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.
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