Reusable handwarmers that get hot by freezing

I want to show you something really cool. Hey, well, it's actually hot. But... well, it's not yet. Here. This thing. It is a small bag filled with a mysterious blue liquid and a small metal disc. When I press that record... something strange starts to happen. There is like a front of light blue crystalline material, almost like ice, emanating from that disk. And now… this is in fashion! What‽ Yes, this is one of my favorite items I've found. This is a

reusable

hand warmer. Reusable? Indeed. I'll show you how in a moment. But first, I'll be honest and wouldn't suggest you rush out and buy some of these because honestly, they're not that good at their job and the reuse aspect of them is tedious and energy-consuming.

But the physical process by which these work is quite fascinating and they have at least one advantage compared to other options. Speaking of other options, here's another option! This is the traditional non-

reusable

one. These products have been around for a long time and are quite affordable. And they usually have a pun-adjacent name like Hothands or Lil’ Hotties. I approve it! To use them, you simply open the plastic packaging which exposes the heater to air and after a few minutes it begins to produce heat. How do they work? Well, I'm afraid I'm a little rusty... Oh wait, sure, it's rust!

How ironic. These contain a mixture of iron and other supporting chemicals that help the iron to oxidize quickly when exposed to oxygen in the air. This redox reaction is exothermic so it releases heat. When you use them, you are literally holding on to a small bag of powders, one of which, iron, oxidizes quickly and generates heat. Over time, all the iron rusts and at that point the heater wears out and can be discarded. These are actually quite interesting on their own, but you can't see the reaction that happens because it happens in this opaque bag that clearly tells you not to open it, puncture it, or tear it.

Luckily I have scissors and a propensity to ignore safety instructions. Um, don't do this at home, always follow the instructions. I do it so you don't have to... but here we go! What is here is a very intense black powder. Activated carbon, also known as activated charcoal, is responsible for the color. That carbon is not part of the reaction, but is used as a source of thermal mass to help evenly distribute the heat produced. When everything is exposed to air like this, the reaction happens a little faster than in the heater with its semi-permeable membrane. In fact, you can see some steam coming out of it - one of the support materials here is there specifically to provide a source of water for the reaction.

It can be any number of things, but a common material is vermiculite, a silicon-based mineral that contains water. In fact, you can see some water droplets forming at the bottom of this container, and they come from this dust and not from the ambient air. We know that because it is heating up, ambient moisture cannot condense on it. Because these bags are filled with only commonly available substances, they are quite inexpensive. And all the materials here are quite inert, which means that you don't need to pay special attention to their disposal. It's basically just a pack of rocks and rust when used up, making it Mostly Harmless.

Although the chemical composition of these varies from one brand to another and even between offers within the same brand. The reaction is very simple, but can be tuned to occur more quickly for faster heat, or more slowly for longer lasting heat, depending on several factors. Therefore, their chemical composition is usually quite similar, but in no case identical. But of course, they are one and done. Once you open it there is no turning back. It's hard to tell through all the activated carbon, but you can definitely see a color difference between the contents of the used hand warmer and the new one.

The worn one looks a little browner, thanks to the fact that the iron is now iron oxide: rust. Macro shots help reveal the rust particles. Once all the iron has rusted, all that remains is to throw it away. These, on the other hand, work through a reaction that is completely reversible. And that's because... it's not a reaction exactly. Instead, it's our friend's latent heat that reappears! Sometimes he prefers you call it enthalpy. Combine it with another fun phenomenon, califragilistic supersaturation, and you have a phase change material capable of delivering heat on demand. What's in this bag is a solution of sodium acetate trihydrate and water.

Sodium acetate trihydrate is a crystalline compound with a melting point of 58°C or approximately 136°F. Believe it or not, I don't set my thermostat that high. It is currently about 18°C ​​in this room, something like 65°F, and yet it is still a liquid. It is a fairly viscous liquid, but liquid nonetheless. This is because this solution is supercooled and supersaturated. It contains more dissolved sodium acetate than is normally 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 freezes but... no, and then when you push it, it freezes almost instantly.

In that case, the water remained liquid below its

freezing

point until it was disturbed. It was supercooled. That's pretty much what's going on here, except instead of water, the substance in question is sodium acetate dissolved in it. Sodium acetate is useful in this situation for two reasons: one, it has a fairly high melting point, and two, in solution, it is remarkably easy to supercool. Once everything melts and dissolves into a solution, that solution remains liquid well below its freezing point, and that's what's happening here. This solution is what is known as metastable. It's in a kind of intermediate state;

Not really frozen, obviously, but not really melted either. Sodium acetate is colder than its melting point but is still molten. And here's the important thing: when it freezes, it will release thermal energy. That's the latent heat of our friend appearing again. Just as there is a latent heat of vaporization, the energy needed to make a substance go from the liquid phase to the gaseous phase, there is also a latent heat of fusion, the energy needed to make a solid substance melt (fusion is another word). ). to melt, see). Materials in the liquid phase contain more energy than in the solid phase, and since it is currently a liquid, but colder than the freezing point of sodium acetate, it has more energy than it really should, in a form of speech.

It managed to cool below its freezing point without freezing, meaning it has not released its latent heat of fusion. The energy that was needed to melt it is still here because it is still molten. However, pressing or bending the metal disc will fix the problem. Remember that it is metastable or stable but just barely. When the small cuts in the metal disc rub against each other, they create a disturbance large enough to cause a solid sodium acetate crystal to form in the solution. This creates a nucleation site, sometimes called seeding, and breaks the metastable state. A chain reaction occurs and a front of crystals quickly forms and radiates from the activation site.

This is sodium acetate that precipitates out of solution and solidifies. In a few moments, everything freezes and, counterintuitively, warms up. When sodium acetate falls out of solution and forms crystals, that is, when it freezes, it releases the stored latent heat of fusion and heats up to almost its freezing point. It can be a little mind-boggling to think that it's cold and hot at the same time, but that's precisely what happens. It's just... the freezing point of sodium acetate is not as cold as ice. In fact, a few years ago there was a strange viral thing called "hot ice"; that was exactly this substance, although not in a bag like the one seen here.

And it generates heat extremely quickly. Take a look at a thermal camera and you'll see that the instant it solidifies, it's hot. It doesn't reach the freezing point of sodium acetate trihydrate, but it's pretty close. Just look at my hands compared to the hand warmer: they are quite a bit colder. Luckily, not everything freezes immediately. Once the crystals have spread, you can knead it and you will find that it is quite soft. It feels almost like lightly compacted snow, but warm. The sensible heat energy released when the initial crystallization occurred brings the entire volume to the freezing point (which again is anything but what we would call cold) and some liquid solution remains.

The latent heat it contains is slowly released as more acetate falls from the solution and continues to freeze. Over time, these things become increasingly rigid, and when used up, they are quite firm and inflexible. Every time I have used one of these it is a fascinating experience. The idea that something can spontaneously release energy when frozen is crazy! And it is a delightful sight to behold. Something about watching crystals spread like this is just amazing. The most recent set I purchased came with a much larger pair of heating pads, and it's fascinating to see this happen.

Just look at that! Seriously, look at that! Wow! One thing I have learned is that not everyone is the same. I was trying to open an interdimensional portal, so I got these too. We haven't had any luck with the portal so far, but the orange ones are just... better. They produce heat for longer and I suspect the reason they do so is because there is more sodium acetate in this solution than in the blue ones. When worn out, they are incredibly solid, practically rock-solid. The blue ones, however, are still a little soft and somewhat pliable, even once at room temperature.

I suspect the blues have more water than the oranges, which keeps them more flexible at the expense of running time. So how are they reusable? Well, you may have already guessed it. You simply need to melt them again. And for this you boil them. Like a strange egg. Boil the water, add a used pad and keep the water simmering until completely melted. I usually spend a few extra minutes because if you don't get every last crystal to melt and dissolve, the remaining crystals will prevent it from becoming oversaturated and it will simply freeze again as it cools.

You know, how things usually are. But, assuming it were to melt completely, as it cools it will sneak past its freezing point without releasing its latent heat of fusion. You'll save it for later, providing an intriguing and effective source of portable heat. By the way, under normal circumstances, for example with running water, when it cools to the freezing point, it will effectively stop cooling as it freezes. No matter how cold the temperature is around you, a certain amount of water will remain at the freezing point until it is all frozen. Only then, after it has released all the latent heat of fusion, can it drop below its freezing point.

This is how latent heat normally behaves, although the rules are modified in all types of circumstances. The biggest problem with these is that, well, they don't work that well. At first they work fantastically! It's almost instant heat at its maximum intensity, which simply doesn't happen with rusty ones, or even with an electric heating pad. But its heat production begins to decline rapidly after only about 10 minutes. In fact, there is a significant, measurable difference after just five minutes. You'll get noticeable heat for maybe half an hour, but by then it will be quite warm and also quite stiff.

Now, as I said, this seems to depend to some extent on the acetate concentration of the heater, but more important than that is its total mass and how it is used. These little ones came with the blue outfit. Disgusting surprise. And they are... almost useless. You get just a few minutes of heat before they run out. There is simply too much surface area for the volume of solution inside. However, the larger orange pads last the longest of all. Once again, though, that comes at the expense of him being a literal rock when it's all over. And the other important factor is how you use it.

If you leave it exposed to the air, say if you just hold it with your bare hands, it will lose its heat quickly and the liquid acetate left behind will freeze faster. Therefore it runs out more quickly. However, if it is somewhat isolated, this is delayed and will last longer. The problem is that, especially at first, they are almost uncomfortably hot against bare skin. And of course, well, eventually you'll have a strange stone in your gloves, if you can even fit it there. Finally, there is the energy necessary to melt them and reuse them. This is extremelyvariable, yes, but they need a fairly long time in almost boiling water to recover.

It takes quite a bit of energy to heat water, especially the amount needed to reset these large packages. And when you get such a poor runtime out of them, it almost seems like an exercise in futility, especially when you consider the minuscule fraction of the energy used to reactivate them that they are actually able to capture and store for later release. That's why I consider them more of a novelty than a truly useful object. That being said, I will give them genuine praise. The heat they generate is clearly different from that of these disposable items. Being something big and dense and solid and hot seems more therapeutic to me, and I think for something like applying heat to relieve pain they are pretty decent, albeit short-lived.

Still, if that's something you need on a regular basis... I would recommend getting an electric heating pad. They are good, but tedious and boiling them gets old quickly. They are not cheap either. This set of 8 hand warmers was $25. That can buy a LOT of disposable hand warmers; In fact, they cost 74 cents a pair and can be had for even less if purchased in bulk. Again, this is just some iron and several rocks. Honestly, I find it quite difficult to justify these reusable gel packs in many circumstances, as they fit an incredibly small niche and require a lot more energy and effort to reuse than I think makes sense, at least for me.

But they are very useful to demonstrate the concept of latent heat. There is hidden thermal energy here because substances in a higher phase of matter have more energy thanks to the difference in intermolecular forces between the phases. It's the process of adding that extra energy that causes things to melt or vaporize. When you go back down the phase ladder, that energy is released as substances condense or solidify. In my opinion, there is nothing that demonstrates this as effectively as these things. Take a liquid at room temperature, watch 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 display of latent heat, and yes, I know you can't feel it, but I hope you see the beauty of it. Do you know where else we find latent heat exploitation? Of course, refrigeration and air conditioning! The same process of absorbing heat in one place and expelling it in another is responsible for how we cool our homes and preserve our food. Yes, okay, it's nothing like boiling something and storing thermal energy in its intermolecular bonds for later use, but the principle of latent heat, that is, the energy absorbed and released as a substance changes phase, is exactly what makes these devices work. .

And in the next video (if time allows, it could be the video after the next) we will talk about heat pumps, using the same principle to heat a space. We have heat pumps everywhere: air conditioners and refrigerators are heat pumps! But only recently, particularly in the United States, have we learned to use that same concept in reverse to provide space heating. Learning to move thermal energy rather than produce it has allowed us to heat our homes more efficiently than ever before, and we will soon learn how, as well as why, it is undoubtedly the heating technology of the future. ♫ enthalpically smooth jazz ♫ When I press that record... nothing happens.

When I press that record…again nothing happens! Sodium acetate is useful in these situations... weooow... because a sodium crystal... Or stable, but barely... precipitates from solution and solidifies in moments. All the - No! Ha ha! And there is some liquid solution left there. Damn! You'll get remarkable heat production for them... Off... off of them. ♫ unprovoked humming ♫ Incredibly small niche and requires... like... Oops! Well, it sure was a different kind of frostbite, right? Frozen but warming up? How strange and imaginative! It sounds like something out of Alice in Wonderland, but it's not! LATENT HEAT STRIKES AGAIN