Science that’ll warm your hands

After trying the hand-warmer my friend gave me for Christmas I thought, “cool, I wonder how this works?”

Here’s the hand-warmer in action:

So what’s going on?

The hand-warmer heats up when you bend the metal disk that’s inside the pouch. Bending the disk causes the liquid inside the hand-warmer to solidify. This change results in an exothermic reaction, meaning heat is released as the liquid solidifies. The heat released is the warmth you feel.

The key to the hand warmer is actually the solution itself, rather than the metal disk. The solution is a mixture of sodium acetate and water. The sodium acetate solution is a supercooled liquid, so it can stay liquid at temperatures below its freezing point (58˚C). The supercooled liquid is metastable or “sorta-stable”, so it needs is a trigger for the spontaneous change from the liquid state to a solid state.

The metal disk just triggers the sodium acetate solution to solidify. It can be any piece of metal as long as it is bendable. Stainless steel is often used for this job.

If you look at the video again, the solution begins to solidify radially outwards from the metal disk. There are slits in the metal disk which trap solid particles of sodium acetate when the hand warmer is being “recharged” (melting from solid back to liquid state).

Here’s an image of the metal disk up-close

These stored particles are “seed crystals” or “starter particles” that initiate the solution to solidify. When the metal disk is bent, the seed crystals trigger the solidification process and the solution starts to harden. But this also means the hand-warmer would stop working if no seed crystals get stored in the metal disk during the recharging step.

Product review
It was very easy to active the hand-warmer. The hand-warmer warmed up almost instantly after I bent the disk. It was decently warm as I could still feel the warmth on my thigh through my jeans. The heat from the hand-warmer lasted for about 15-20 minutes. So I think it’d be warm enough for my coat pocket or in the car for temporary uses.

However, it was a bit trickier “recharging” the hand-warmer back to its original form. Following the instructions, I wrapped the hand-warmer in a towel and immersed it in a pot of boiling water for 15 minutes. Maybe because I turned off the stove after the water came to a boil, but some sodium acetate crystals were left in the hand-warmer and the hand-warmer eventually re-solidified. I tried it again with the water continuously boiling and this time the sodium acetate crystals fully melted.

Overall, the hand-warmer was easy to use and is small enough to carry around. It’s also reusable which is nice. I’m sure the hand-warmer will eventually wear out and leak, it is in boiling water a lot. But the hand-warmer is non-toxic since it’s just sodium acetate, a food additive, mixed with water.

So thank you Christina for an awesome gift that’ll keep me warm this winter!

—–
Sandnes, B. (2008). The physics and the chemistry of the heat pad American Journal of Physics, 76 (6) DOI: 10.1119/1.2830533

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13 responses to “Science that’ll warm your hands

  1. Thanks for clearing up the science behind the warmers, I was afraid I was only going to find spiritual hot crystal massages when searching for this. 😛

  2. I also have a handwarmer with sodium acetate and a metal disc. When I heated the hand warmer recharge it and to dissolve the sodium acetate, I then let it sit to cool to room temp. During the cooling process a small amount of the sodium acetate crystalized, even though I had fully dissolved it. Even with the partial crystallization, it fully crystalized and released heat after I snapped the disc. However, I couldn’t get it to crystalize without snapping the disc. I thought agitating the crystals that formed at room temp would be enough to crystalize the entire hand warmer. Do you know what makes the crystals trapped in the disc different than the crystals that form while the hand warmer cools?

  3. There is no difference, basically the sodium acetate that sticks in the slits of the disc have to be released for the chemical reaction to take place is all. As the writer stated if the sodium acetate in the the disc some how melts during the recharging process than the heating pad will no longer work.

  4. There are no crystals stored in the metal disk.

    Sodium Acetate is already well below its freezing point, which is like you said, somewhere around 130 degrees F, or 57 degrees C

    It’s like when you have water that’s already at it’s freezing point, but not yet cold enough to spontaneously start the crystallization process, you you flick the bottle, and it starts a chain-reaction.

    It’s the same with sodium acetate, only sodium acetate is a far more stable compound that needs a much larger difference between its freezing point and the point where it will start spontaneously crystallizing on its own.

    I don’t know what the mechanism is, but something about the stainless steal act of flexing moving against itself is that starts the crystallization process. Perhaps the heating and cooling of compression and relaxing that compression is what causes it. Perhaps the release of the compression cools a few molecules of the metal well below what sodium acetate needs to crystallize, causing a few adjacent molecules of sodium acetate to crystallize, when cascades into an domino effect that eventually crystallizes the entire solution. I don’t know if this is the case, it’s just a guess. But thinking that somehow crystals avoid melting that stick to the metal disc seems extremely unlikely considering this property of sodium acetate not freezing till some time before it’s freezing point is a common property with liquids.

  5. There are no crystals stored in the metal disk.

    Sodium Acetate is already well below its freezing point, which is like you said, somewhere around 130 degrees F, or 57 degrees C.

    It’s like when you have water that’s already at its freezing point, but not yet cold enough to spontaneously start the crystallization process, you then flick the bottle, and it starts a chain-reaction.

    It’s the same with sodium acetate, only sodium acetate is a far more stable compound that needs a much larger difference between its freezing point and the point where it will start spontaneously crystallizing on its own.

    I don’t know what the mechanism is, but something about the stainless steal act of flexing or moving against itself is what starts the crystallization process. Perhaps the heating and cooling of compression and relaxing is what causes it. Perhaps the release of the compression cools a few molecules of the metal well below what sodium acetate needs to crystallize for a brief moment causing a few adjacent molecules of sodium acetate to crystallize that then cascades into a domino effect that eventually crystallizes the entire solution. I don’t know if this is the case, this is just a guess. But thinking that somehow crystals avoid melting that stick to the metal disc seems extremely unlikely considering this property of sodium acetate not freezing till some time after the temperature has lowered below its freezing point is a common property with liquids.

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