Reusable handwarmers that get hot by freezing

I want to show you something really cool. Eh, well, actually it’s hot. But.. eh well it’s not yet. Here. This thing. It’s a little pouch filled with a mysterious blue liquid and a little metal disc. When I squeeze that disc… something weird starts happening. There’s like a front of light blue crystal material, almost like ice, emanating from that disc. And now… this thing is hot! What‽ Yes, this is one of my favorite objects I’ve ever encountered. This is a reusable hand warmer. Reusable? Indeed

. I’ll show you how in a bit. But first - I’ll be frank and would not suggest you rush out and buy some of these because honestly they’re not that great at their job and the reusability aspect of them is both tedious and energy-intensive. But the physical process by which these work is quite fascinating, and they do have at least one advantage compared to other options. Speaking of other options, here’s another option! This is the traditional not-reusable one. These products have been around for

a long while and are quite inexpensive. And usually have a pun-adjacent name like Hothands or Lil’ Hotties. I approve! To use them, you simply tear open the plastic packaging which exposes the warmer to air, and after a few minutes it starts producing heat. How do they work? Well, I’m afraid I’m a little rusty… Oh wait, right, it’s rust! How ironic. These contain a mixture of iron and other support chemicals which help the iron quickly oxidize when exposed to oxygen in the air. This redox react

ion is exothermic so it gives off heat. When you use these, you literally hold onto a little pouch of powders, one of which - the iron - quickly rusts, creating heat. Eventually all the iron is oxidized and at that point the warmer is spent and can be disposed of. These are really quite interesting on their own, but you can’t see the reaction occurring because it happens in this opaque pouch which it clearly tells you not to open, puncture, or tear. Fortunately I have scissors and a propensity t

o ignore safety instructions. Um, don’t do this at home, always follow instructions I’m doing it so you don’t have to... but here we go! What’s in here is a very deep black powder. Activated carbon, aka activated charcoal is responsible for the color. That charcoal isn’t part of the reaction, instead it’s used as a source of thermal mass to help evenly distribute the heat produced. When it’s all exposed to air like this the reaction happens quite a bit more quickly than it does in the warmer wit

h its semi-permeable membrane. You can actually see some steam coming from it - one of the support materials in here is there specifically to provide a source of water for the reaction. It can be any number of things, but one common material is vermiculite, a silicon-based mineral that contains water. You can actually see some water droplets forming on the bottom of this container, and they are coming from this powder and not the ambient air. We know that because this is getting hot, thus ambien

t moisture cannot condense on it. Because these pouches are filled with nothing but commonly-available substances they’re quite inexpensive. And, the materials in here are all fairly inert, meaning that no special consideration needs to be given to their disposal. It’s basically just a packet of rocks and rust when it’s used up, making it Mostly Harmless. Although the chemical makeup of these does vary from brand to brand and even between offerings within the same brand. The reaction is very sim

ple, but can be tuned to happen more quickly for faster heat, or more slowly for longer-lasting heat, depending on various factors. So the chemical composition of these is usually quite similar but by no means identical. But of course, they are one and done. Once you open it there’s no going back. It’s hard to notice through all the activated carbon but you can definitely see a difference in color between spent and fresh hand warmer contents. The spent one looks a bit browner, thanks to the iron

now being iron oxide - Rust. Macro shots help reveal the oxide particles. Once all the iron’s been oxidized, there’s nothing you can do but throw it away. These, on the other hand, work through a reaction that’s entirely reversible. And that’s because… it’s not a reaction, exactly. Instead it’s our friend latent heat popping up again! Sometimes it prefers you call it enthalpy. Combine it with another fun phenomenon, dotiouscalifragilisticsupersaturation, and you’ve got a phase-change material c

apable of delivering heat on demand. What’s in this pouch is a solution of sodium acetate trihydrate and water. Sodium acetate trihydrate is a crystalline compound with a melting point of 58°C or about 136°F. Now believe it or not, I don’t set my thermostat that high. It’s currently about 18°C in this room, something like 65° F, and yet this remains a liquid. It’s a fairly viscous liquid, but liquid nonetheless. That’s because this solution is both supercooled and supersaturated. It contains mor

e dissolved sodium acetate than is ordinarily possible at this temperature, and more importantly the solution is below the freezing point of sodium acetate. This supercooling can happen to many substances- you’ve probably seen one of those viral videos where a water bottle is frozen but… not, and then jostling it causes it to nearly instantly freeze. In that case, the water remained a liquid below its freezing point until it was disturbed. It was supercooled. That’s more or less what’s going on

right here, except rather than water, the substance in question is the sodium acetate dissolved in it. Sodium acetate is useful in this situation for two reasons: one it has a rather high melting point, and two in solution it’s remarkably easy to supercool. Once it’s all melted and dissolved in a solution, that solution remains liquid well below its freezing point, and that’s what’s happening here. This solution is what’s known as metastable. It’s in a sort of in-between state; not really froz

en, obviously but not really melted, either. The sodium acetate is colder than its melting point yet still molten. And here’s the important thing: when it does freeze, it will release heat energy. That’s our friend latent heat showing up again. Just as there’s a latent heat of vaporization, the energy needed to get a substance to move from the liquid phase to the gaseous phase, there’s also a latent heat of fusion, the energy needed to get a solid substance to melt (fusion is another word for me

lting, see). Materials in the liquid phase contain more energy than they do in the solid phase, and since this is currently a liquid, yet it’s colder than the freezing point of sodium acetate, it’s got more energy in it than it really should, in a manner of speaking. It managed to cool below its freezing point without actually freezing, which means it hasn’t released its latent heat of fusion. The energy it took to get it to melt is still here because it’s still molten. Pressing on or bending th

e metal disc, though, will fix that. Remember it’s metastable, or stable but barely. When the tiny cuts in the metal disc scrape past each other, they create a disturbance great enough to cause a crystal of solid sodium acetate to form in the solution. This creates a nucleation site, sometimes this is called seeding, and this breaks the metastable state. A chain reaction occurs and a front of crystals quickly forms radiating away from the activation site. This is the sodium acetate precipitating

out of the solution and solidifying. In moments, the entire thing is frozen and, counterintuitively, hot. As the sodium acetate falls out of the solution and crystals form, in other words as it freezes, it releases the stored latent heat of fusion and heats up right about to its freezing point. It can be a little mind-bending to think of it as freezing while also getting hot, but that’s precisely what happens. It's just… the freezing point of sodium acetate isn’t cold like ice. As a matter of f

act there was a weird viral thing going around some years ago called "hot ice" - that was this exact substance, though not in a pouch like you see here. And it generates heat extremely quickly. Take a look in a thermal camera and you’ll see that the instant it solidifies, it’s hot. It doesn’t quite reach the freezing point of sodium acetate trihydrate, but it’s pretty close. Just look at my hands compared to the hand warmer - they’re quite a bit colder. Luckily, it doesn’t all freeze immediately

. After the crystals have spread out, you can knead this thing and you’ll discover that it’s pretty soft. It feels almost like lightly packed snow, but warm. The sensible heat energy released as the initial crystallization occurred brings the entire volume up to the freezing point (which again is anything but what we would call cold) and some liquid solution hangs around. The latent heat it contains is slowly released as more acetate falls out of solution and continues freezing. Over time these

things get stiffer and stiffer, and by the time they’re all used up it’s quite firm and inflexible. Every single time I’ve used one of these it’s a fascinating experience. The idea that something can just spontaneously release energy by freezing is wild! And it’s a delightful sight to behold. Something about seeing the crystals propagate like this is just amazing. The most recent set that I got came with a couple of much larger heating pads, and it’s mesmerizing to see this occur. Just look at t

hat! Seriously, look at that! Wowzers! One thing I’ve learned is that these aren’t all created equal. I was trying to get an interdimensional portal going so I got these ones, too. So far no luck with the portal but the orange ones are just… better. They produce heat for longer, and I suspect the reason they do is that there’s more sodium acetate in this solution than in the blue ones. When these are spent, they’re incredibly solid - practically rock-like. The blue ones, though, remain a bit sof

t and somewhat flexible, even once at room temperature. I suspect the blue ones have more water in them than the orange ones, which keeps them more flexible at the expense of runtime. So how are these reusable? Well, you may have already guessed. You simply need to melt them again. And to do that, you boil them. Like a weird egg. Bring water to a boil, add a used pad, and keep the water simmering until the entire thing has melted. I usually go for a few minutes more because if you don’t get ever

y last crystal to melt and dissolve, the remaining crystals will prevent it from supersaturating and it will simply freeze again as it cools. Ya know, like things normally do. But, assuming it did melt completely, as it cools down it’ll sneakily slip right past its freezing point without releasing its latent heat of fusion. It’ll save that for later, providing an intriguing and effective source of portable heat. By the way, in normal circumstances, for instance with plain water, when it cools to

the freezing point it will effectively pause getting colder as it freezes. No matter how cold the temperature is around it, a given quantity of water will just stay at the freezing point until it’s all frozen. Only then, after it's released all the latent heat of fusion, can it get below its freezing point. That’s how latent heat normally behaves, although the rules are bent in all sorts of circumstances. The biggest trouble with these is that, well, they don’t work all that well. Initially the

y work fantastically! It is near-instant heat at its full intensity, which simply does not happen with the rusty ones, or even an electric heating pad for that matter. But their heat output starts falling rapidly after only about 10 minutes. In fact, there’s a significant measurable difference after only five minutes. You’ll get noticeable heat out of them for maybe a half hour but by then it’s already pretty tepid, and also quite stiff. [creaky sounds as warmer is flexed] Now as I said this doe

s seem to depend somewhat on the warmer’s acetate concentration, but more important than that is its overall mass and how you use it. These tiny ones came with the blue set. Shocker. And they’re… next to useless. You get mere minutes of heat before they’re used up. There’s just too much surface area for the volume of solution inside. The larger orange pads, though, last the longest of any of these. Again, though, that comes at the expense of it being a literal rock by the time it’s all over. And

the other greatest factor is how you use it. If you leave it exposed to air, say if you’re simply holding on to it with bare hands, it will lose its heat rapidly and whatever liquid acetate remains will freeze faster. Therefore it gets used up more quickly. If it’s somewhat insulated, though, this is delayed and it will last longer. The trouble there is, especially in the beginning, these are borderline uncomfortably hot against bare skin. And of course, well, eventually you’ll have a weird roc

k in your gloves, if you could even fit it in there. Lastly, there’s the energy needed to melt them for re-use. This is extremely variable, mind you, but they need a fairly long time in near-boiling water to be reset. It takes a fair bit of energy to heat water, especially the quantity needed to reset these large packs. And when you get such poor runtime out of them, it seems almost a futile exercise, especially when you consider the miniscule fraction of the energy used to reactivate these that

they are actually able to capture and store for later release. Therefore I consider these to be more of a novelty than a truly useful object. That said, I will give them one bit of genuine praise. The heat they generate is distinctly different from that of these disposable things. Being a large, dense, solid hot thing feels more therapeutic to me, and I think for something like applying heat for pain relief these are pretty decent, if short-lived. Still, if that’s a thing you regularly need… I

would advise getting an electric heating pad. These are neat, but tedious and boiling them gets old fast. They’re also not cheap. This set of 8 hand warmers was $25. That can buy a LOT of disposable hand warmers, in fact these were 74 cents a pair and can be obtained for even less when bought in bulk. Again - this is just some iron and various rocks. I find it pretty hard to justify these reusable gel packs in a lot of circumstances, honestly, as they fit an incredibly small niche and require f

ar more energy and effort to re-use than I think makes sense, at least to me. But, they are very useful at demonstrating the concept of latent heat. There’s hidden heat energy in here because substances in a higher phase of matter have more energy in them thanks to the difference in intermolecular forces between the phases. It’s the process of adding that extra energy that gets stuff to melt or vaporize. When you come back down the ladder of phases, that energy gets released as substances conden

se or solidify. In my eyes, there’s nothing that demonstrates this quite so effectively as these things. Take a liquid at room temperature, see it instantly fall from the liquid phase to the solid, and feel the heat it releases as it does so. It’s a remarkably tangible demonstration of latent heat, and yes I know you can’t feel this, but hopefully you see the beauty of this. Ya know where else we find exploitation of latent heat? Why, refrigeration and air conditioning, of course! The same proce

ss of absorbing heat in one place and expelling it in another is responsible for how we cool our homes and keep our food preserved. Yeah, OK, it’s nothing like boiling a thing and storing heat energy in its intermolecular bonds to be used for later but the principle of latent heat, that is the energy absorbed and released as a substance changes phases, is exactly what makes these devices work. And in the next video (weather permitting - it might be the video after the next) we’ll talk about heat

pumps - using that same principle to make a space warmer. We have heat pumps everywhere - air conditioners and refrigerators are heat pumps! But only recently, particularly in the US, have we been learning to use that same concept in reverse to provide space heating. Learning to move heat energy rather than produce it has allowed us to warm our homes more efficiently than ever before, and soon we’ll learn how as well as and why it is undoubtedly the heating technology of the future. ♫ enthalpi

cally smooth jazz ♫ When I squeeze that disc… nothing happens. When I squeeze that disc… again, nothing happens! [frustrated laughter] Sodium acetate is useful in this situations… weooow ...cause a crystal of soble… [weird noise] Or stable, but barely [weird throat clear] ...precipitating out of the solution and solidifying in moments. The entire - No! [laughs] Gah-ha! And some liquid solution remains hangs around. Dang! You’ll get noticeable heat output for them... Out.. out of them. ♫ unprov

oked humming ♫ Incredibly small niche and require… as… Oops! Well that sure was a different kind of freezing, wasn't it! Freezing but getting hot? How strange and imaginative! Sounds like something out of Alice in Wonderland but it's not! LATENT HEAT STRIKES AGAIN