling, then drop anything into it (sugar, coffee, a teaspoon... pretty much anything) it will very energetically, instantly boil.. often vertically... be careful performing this experiment. Edit: here this will help explain it https://www.animations.physics.unsw.edu.au/jw/superheating.htm https://www.animations.physics.unsw.edu.au/jw/superheating.h...
- kosievdmerwe 6y agoWhy I never boil anything in a microwave. Because as you say it explodes when you interact with it and are therefore are close to it rather than when it's sitting safely inside the microwave.
- social_quotient 6y agoSupercooled - https://van.physics.illinois.edu/qa/listing.php?id=1618 https://van.physics.illinois.edu/qa/listing.php?id=1618
- yellow_lead 6y agoNot sure if it's related to the article but this is called superfreezing or supercooling. You can do it with distilled water at home too I believe. https://en.m.wikipedia.org/wiki/Supercooling https://en.m.wikipedia.org/wiki/Supercooling
- air7 6y agoThis is one my favorite science stories. A tale of how truth eventually surfaces and holds in front of ridicule. https://en.m.wikipedia.org/wiki/Mpemba_effect https://en.m.wikipedia.org/wiki/Mpemba_effect See the section called "Mpemba's Observation"
- RcouF1uZ4gsC 6y agoSometimes when we are so familiar with something, we forget how special it is. Water is really amazing. It expands when it freezes. Hot water freezes faster than cold water. It dissolves almost anything, but not too fast.
- saeranv 6y agoPerhaps it has to do with the increased turbulence in the hotter fluid? This would be a result of the higher saturated vapor pressure of the liquid leading to greater evaporation/boiling. Even as it cools down the higher temperature difference between the hot fluid and room temperature should produce a more turbulent boundary layer along the surface of the water. The turbulence will reduce the thermal resistance associated with laminar surface air films on colder surfaces, which would normally be insulating surface, so you'd get a faster heat loss then with a more stagnant layer of cold water.
- aaron695 6y ago> Hot Water Sometimes Freezes Faster Than Cold Water No it doesn't. Unless you want to word play between English and really specific small technical processes, perhaps. What's interesting is it's an old urban legend the predates Usenet. Usenet did kill the glass is a liquid legend though.
- aaron695 6y agoWe all get the Mpemba effect is totally fake right? You all own freezers so you can test it for yourself, it was found by a child making icecream so it will be easy! And if you're the !first! to put it on Youtube that'd be cool. Sciencium talks about it here if you want an ok run down and how it's not real - https://www.youtube.com/watch?v=SkH2iX0rx8U https://www.youtube.com/watch?v=SkH2iX0rx8U
- irrational 6y agoWe tried to replicate this for one of the kid's elementary school science fair project. We were unsuccessful, but the judges really liked it anyway.
- pdm55 6y agoAll the talk in the comments about dissolved oxygen brings back a few memories of student lab work that went wrong. I was given a freeze-pump-thaw protocol to follow in order to remove oxygen from a copolymer solution. Basically we used liquid nitrogen to freeze the mixture, then used a vacuum pump to remove the gases above the mixture. When we thawed the mixture, the dissolved oxygen would escape, in order to equilibrate with the amount above the surface. I repeated this for a few cycles. It was all done with sealed glassware. Unfortunately, the glassware kept cracking during the thaw stage. This went on for a couple of months, till finally my supervisor reviewed the procedure, and realised that, whether cracking occurred, depended upon the relative concentrations of the two polymers. If I recall correctly, the problem occurred when the polymer with the lower freezing point (which, thereby, thawed first) was present at a lower concentration. It was basically trapped inside the other still-frozen polymer, and expanding, as it thawed ... till crack! The solution was surprisingly simple. Change the method. Bubble nitrogen through the mixture to drive out (sparge) the oxygen. Replacing the oxygen with nitrogen was beneficial, as nitrogen didn't interfere with the copolymerisation reaction. The main problem was that I had lost two months of experimental time, and this was just the start of a series of experiments and instrumental analyses. It was now Easter and the deadline to hand in my student thesis was early December. Please don't believe anyone who says students have an easy life.
- squarefoot 6y agoA marginally related question for the experts here: I love extremely cold water, juices and drinks, so I often keep them in the freezer in plastic bottles. Now, if I take out a bottle of something that is just about to freeze, as soon as I open the bottle, it almost immediately freezes although I am certainly not removing heat from it but actually doing the opposite by keeping it with my hands, not to mention the warmer environment in which it is now. I couldn't explain it in any other way than with the different pressure, that is, the water expands when it freezes, so the almost freezing liquid is already under higher internal pressure compared to the moment it was closed, but as soon as I open it the pressure drops abruptly, which would be enough to compensate for the higher temperature, de facto freezing the water. Is this correct?