Heat bag - how does it work?
December 22, 2005 9:52 PM   Subscribe

My employer has a knack for finding the oddest Christmas presents for the staff. This year: a heat bag. How does it work?

It's a plastic bag filled with a clear, heavy liquid. Inside the bag a metal chip is floating. Bend the chip and the liquid solidifies (it looks like candle wax) and gets warm. To use the bag again, it must be "recharged" by boiling it (the contents of the bag turns liquid again). For an non-scientist, this is pretty mysterious.
posted by Termite to Grab Bag (14 answers total)
My dad has one of these and loves it after a tennis game ... It's amazing. I don't know any science, but I understand the laws of thermodynamics, so I assume the energy from the boiling is somehow stored when the stuff turns solid and released by a chemical reaction with the metal. I hope someone else here can explain the details of this, because I've been wondering for years.
posted by luriete at 10:21 PM on December 22, 2005

From Here:

How Do these Heat Pads Work?

What is happening here is strange but can be understood if you think about water freezing. Water freezes at 32 degrees F or 0 degrees C. If you were to stick a thermometer in a cup of water and put the cup in the freezer, you would find that the temperature of the water falls to 32 degrees F and then hangs there until all of the water is completely frozen. Then the temperature of the solid water falls to the temperature of the freezer.

What if you could somehow supercool the water? That is, say that you could get the water's temperature to 10 degrees below freezing without it crystallizing into a solid (you can sometime do this using a very clean glass and distilled water so there are no points for the water to begin crystallizing). In this condition you can tap the glass and the temperature of the water will jump up to 32 degrees F/0 degrees C and the water will solidify quickly.

Our magic heat packs contain sodium acetate and water. It turns out that sodium acetate is very good at supercooling. It "freezes" at 130 degrees F or 54 degrees C, but it is happy to exist as a liquid at a much lower temperature and is extremely stable. Clicking the disk, however, has the ability to force a few molecules to flip to the solid state, and the rest of the liquid then rushes to solidify as well. The temperature of the solidifying liquid jumps up to 130 degrees F or 54 degrees C in the process.

When you boil the solid you melt it back to the liquid state. You have to completely melt every crystal or the liquid will quickly re-solidify. You can repeat this cycle thousands of times, just as you can freeze and melt water as many times as you like.

posted by stavrogin at 10:25 PM on December 22, 2005

I should have paid more attention in chemistry class, because googling this has proven surprisingly difficult. Victory: How do sodium-acetate heat pads work?
posted by Firas at 10:28 PM on December 22, 2005

Gah, obviously I should've searched for "heat pad" or "heat pack" rather than stuff like "physical reaction reverse crystalize exothermic metal alloy"...
posted by Firas at 10:31 PM on December 22, 2005

Supersaturated water. The click-disc is a convenience; you can also start it by flicking it or dropping it.
posted by five fresh fish at 11:49 PM on December 22, 2005

That is really, really cool. I might have to find one of these just to play with it.
posted by MadamM at 12:48 AM on December 23, 2005

Awesome! My sister was telling me about this today, and I thought I would have to ask Mefi to find out about it. I come here - lo and behold...!

Now, where to get one?
posted by tomble at 4:46 AM on December 23, 2005

I was on the hunt for these for my family last x-mas. It was difficult but I eventually found one brand by searching for the term "EZ Heat Reusable Hand Warmer." They really are fantastic ...
posted by hogweed at 7:32 AM on December 23, 2005

no-one has really tried to explain the physics, so here goes nothing....

"stuff" is made up of atoms or molecules. molecules are collections of atoms held together by chemical bonds. for this explanation we can ignore atoms and just assume everything is made of molecules.

so, starting again, "stuff" is made up of molecules. in a gas each molecule is flying off pretty much as it pleases. in a solid they are all stuck together in a mass.

often, when stuck together as a solid, they join up in regular patterns (just like if you pack a bunch of similar shaped things carefully together in a box). then you get crystals. sometimes they stick together pretty much at random.

anyway, the fact is that molecules like to stick together. so by default they want to be in a solid. the usual way to break them up is to heat them.

heating things, at the molecular level, means shaking them. seriously. the molecules in hot things are shaking around. if you heat something enough then the molecules fly off and you get a gas.

and you can measure how much molecules jiggle. it turns out that this is temperature. yep. temperature is just "jiggliness".

so far i've ignored liquid. for some reason that i don't completely understand (ok, that i don't understand at all), there's a kind of compromise state that often occurs, where the molecules jiggle around so much they move relative to each other, but they stay close.

so typically, when you heat something up, starting with a solid, the molecules start in a fixed place, then start vibrating in place, then start to slip past one another, then move around freely, but stay in contact with each other (a liquid) and finally fly off at random (a gas).

imagine a whole pile of ping-pong balls in a box. when still, they sit in a regular pattern. if you you move the box a bit they jiggle around. move the box like crazy and they fly all over the room.

and note how temperature being jiggliness makes sense. nearly always you have to heat up a solid to make a liquid (melting it). and you have to heat up a liquid to make a gas (boiling it). so jiggliness goes up as temperature goes up, just like you would expect.

ok, so that's the basics of solid/liquid/gas (and temperature), but there's more to it than that. when i said that the molecules "like" to stick together in a solid, you can think of it a bit like magnets. there's a real "clunk" as they stick together. and to pull them apart (to get back to a liquid, say), you need to put some effort into pulling those molecules apart from each other.

now this isn't magnetism. but it's the same kind of thing. and it means a certain amount of energy (effort) is needed to go from solid to liquid. this effort isn't reflected in how much the molecules are moving - it's extra effort.

so do a little thought experiment. imagine two magnets sitting near each other. they're still. now you nudge them just a tiny bit closer. if you're at just the right distance, they'll "notice" each other and fly together. and you'll hear a "click", and perhaps they'll roll along the table a bit (if they're round).

think about that. you started off with just a tiny nudge of the magnets and you ended up with them flying around, going "click" and perhaps rolling.

in other words, when they stuck together you got some energy out, that ended up making them "jiggle". in molecular terms, when the molecules stick together to form a solid, they actually jiggle a bit more.

now translate that back into temperature. what we just said is that when molecules form a solid, the temperature goes up! and that's what is happening in this bag.


you can think about this going backwards, too. a while back we had a thread about cooling drinks, and people were surprised to learn that ice cubes cool well because heat is "absorbed" as they melt. in fact, that's just the same as we see here, but backwards.

with ice cubes, energy is used up to pull the molecules apart. just as you need to put some energy into pulling magnets apart. and that energy has to come from somewhere - it comes from the drink you're cooling, which means that the molecules in the drink jiggle a bit less. and so the drink's temperature has gone down.

imagine it this way: imagine looking at an ice cube in a drink using a really powerful microscope. you can see the molecules in the drink. moving around. and you can see the molecules in the ice cube, all stuck together. now a particularly excited molecule in the drink goes crashing into the ice cube. there's an almighty crash and it knocks off a molecule from the ice cube. that almost brings the drink molecule to a halt. it's jiggling a lot less (it's temperature has gone down), but the ice cube is now one molecule smaller - the broken off molecule that was part of the ice cube is now "melted away" as water.


but that still doesn't explain the "magic" about this thing going solid when you click the disk. that's actually pretty strange, and harder to describe. in fact, i don't think i can, because i don't understand it well myself.

but what's happened is that the stupid molecules have cooled down - are jiggling less and less - and should have got to the point where they stick together as a solid. but for some reason, they haven't.

typically this is because (and this sounds stupid, but is more or less straight science) none of them wants to make the first move. so you end up with the equivalent of a lot of magnets all way closer than they should be, but none of them "wanting" to be the first to stick together. aw, shucks.

clicking the little disk, or hitting the bag, or whatever, knocks a few together whether they want it or not and - KERBLAM! they all stick together. and, as you can imagine if you think of a load of magnets slamming together, everything starts to vibrate/jiggle, and, of course (you know this by now) that means that the temperature goes up...

and now i really have to go out and get an xmas pressie!
posted by andrew cooke at 8:23 AM on December 23, 2005

One warning: I ruined one of these by trying to microwave it to reset its heating abilities. Strongly suggest you follow the directions and boil it.
posted by jellicle at 10:18 AM on December 23, 2005

I ruined one by dropping it when it was frozen solid after a day of skiing. The vinyl cracked open. It was a sad event.

You can pick 'em up or order online at MEC or Lee Valley Tools.
posted by five fresh fish at 1:18 PM on December 23, 2005

These things should be carried at any outdoors supply or anywhere that carries camping goods. I used to get them in the Boy Scouts for winter campouts. Winter. Campouts. What was I thinking???
posted by ScottMorris at 4:49 PM on December 23, 2005

Andrew: heh, I've never heard molecular state during phase change described like a big seductive dance before. Props for the description.
posted by Firas at 5:12 PM on December 23, 2005

I bought one of these for my girlfriend yesterday. Now I can bore her and her family with an explanation of how it works, and preserve my reputation for knowing everything. Thank you, Metafilter.
posted by Hogshead at 6:31 AM on December 24, 2005

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