Does hot or cold water freeze faster?

00:00:08
Speaker 1: Hey, Daniel, do your kids ask you a lot of physics questions?

00:00:11
Speaker 2: Mmm?

00:00:12
Speaker 3: Sometimes, but actually they're mostly interested in other stuff, like horses or chemistry.

00:00:17
Speaker 1: Whoa, it sounds like they're doing experiments on horses.

00:00:22
Speaker 3: Not chemical experiments, fortunately.

00:00:25
Speaker 1: Just the romantic chemistry kind between horses.

00:00:29
Speaker 3: No comment.

00:00:30
Speaker 1: Well, do there questions ever give you good ideas like for your science?

00:00:34
Speaker 3: Not yet, but I'm waiting for the day they inspire some new physics ideas in me.

00:00:39
Speaker 1: Would you give them credit or did you just say called the whites in theory? Then it can apply to both.

00:00:45
Speaker 3: No, of course I'd give them credit. I'd love to have a paper with my kids.

00:00:48
Speaker 1: Whitesin and whiteson doubule whites in.

00:00:51
Speaker 3: Yeah, though I guess we might have an argument about whose first author ooh.

00:00:56
Speaker 1: Or whose last author? What are your kids? Window? Don't kids always when.

00:01:00
Speaker 3: I always put them first, even on author lists.

00:01:03
Speaker 1: There you go.

00:01:19
Speaker 4: Hi.

00:01:19
Speaker 1: I'm hoora ma cartoonists and the author of Oliver's Great Big Universe.

00:01:23
Speaker 3: Hi, I'm Daniel. I'm a particle physicist and a professor at uc R Vine, and I love answering kids' questions.

00:01:29
Speaker 1: But do kids love the answers? Though that's always the question.

00:01:33
Speaker 3: As long as I can keep them brief, they're pretty happy to hear them.

00:01:38
Speaker 1: Doesn't it depend on the kind of question, like, Hey, Dick, can I play more Vita games?

00:01:42
Speaker 2: No?

00:01:43
Speaker 1: Can I play more Vida games? No? At some point, I don't think you enjoyed those questions, do you.

00:01:48
Speaker 3: That's not the kind of question I mean. I mean when they're trying to understand something, when you can see their brains have chewed on something and it doesn't quite fit together and they want to know what the resolution is.

00:01:58
Speaker 1: Do they ever regret asking questions like I have a question, I'm curious, but oh wait, I forgot I'm going to get a lecture from a physics professor.

00:02:05
Speaker 3: Anything longer than like thirty seconds. And you can see them start to tune me out.

00:02:11
Speaker 1: You can see them start to think, maybe I should have asked Wikipedia instead, not that Apedia.

00:02:19
Speaker 3: Exactly. But hey, that trains me to keep my answers brief.

00:02:22
Speaker 1: Oh there you go, to get it down to yes, no answers.

00:02:26
Speaker 3: What is quantum gravity? Yes?

00:02:28
Speaker 1: Yes and no. There you go. It's about true and false. Do they ever ask you questions about the like everyday life that you can't answer.

00:02:37
Speaker 3: Yeah, I mean they asked me questions about how to navigate tricky situations, and sometimes there isn't the perfect answer.

00:02:42
Speaker 1: No, I mean like everyday phenomenon that happens to them about physics.

00:02:45
Speaker 3: Yeah, all the time. There's lots of things in kids' lives that they don't understand. Why a rainbow follows them around, or why you can smell the rain before it comes. All sorts of things inspire questions.

00:02:56
Speaker 1: And what if you don't know the answer, or what if there isn't an answer?

00:02:59
Speaker 3: Those are my favorite because then I get to show them how little we understand about the universe and how close they are to the forefront of knowledge.

00:03:06
Speaker 1: What if there is an answer but you don't know it, you have to take a little Wikipeda break.

00:03:11
Speaker 3: Yeah, then we try to figure it out together exactly, and I show them how to find their own answers.

00:03:15
Speaker 1: Oh, well, there you go, how to google together in case they don't know how to do it already.

00:03:21
Speaker 3: A crucial skill in today's world.

00:03:23
Speaker 1: But it is interesting how sometimes there are still mysteries that you find even in our everyday lives, right in small effects that it kind of surprises you. The scientists don't know the answer to.

00:03:33
Speaker 3: Yeah, a lot of people have the impression that we've mostly figured out the universe, that your everyday experience is totally cracked and it's just like tiny little questions like what's inside a black hole or what's dark matter made out of the physicists are struggling over. But there's a lot of things in your everyday experience that are still pretty tricky.

00:03:51
Speaker 5: Yeah.

00:03:51
Speaker 1: I remember we had the podcast episode about ice skates, right, isn't that a big mystery still how ice skates work?

00:03:57
Speaker 3: Yeah, ice skates and bicycles and all sorts of stuff.

00:04:01
Speaker 1: And sometimes we like to tackle these questions on our podcast to hopefully demystify or at least explain the mystery to people.

00:04:09
Speaker 3: And some of the questions that scientists are still struggling over we're inspired by actual children, teenagers.

00:04:15
Speaker 1: Even so, today on the podcast, we'll be asking the question does hot or cold water freeze faster? This seems a bit a left field for our podcast, why because it's chemistry kind of or it doesn't involve some distant planet or some crazy microscopic effect.

00:04:38
Speaker 3: Well, there are crazy microscopic effects here, and it is really fascinating, But in the end, this is what physics is about, is explaining our everyday world, what we can see in the sky, but also what we see under our feet. And it's a huge challenge to bring it up from microscopic particles to explaining the fabric of our reality. So sometimes I'd like to show how difficult it is to really bridge that gap.

00:05:00
Speaker 1: To explain it all here, even the things inside of your fridge which may seem inexplicable and mysterious.

00:05:07
Speaker 3: And this particular story really was inspired by an experiment done by a Tanzanian teenager who wrote the first paper on the topic.

00:05:16
Speaker 1: Now, this is an interesting question. Does hot or cold water freeze faster? Because I guess, at first glance, it seems kind of obvious that the colder water is going to freeze faster if you put them in a freezer.

00:05:27
Speaker 3: Exactly, But it turns out the universe is not so simple, or your freezer is not that simple. Your freezer is part of the universe.

00:05:32
Speaker 1: So yeah, although sometimes it seems like it's from another dimension. If you look inside my freezer, there's things going in there that have a supernatural color to them.

00:05:42
Speaker 3: I think my freezer might have additional dimensions because I keep putting ice cream in there and then it's gone.

00:05:47
Speaker 1: Oh wow, yeah, it goes into a black hole maybe called your stomach or my teenager. But anyways, welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio.

00:05:57
Speaker 3: In which we tackle mysteries big and small, all hot and cold, weird and.

00:06:02
Speaker 1: Wacky mysteries that you encounter every day, and mysteries that sometimes nobody may ever encounter. We cover the whole range. But anyways, this is kind of a seemingly simple question, and we were wondering how many people out there had thought about this and whether they know whether the answer is yes or no.

00:06:19
Speaker 3: Thanks very much to everybody who answers these questions. We love this audience participation segment and we'd love to hear your voice as part of the chorus. So please write to me two questions at Danielanhorge dot com and I'll set you up.

00:06:31
Speaker 1: So think about it for a second. Which one do you think freezes faster? Hot or cold water. Here's what people have to say.

00:06:37
Speaker 4: I've heard that hot water phrases faster. I'm not sure why. Possibly, I think maybe the atoms have more speed and are more active, so maybe they lose their heaked a little bit quicker than colder water.

00:06:52
Speaker 3: So hot water definitely freezes faster.

00:06:55
Speaker 6: And I know this because in Colorado you can take boiling water and throw it in the air on a really cold day and it turns into freezy dusts really fast.

00:07:04
Speaker 2: Well, I would imagine cold water freeze faster because freezing is the process of slowing down your molecules to the point where they would be measured cynthigrade. So hot water a lot more energy than the molecules they are moving a lot quicker, so more energy needs to be applied over a longer duration to slow those molecules down. However, I suspect the correct answer is probably hot because it's the less obvious one.

00:07:31
Speaker 5: It's funny because I've seen so many science shows demonstrate this, and I clearly remember that hot water freezes faster, but I do not remember exactly why. I think it's because there's more energy in hot water to facilitate the phase transition, but that's just my guess.

00:07:48
Speaker 7: Well, I believe it's hot water, but I don't really know why, other than it's always been a wives tale of sorts. Maybe because of hot water is as more pressure, and I know pressure is a big part of phase changes as much as temperature.

00:08:04
Speaker 8: Well, logic would say cold water freeze faster because it's already closer to the freezing point. But you wouldn't ask this question if it would be that easy.

00:08:15
Speaker 9: And I seem to recall that some people were chutting boiling water into freezing air on YouTube a while back. So my final answer is hot water freezes faster. But I cannot tell you why.

00:08:28
Speaker 6: I always thought that boiled water freezing faster was just a myth. But you can boil water at room temperature by lowering the pressure, so perhaps if you increase the pressure maybe.

00:08:38
Speaker 1: All right, well, let's dig into this strange question, Daniel, What does this question even mean? What's the setup?

00:08:44
Speaker 3: I love this question because it seems so simple, both experimentally, like actually measuring this and theoretically, but it turns out to be much more complicated in both aspects. From a basic point of view, it's a very simple experiment you could do while you listen to this episode. I mean, take two cups of water, one of them hotter than the other, put them both in the freezer and just measure how long does it take for each one to freeze? And now what answer the question? Does hot or cold water freeze faster?

00:09:12
Speaker 1: Which one turns into a cube of ice first?

00:09:15
Speaker 3: Yeah?

00:09:15
Speaker 1: Exactly does it depend on what you mean by freeze?

00:09:18
Speaker 3: M We're going to get into that.

00:09:19
Speaker 1: Yeah, But I guess the question is which of the two cups, the hot one or the cold one, turns into ice first? And so intuitively you'd assume that the cold one freezes faster because it's closer to freezing temperature.

00:09:32
Speaker 3: Exactly, if you can describe water basically in terms of just one variable, like it's temperature, then the hot water has to travel further along that temperature line, and so it should take longer. Like eventually the hot water will become the cold water, and they'll still have to progress to freezing. So if the hot water has to pass through the cold water point, then it's like riding the bus home. If you get on at a further stop, it's going to take longer to get home.

00:09:54
Speaker 1: Right, Like, if you have hot water one hundred degrees and cold water at fifty degrees, Eventually, after a while, the hot water is going to be fifty degrees, at which point it's basically the same starting point as the cold water.

00:10:05
Speaker 3: Yeah, and if it took time to get there, then obviously it's going to take longer overall to get to the freezing point.

00:10:11
Speaker 1: Right. So you would think that the cold water freezes faster because the hot water has to become the cold water first anyways, exactly, But it sounds like physicists have other ideas.

00:10:22
Speaker 3: Yeah, and not just physicists. It's sort of an old wives tale that people have been repeating for like centuries or millennia that hot water freezes faster. You hear plumbers in the northeastern part of the United States often saying things like that hot water pipes freeze more often in big snowstorms, like they will burst more often than the cold water pipes. There's writings by Aristotle in the fourth century BC who says, many people, when they want to cool water quickly, begin by putting it in the sun.

00:10:52
Speaker 2: Mm.

00:10:52
Speaker 1: Interesting, give it a sunburned first, and that will freeze it faster. Now does Aristotle count as an old wife? Is he like the og old wife?

00:11:02
Speaker 3: He's definitely one of the og physicists. But you know, in the end it comes down to actually measuring this, like doing the experiment. And for a long time this is something people sort of just discussed and assumed that the cold water freezed faster. But then there was this event in the nineteen sixties that involved ice cream that sort of changed the course of this question forever.

00:11:21
Speaker 1: Oh my goodness, did somebody stop to eat a snag er and accidentally drop that left some cold water in the freezer.

00:11:27
Speaker 3: No, there was a Tanzanian teenager named Erasto Pemba. He and his science class were doing the exercise of making ice cream, but apparently there weren't going to be enough slots in the refrigerator. While most students were letting the ingredients cool to room temperature, he just jammed his in the freezer to get a spot, and he saw that his concoction froze faster than other kids who put it in at the same time and started cooler. So he thought, hmm, maybe hot water does freeze.

00:11:53
Speaker 1: Faster, so he did it with ice cream. But that's not really how you make ice cream, is it. You don't just freeze cream, do you know.

00:11:59
Speaker 3: You have to freeze it and you have to mix it at the same time. So I'm not sure if he qualifies as ice cream.

00:12:04
Speaker 1: So maybe he was just making popsicles exactly creamsicles. I guess I'm not just trying to debug this story figure out what is really true here, Daniel. We're doing some heart investigating here.

00:12:14
Speaker 3: That's just sort of what inspired him. And he asked his physics teacher about it, and his physics teacher told him, no, no, no, you're confused. That cannot happen, is physically impossible. And then later on a physicist came to visit his school, and he raised his hand and he told his story, and he asked the physicist about it, and the physicist was interested enough to invite him back to his lap where they did a bunch of experiments confirmed the result. They saw hot water freeze faster than cold water, and then they wrote a paper together and that's why this is now called the Pemba effect.

00:12:42
Speaker 1: WHOA, So they actually did an experiment in a physics lab.

00:12:46
Speaker 3: They actually did an experiment in a physics lab.

00:12:48
Speaker 1: And what did this experiment look like?

00:12:50
Speaker 3: The experiment is just two cups of water in a freezer with thermometers in them.

00:12:54
Speaker 1: M I see, pretty basic.

00:12:55
Speaker 3: It was pretty basic but the paper is pretty fun to read because it's written by a teenager. Like the opening line of the paper is my name is Erastum Pemba, and I'm going to tell you about my discovery, which was due to misusing a refrigerator.

00:13:09
Speaker 1: Nice, nice and direct, nice and direct, opposed to how an adult academic would do it, which is would take you the seven paragraphs to get to the same point exactly.

00:13:19
Speaker 3: But the question you asked a minute ago about the details of the experiment turn out to be crucial because these experiments done in the sixties sort of established the effect, and lots of people have tried to reproduce it in the decades following with mixed success.

00:13:33
Speaker 1: Wait, but if it's a simple experiment just putting two cups into a freezer, you're saying people have done the same thing, but some of sometimes the cold water beats the hot water, and sometimes the hot water beats the cold water to the freezing point.

00:13:45
Speaker 3: Exactly. The experiments seem to be very sensitive to the conditions in a way that we do not fully understand because it's not just two cups of water in a freezer. It's like the questions of the purity of the water or the blowing of the fan through the freezer. Something is going on which makes this very difficult to reproduce.

00:14:03
Speaker 1: It's not just about putting two cups inside of a freezer. It's about like how they're being cooled and what they're sitting on.

00:14:08
Speaker 3: Maybe perhaps I read one study that saw hot water freeze faster and they discovered that it was because the shelves of the freezer were covered in frost, and the hot water basically melted that frost, which improved the thermal contact with the shelf, the metal shelf, and that helped it cool faster. So all sorts of little details like that can make a difference in how you do this experiment.

00:14:31
Speaker 1: Well, but I guess you know, the main headline is that this is kind of an undecided question. It seems like people have tried it, and sometimes the hot water does beat the cold water to freezing.

00:14:42
Speaker 3: Yeah, it's something which is not experimentally decided, like we cannot reliably reproduce this result. It turns out to only happen under certain conditions and be very sensitive to those conditions, and nobody has quite isolated those and controlled them.

00:14:55
Speaker 1: But I guess my question is how hard have people tried, Like, have you spent billions of dollars you have in the leec to figure out this question? Or is this one of those questions that you only see, you know, physicists publish at the on their websites.

00:15:10
Speaker 3: No, there are real labs doing detailed studies because it turns out this is a really interesting question on the energetics of water, and so people have really tried. I read another review of this which said, quote, there is a wealth of experimental variation in the problem. So any laboratory undertaking such investigations is guaranteed different results from all the others. So this is something serious physicists and chemists are trying to nail down because the chemistry of water is very important. It's very important to life, it's important to climate change, it's important to exoplanets like water is important stuff.

00:15:41
Speaker 1: Yeah, I hear, it's important for horses too.

00:15:45
Speaker 3: I'll have to ask my daughter about it.

00:15:49
Speaker 1: All right, Well, let's dig into the details of the chemistry of water and the details of these experiments and how they might affect who wins first cold water or hot water to freezing. So let's do that. But first let's take a quick break or right, we're asking the simple question does hot or cold water freeze faster? And it seems like the answer is it depends, which is wild like the idea that hot water can freeze faster than cold water.

00:16:28
Speaker 3: It comes down to lots of tricky little details. One question you asked earlier is like, well, what do you mean by freezing? Right? Is that the formation of crystals. It's possible that like hot water could start forming crystals sooner, but actually takes longer to get to zero C. Water is very complicated stuff and it can exist in different phases at the same temperature. Or do you really just mean thermodynamically getting to zero C?

00:16:53
Speaker 1: Well, I guess maybe I would guess that what most people think of as freezing is when water turns into a solid. So are you saying sometimes the hot water turns completely into a solid.

00:17:02
Speaker 3: First, Yeah, some of these experiments see hot water forming these crystals first, right, number water is a sort of a disordered crystal. It's possible for the water, which starts out hot, to start forming those crystals, and crystal formation is very stochastic.

00:17:16
Speaker 2: Right.

00:17:16
Speaker 3: Once you get a seed going, then you can rapidly form more crystals, So it sot of depends on like when you get that first seed. So hot water might be able to like first form that seed, which gives it an advantage even if it's still at a higher temperature.

00:17:30
Speaker 1: Right. And I think freezing also kind of depends on pressure as well, Right, But I guess you're assuming they're both under the same amount of pressure.

00:17:37
Speaker 3: Yeah, I think we're assuming that they're in a refrigerator, which is basically like an infinite thermal bath and can provide the pressure. But there are these little details also like evaporation, right, like did you seal your container, because hot water will also evaporate, which lowers its volume, which makes it easier to freeze.

00:17:54
Speaker 1: I see, So the hot water, even if you stack it in the freezer, it's going to be hot for a while, and so some of it's going to vapor rate out into the freezer, which means that the amount of water left in the cup will be less than the amount of water in the cold cup, which means that it might be easier for it to freeze.

00:18:09
Speaker 3: Mm exactly. And there's other complicated experimental issues too, like one study discovered that it dependent on where you put the thermometer in the cup that sometimes they could get false readings of the Impemba effect because they put the thermometer like in the wrong place.

00:18:25
Speaker 1: What do you mean, what's the right place to put it in the cup of water?

00:18:28
Speaker 3: Basically you have to have them in the same place. Really, you should put it at the core so that you're measuring the time it takes the core to get to zero C for both cups. You mean for both cups. But if you don't place the thermometer exactly in the center for both of them, you can get a false reading of the Impemba effect. So there's two things going on here. One is like how treaty it is to reproduce this effect? And the other is are all the reported cases of the Mpemba effect actually real or are some of them experimental mismeasurements.

00:18:55
Speaker 1: Well, let's break it down. In the case is that we think or people have thought that hot water fast or what do you think might be going on?

00:19:02
Speaker 3: It all comes down to this question of describing water with one number temperature. You imagine what's happening is you have this cube of water and the outer layers in contact with a freezer, and so it starts to chill, and then it chills the inner layer, and then that chills the inner layer, and eventually the whole thing cools down. That is the simple model of the temperature dropping, and you're really just describing the water in terms of one number. But this is tricky because temperature is a very slippery topic and it's not actually well defined. Out of equilibrium, equilibrium means there's no heat transfer, right Like, when the water is cool and it's no longer changing, then you can define it to be at the same temperature as the rest of the freezer. But if things are changing, then temperature is not technically defined. It's only defined for a system in equilibrium. Out of equilibrium, things get very complicated very quickly, and you need much more information to actually describe the whole system.

00:19:51
Speaker 1: Well, I see, we're saying, like the idea of temperature it only works if nothing's changing. But when something's freezing from hot water or cold water into ice, things are changing. So if you're just going by measuring freezing as some sort of temperature reaching a point, then you're already kind of measuring the wrong thing.

00:20:13
Speaker 3: Yeah, exactly. And the things you remember is that temperature is a macroscopic thing. It's like our experience. It's something we can measure using thermometers, and we try to connect it to the microscopic to say, what's actually happening in hot water or in cold water? What's the difference between those two. And there's a bunch of different descriptions, a bunch of different ways to try to explain and understand our intuitive macroscopic experience of temperature in terms of the microscopic particles. Like one very common one is the kinetic theory says in hot water molecules move faster and in cold water molecules move slower. That's rough because molecules can also do other things like spin and vibrate. That's one basic idea. But zooming out from that microscopic picture of the little molecules and their speeds determined in the temperature assumes that it's in equilibrium. There's a bunch of steps there, the mathematics involved to go from like zillions and zillions of little particles out to a single number that describes it. Assumes that it's an equilibrium, and if it's not, it just can't do that.

00:21:12
Speaker 1: So I think what you're saying is if you're going by freezing as when it reaches, for example, zero degree celsius, then even if you measure it to be zero degree celsius, maybe it's not done changing, or maybe the molecules are still moving around.

00:21:26
Speaker 3: I think I'm saying more generally that describing the whole system as like having to move through one path to get from hot to freezing is not an accurate way to think about it. In reality, there's lots of different paths. It's not like the hot water is getting on the bus further from home. It's like the hot water might be able to take a short cut. There's not just one path. From one hundred degrees to zero doesn't necessarily have to pass through fifty because temperature isn't even really well defined for a system that's changing. It can be one hundred, it can later be zero, and it could have never been fifty.

00:21:58
Speaker 1: Well, it sounds like you you're basically disqualifying temperature as a gauge for this experiment, like basically don't use temperature to measure whether something freezes or not.

00:22:09
Speaker 3: Well, I think if for the starting point and the ending point, you can use it like is it frozen, yes or no? But while it's out of equilibrium. As it's changing. There's a lot more complicated stuff that's going on.

00:22:19
Speaker 1: Oh I see, they may trace the same history of temperature, but actually what's going on inside the cup might be totally different.

00:22:26
Speaker 3: Yeah, exactly. The hot water cooling and the cold water cooling might be very very different processes. Remember that water is very very complicated stuff. It has all these strange bonds, these hydrogen bonds between the molecules, which create lots of counterintuitive effects. Like famously, water is less dense when it's a solid than when it's a liquid. Right, water floats on top of liquid water. It's like one of the only substances that will do that. You mean ice floats, right, Yes, solid water floats on top of liquid water. Ice floats, right, It's one of the only things that will do that. And so it has all of these weird counter into effects.

00:23:01
Speaker 1: Like what, how would that affect how fast it freezes or not.

00:23:04
Speaker 3: So these hydrogen bonds, the sort of weak bonds between the molecules, are crucial for forming the crystal, and if you heat the water up, it can like destroy all the existing ones, freeing them up to rearrange themselves. So hot water is like more active and more loose, which allows it to like explore the possible configurations more quickly. Like imagine you have a box of legos and you're shaking it around. The more you shake it around, the more likely you are for it to end up in some configuration. Then if you're shaking it less because you're exploring like more combinations of legos bumping against each other. So hot water sort of like explores all those configurations faster, and it might find accidentally some configuration which causes a seed of structure, a little mini crystal, which then flourishes and forms a larger crystal.

00:23:49
Speaker 1: But if the whole thing is hotter wooden, those eventually break. Like even if if you're shaking the container of lego, they might you know, accidentally or coincidentally you know, build themselves, but you're still shaking, you're still shaking the canisters, they wouldn't break apart.

00:24:06
Speaker 3: Yeah, you're absolutely right. So we have to add one more little thing to our model, which is that water does like to stick to itself, right, there are hydrogen bonds, So it's sort of like a box of sticky legos and you're shaking them and some pieces come together and like to stick together, and that's what happens with water. Sometimes the pieces end up in exactly the right configuration and boom, they're bound together. So even though they were hot, now they like to stick together. That's a lower energy configuration.

00:24:30
Speaker 1: So that's one way that maybe hot water can get to being a solid faster than cold water exactly.

00:24:35
Speaker 3: That's one idea that's out there, and people have done a bunch of experiments to try to confirm this. They're very artificial sounding experiments. They like take beads of glass and shoot them with lasers and study these very artificial situations where the bead of glass is like different energy levels it can be in, but in generally prove the principle that like high energy beads of glass find the minimum faster. They like explore all the possible figurations more quickly, and end up finding that minimum that most relaxed state more quickly than a slow bead of glass. And so some people think that that sort of proves that that's possible in principle, though that doesn't mean it's what's actually happening, like in water.

00:25:15
Speaker 1: The way they don't think this is helping hot water freeze faster.

00:25:18
Speaker 3: A lot of people think, okay, that proves in principle that hot things can relax faster. But you know, water is a very complex molecule, and so like very simple models of it don't always describe the behavior in reality. I talked to one water chemist who said, you can't believe anything that doesn't have a lot more details included in the simulation.

00:25:37
Speaker 1: So why don't they do these more detailed simulations.

00:25:39
Speaker 3: They are doing these simulations, and people already do lots of simulations of water for reasons We talked about like how does water form, and what happens when asteroids hit the atmosphere and contain ice in them? Is there a possibility for making like basic amino acids and organic molecules. All sorts of people are studying water for lots of reasons, but it's hard because there are lots of interaction actions, and so it takes like supercomputers basically to model a little bit of water.

00:26:05
Speaker 1: I guess maybe paint us a little bit of a picture. When you say that sometimes hot water freezes faster, is it by a lot or is it like super close to the cold water.

00:26:15
Speaker 3: So if you look at the plot in the original paper or the one by in Pemba. Then the variation in the time to start freezing is in like tens of minutes. So if you start it like eighty C, he says, it takes thirty minutes to cool his glass, where if you start it like twenty C, it takes one hundred minutes. So it's a very strong effect, at least in this original and Pemba paper.

00:26:39
Speaker 1: Oh wow, Yeah, that's a big difference. And I guess it's interesting how in his experiment the hot water wasn't that much harder, Like it wasn't boiling water. Would this happen with boiling water?

00:26:50
Speaker 3: He did try a bunch of different temperatures from twenty to eighty C, but he didn't try boiling water. No, Boiling water is much harder to control, right, It's like actively vaporizing, and so you can't really control the volume as easily.

00:27:02
Speaker 1: I see, all right, So it seems like that's one idea about what could be going on. What else physicists thought about could explain this.

00:27:08
Speaker 3: There's a whole lot of little details that people are thinking about, Like it could be the impurities cold water tends to contain more dissolved gases in it just because of the chemistry of water, and that can help actually lower its freezing point, which means that you're not really comparing two things that are the same. You're comparing one which has a little bit more gas dissolved in it, which can totally affect how long it takes to freeze.

00:27:32
Speaker 1: Wait, what can you explain that again?

00:27:33
Speaker 3: So cold water can have more dissolved gas in it, Like if you try to make bubbly water and try to get CO two dissolved into your water, it's hard to get that CO two in unless the water is cold. So cold water will absorb COEO two more than hot water, and so cold water likely has more CO two in it and other gases dissolved, and that will change the freezing point of that water.

00:27:57
Speaker 1: Wouldn't the hot water absorb gas? Is it cool?

00:28:00
Speaker 2: Yeah?

00:28:00
Speaker 3: Absolutely. Hot water will absorb gas once it cools, but it doesn't have as much time. So if you've had water that's been cold for a long time, it will have absorbed a bunch more CO two then water that's just been hot, like five minutes ago.

00:28:14
Speaker 1: And so the gas in the water might help or make it more difficult for the water to freeze. Yeah, or this is I guess it makes it more, it makes it harder. Why would it make it harder.

00:28:23
Speaker 3: Well, you know, if water has to form these crystals, and if there's a bunch of CO two involved, then it inhibits the ability of those water molecules to like find each other and to make those bonds.

00:28:32
Speaker 1: There's just more stuff going on that's not water.

00:28:35
Speaker 3: Yeah, exactly, the same way that like adding salt to water will change its freezing point.

00:28:39
Speaker 1: So the idea is that maybe when you heat it up the water to put into your experiment, it lost a bunch of gas.

00:28:44
Speaker 3: Yeah exactly. Then there's people who argue the opposite, say that actually having impurities in the water should seed crystals that as it cools, if one of the two glasses has more impurities in it, that those impurities become like the nucleation side for crystal formation. Remember, crystal formation is sort of a stochastic thing, Like you have a bunch of molecules at different temperature and they have to sort of like click together in order to start that crystal. And we talked earlier about how maybe hot water has an advantage because it tries more combinations per second, but could also just be a difference experimentally in the impurities in that water, And as you heat it up, you might have boiled off some of those impurities, or you may be concentrating those impurities as you heat it up the water. So if the hot water has more impurities in it, that could actually provide more sites for the nucleation of those crystals, which would lead to it forming a solid faster, even if it's not at the same temperature.

00:29:39
Speaker 1: But I feel like maybe we're talking a lot about things that might affect the formation of ice crystals. But from a sort of a macro point of view, doesn't the hot water just have more energy, and so wouldn't it just technically take longer to get rid of that energy.

00:29:56
Speaker 3: It's a compelling argument, and it's compelling because it's a simple model when it basically says is a single number you can use to describe the system. But imagine if instead of just having energy, you also have energy in like momentum of heat loss. What is starting from a higher temperature means that you start losing energy more quickly because there's a higher difference between the heat of the water and the freezer, and that high rate of energy transfer is somehow remembered. So now when you pass through that fifty degree mark, you're losing heat faster than the cold water did when it was there. It's sort of like having somebody start a RaSE ten or fifteen meters behind the starting line, but they get to accelerate up to their top speed before they cross the starting line.

00:30:39
Speaker 1: Right. That's an effect and might explain it. But it's a bit crazy, isn't it. The idea that you can have like temperature inertia.

00:30:45
Speaker 3: Yeah, that is a bit crazy, and it totally violates our a simple model where you can describe things just in terms of like temperature and pressure and volume. And that's because that only applies in equilibrium. Out of equilibrium, things are crazy, and none of these approximations we used to, like apple with all the crazy details of those particles, are really applicable. And so there's lots of things that can happen to complex liquids, much more than can be summarized in just a few numbers.

00:31:10
Speaker 1: Well, I think maybe you're not saying that the water itself has some sort of temperature inertia, but you're saying that maybe like the details or the complexity of how the cup interacts with the air or around it, that might have some effects that look like temperature inertia.

00:31:26
Speaker 3: It could be both of those things, right. It certainly could be dependent on the details of the experimental setup. But also we just do not understand out of equilibrium chemistry very well, So temperature inertia could be a real thing. But it could also depend on the details of the molecular structure, like maybe only happens for more complex substances, maybe only happens for things with very specific kinds of bonds that are possible.

00:31:49
Speaker 1: All right, well, let's get into some of these other ideas for what could be making hot water freeze faster than cold water and what that means about our understanding of water chemistry and horses. So to dig into that, but first let's take another quick break. All right, we're talking about Daniel raiding the freezer for ice cream and how it seems to disappear magically, as if by magical horses.

00:32:28
Speaker 3: That's right, when my teenager leaves ice cream out on the counter, why does it cool faster?

00:32:33
Speaker 1: M Wait, could he say then that he's just trying to freeze it faster, because then he's taking it out, leaving it out so it heats up, so then when you finally put it in the freezer, it'll freeze faster.

00:32:46
Speaker 3: Yeah, I'm sure he's just doing a chemistry experiment, right.

00:32:49
Speaker 1: Yeah, that's right. He's maybe just trying to raise your boiling point.

00:32:53
Speaker 3: But you know, it's amazing to me that this is still an open territory, both theoretically and experimentally. Lots of people trying to understand under what conditions you can make this happen, and a bunch of other people are trying to understand whether it makes sense and what it means about like the nature of matter and phases and being a liquid.

00:33:11
Speaker 1: Yeah, it's pretty interesting that we don't have an answer to this. It sounds like we need to spend a couple more billion dollars on this question.

00:33:18
Speaker 3: That's right, billions of dollars. Where of ice cream must be purchased.

00:33:23
Speaker 1: That's right, Yes, white chocolate ice cream with bananas. Okay, So then I don't think we're quite done. You said there might be other factors that might be going on that might explain why the hot water freezes faster.

00:33:35
Speaker 3: Yeah, another theory I was reading about, and there's like no shortage of theories impossible at Explanations for what might be going on here has to do with convection, how the water moves through the glass as it's cooling. So like, as the water is cooling, it's going to definitely have some currents within it, even if it started out totally stable, because density decreases with increasing temperature. So the surface of the water is going to be warmer than the bottom, which sometimes they call it hot top. Now the water primarily loses its heat to the surface, then water with the hot top will lose heat faster than we would expect based on its average temperature. When they initially warmer water then cools down a little bit to the initial temperature of the other cooler water, it's going to have a hot top, and its rate of cooling will be faster than the rate of cooling of the other water at the same temperature. Anyway, it's complicated, but it has to do with like how the water changes density in which chunk of the water is essentially exposed to the cool air.

00:34:32
Speaker 1: Oh, I see what you're saying. Like maybe you have a hot cup and a cold cup, and you're measuring the temperature, and maybe they both have the same temperature in the middle of the cup, but you're saying maybe the warmer cup has a hotter surface or water on the surface of it, which means it's losing heat faster, which means it's you know, basically running faster towards freezing than the cold water.

00:34:54
Speaker 3: Yeah. It's like the hot cup is constantly exposing the hotter water to the surface, which helps cool Where there's the colder cup, the water is moving more slowly through it, and so it's not like as efficient at exposing the hottest parts of itself. The hottest parts of itself stay in the center for the cooler cup rather than the hot cup. And that depends on like concurrence, which depends on like fluid dynamics, which we all know is a nightmare.

00:35:18
Speaker 1: But then wouldn't the hot cup need a colder bottom?

00:35:21
Speaker 3: Yeah, and this is one of the reasons why this depends on like where are you measuring this in the cup?

00:35:25
Speaker 1: But how did it get a colder bottom?

00:35:27
Speaker 3: Well, the hot water is going to rise to the surface because density decreases with increasing temperature.

00:35:32
Speaker 1: Hmmm, So it might be like more dense than at the bottom of the hot cup than at the bottom of the cold cup, exactly. I think there's also a theory about heat conduction, right, like maybe that when you initially put the cup into the freezer, like, it melts the things around it a little bit more, which helps it maybe connect to the coldness in the freezer better.

00:35:56
Speaker 3: Exactly, because heat conduction is very different from material to material. Between water and air surfaces it's one number. Between water and water it's another. And so as we were saying earlier, like if you put the cup on a frosty shelf and that melts the frost near the cup, then you now have like a water connection to that shelf. It can very rapidly cool it. And so all sorts of these little details. You can even pull frost out of the air and then melt it. You can even pull water vapor out of the air and it can accumulate on the sides of the cup. All these things could have a big difference on the experimental measurements. So I think first we have to like nail down what are the conditions we need to make this happen. Then we can understand on the theoretical side like, do we understand why it happens in one configuration and not another.

00:36:40
Speaker 1: Meaning like, if you stick a cold cup in the freezer and it's like my freezer, which is full of frost, it will basically be like putting a cold cup of water on a bank of snow, and because the water is cold, it's not going to melt the snow, and so it's basically going to be sitting kind of an insulation of fluffy snow. Whereas if you put in a hot cup, it's going to be hot enough to melt that snow, which basically melts and then it basically it turns into ice around it, which means that the hot cup's going to have basically like a fast track for the heat to escape.

00:37:14
Speaker 3: Exactly for the same reason that like wet jeans will make you colder on a ski hill than snowy dry jeans, right, that water will conduct heat away from you much faster than the snow will. So if you melt the surrounding area, then very quickly your heat is going to bleed out.

00:37:29
Speaker 1: And that's the reason you don't ski, right.

00:37:31
Speaker 3: That's one reason I don't ski. Absolutely. There are so.

00:37:34
Speaker 1: Many Well we can fix that one pretty quick then and just use a special socks.

00:37:41
Speaker 3: No waterproof pants are the answer to that one.

00:37:43
Speaker 1: Yeah, yeah, those ski and jeans.

00:37:44
Speaker 3: But the physicist in me is more interested in, like, what's happening to these particles, and how do you zoom out from all these tiny little particles doing their thing individually to an explanation of what's happening to the water, right because in the world enteract with, there's not individual particles of water in their hydrogen bonds, but like the water in your cup, or you want to freeze your ice cream. And in the end, what we're trying to do is explain that universe. And it's very very difficult sometimes to connect this picture of the microscopic particles with our actual experience and like things we can measure.

00:38:17
Speaker 1: But I guess maybe I wonder if the point is that if you have a very simple model, or just assume that it's like some molecules floating in a simple canister or something, then you do expect the cold water to freeze faster. But because we live in the real world and there's all these different things that can happen, and the way that the water freezes and you know, the heat conduction depends on surfaces and what those surfaces are touching or which direction they're pointing. Then the phenomenon of a freezing is much more complex.

00:38:50
Speaker 3: Yeah, that's exactly right. We're trying to describe a complicated emergent behavior, and we usually start with a simple model because we hope that works. And often this simple model, which just has like temperature, pressure, volume, does work. It works amazingly well, it's incredible. But we can also learn from when it fails. And when it fails this tells us that other details that we've ignored in our model are now important. They may even be crucial. They may even determine the total outcome. And that's an opportunity to learn something, to go back and say, Okay, well, what is it that we didn't include in our description that give us that simple story, and how do we need to make it more complicated to describe what we're actually doing. Science is all about that, right, Start simple, and then refine, refine, refine.

00:39:31
Speaker 1: And I feel like this question is interesting, as you said before, because it taps into the chemistry of water and the dynamics of water, which might be related to how life started right here on Earth. And maybe in other planets. Like, if we understand more how what's happening at the molecular level with water, maybe we can understand how likely or less likely it is or necessary it is for water to be there for the molecules that make life deform.

00:39:57
Speaker 3: Yeah. One theory about how organic molecules we're created on Earth involves like asteroids slamming into the Earth, and if those asteroids have ice in them, then those very high temperature, high pressure impacts could have created conditions needed for that kind of chemistry to happen. So understanding exactly how water works and all the weird forms that it can take is crucial for understanding how life could have started here and also what the conditions might be like on exoplanets. We're like right on this exciting cusp of being able to detect water vapor in the atmospheres of planets around other Solar systems, and understanding the chemistry of that gives us a deeper and richer picture of what it's like on those surfaces and what it might be like to be a squigly blob crawling around and waiting for your ice cream to cool.

00:40:44
Speaker 1: Yeah, or not even maybe that far like even in our solar system. The moon, one of the moons of Jupiter Europa, Right, it's made out of ice on top, and it's got an ocean of liquid water inside. So we kind of want to know, is it possible for life to exist there?

00:40:58
Speaker 3: Oh yeah, you're absolutely right. I'm that close to home. There's exciting water chemistry happening in our solar system. It's crazy to imagine this, like frozen crust of ice under which there's like a mile deep ocean of super cold water in which maybe life has started.

00:41:14
Speaker 1: We just don't know, Yeah, because we know there's life underneath, like the ice shelf in Antarctica. But maybe, like you need some special conditions for life to start. Maybe they can start in somewhere like EUROPEA.

00:41:28
Speaker 3: Yeah, I wonder if you take two moons of Jupiter and one of them is hotter and one of them is colder, which one will freeze first?

00:41:33
Speaker 1: Yeah, you need a pretty big freezer for that bo like maybe space.

00:41:39
Speaker 3: Now, imagining a moon size chunk of ice cream.

00:41:42
Speaker 1: There you go, a giant scoop the ship the size of the moons. That's no moon, that's a giant ball of ice cream.

00:41:51
Speaker 3: That'll be the death of me.

00:41:53
Speaker 1: I can hear the cries of a thousand white suns or a billion white sons crying out fur their ice.

00:41:59
Speaker 3: Cream as our cholesterol rises and temperature drops.

00:42:03
Speaker 1: That's right, and our mass increases as well. Okay, So what would you do, Daniel if your daughter asks you what would happen if you stuck a cold horse and a warmer horse in a freezer? Would you be concerned? Would you be like, oh, interesting questions? Sit down, let me explain this to you.

00:42:20
Speaker 3: I try to summarize this entire podcast in thirty seconds, and I would fail. It's hard, man, it's hard.

00:42:27
Speaker 1: Well, yeah, it kind of has an impact on horses and cold laces, right.

00:42:31
Speaker 3: Yeah, absolutely No. I tell her that we don't know the answer to that, that it's too complicated, and maybe she should grow up and figure it out.

00:42:38
Speaker 1: Mmm, become a horse chemist.

00:42:42
Speaker 3: Better than a horse particle collider.

00:42:44
Speaker 1: Yeah, that would be a nea career you want to pursue.

00:42:51
Speaker 3: That's not science, that's just horsing around.

00:42:53
Speaker 1: Yeah. Yeah, don't look at gift horse and the particle collider.

00:42:58
Speaker 3: I don't know what that means, but I laughed anyway.

00:43:01
Speaker 1: I don't know either, it's horse nonsense, all right. Well, another reminder that there are still mysteries in the universe, and some of them that you could even do at home and experiment and see for yourself how weird things can happen and how complicated even something as simple as making ice can be.

00:43:18
Speaker 3: Teenagers out there, your questions to your science teacher could kick off a multi decade exploration and reveal deeper understandings of the natures of liquids and solids. So keep thinking, keep asking questions, and keep eating ice cream.

00:43:33
Speaker 1: We hope you enjoyed that. Thanks for joining us, See you next time.

00:43:41
Speaker 3: For more science and curiosity, come find us on social media where we answer questions and post videos. We're on Twitter, Discboard, Instant and now TikTok. Thanks for listening and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iheartradi, your app, Apple Podcasts, or wherever you listen to your favorite shows.

00:44:08
Speaker 4: Mm hmm.