chemical hand warmers

captain swoop · #1 (**permalink**) 12-January-2006, 05:01 PM

I have a couple of chemical hand warmers I use when out mountain biking. they are clear, flexible plastic 'bubbles' filled with a liquid. When the little metal 'widget' inside is flexed so it 'clicks' the liquid starts to turn into a solid and gets very hot. Simmering it in a pan of boilin water returns it to the liquid state.

What is the chemical inside the bubble and how does it work?

Grey · #2 (**permalink**) 12-January-2006, 05:28 PM

Quote:

Originally Posted by captain swoop

What is the chemical inside the bubble and how does it work?

Apparently sodium acetate and water. They aren't too clear on the details, but from the clue that it's a crystallizing reaction, and that when boiling it you have to make sure that all the crystals are re-dssolved before letting it cool down, I can make a good guess. First, they picked something that would have an exothermic crystallization reaction, so that's how it produces heat. It's a supersaturated solution, and so it's ready to begin crystallizing at any moment. But any reaction like that needs some impurity, or rough spot, or something to act as a site to start crystallizing (this is the same reason that raindrops and snowflakes form around dust specks and the like, or you can make rock candy by dipping sticks in a supersaturated sugar solution). So pressing the button must do something that introduces a crystallization site, but without an example, I can't think of what.

However, I think I may need to get some of those. They look pretty cool!

[edit to add]
It looks like I was more or less correct. Here's some more information, and here's a patent on the activator (or at least a similar one).

captain swoop · #3 (**permalink**) 13-January-2006, 09:43 AM

Thanks, that's a perfect reply.

Grey · #4 (**permalink**) 13-January-2006, 02:01 PM

No trouble. I'm impressed with the technique, and I'm happy to hear about them. I spend time outdoors in northern Michigan for work, and that looks like a perfect item to have.

captain swoop · #5 (**permalink**) 13-January-2006, 03:14 PM

Heat when u need it, usualy at the top of the moors before I start the run down again.

trinitree88 · #6 (**permalink**) 15-January-2006, 06:46 PM

That's an interesting little invention. Evidently the flexible clicker breaks off a tiny particle of garnet or silica which is sufficient to initialize the crytallization of the supercooled liquid.Neat!
Used to use sodium thiosulphate for the same effect in class. Using a new test tube helped prep the solution. Tiny scratches left on the interior surface of old, scrubbed tubes often initiated crystallization as you tried to chill them in ice water.
For fun with munchkins (little people) try a glass of beer or ginger ale. Pour slowly. Take a stainless steel fork. Scratch their initials or a happy face on the walls of the interior of the glass, and you'll nucleate bubble formation on your design on the microsctratches.

Same effect causes bubbles to seem to appear at the same impurity on your beer glass wall. Donald Howard Glaser noticed that in his beer...developed the hydrogen bubble chamber....got the Nobel Prize for it.

......and to think they don't think scientists get anything done while having a few cold beers.!
Hydrogen held under pressure as a liquid, has the pressure reduced by an oscillating piston....particles stream through the liquid...nucleation occurs on the ionization trail...a strobe flashes...takes a picture...the piston repressurizes the chamber...the bubble trail disappears. They do it again. See Gargamelle.

captain swoop · #7 (**permalink**) 17-January-2006, 10:40 AM

Carling Black Label is served in the UK in 'Spot On' Glasses that keep in 'lively' until the end of the pint. they work by having a rough patch on the bottom of the glass forming the words 'Spot On' Seems people think the more it 'fizzes' the fresher it is.

captain swoop · #8 (**permalink**) 17-January-2006, 10:41 AM

So why when the hand warmer is boiled and returns to liquid doesn't it just re crystalize on the partical broken off when it was activated?

Grey · #9 (**permalink**) 17-January-2006, 02:02 PM

Quote:

Originally Posted by captain swoop

So why when the hand warmer is boiled and returns to liquid doesn't it just re crystalize on the partical broken off when it was activated?

I can't think of a reason for that.

trinitree88 · #10 (**permalink**) 18-January-2006, 05:40 PM

Quote:

Originally Posted by Grey

I can't think of a reason for that.

Good Question. Not sure this is right...but sometimes supersaturated solutions will crystallize out if you tap gently on the external surface of a test tube, or expose them to mechanical shock, like banging them down on the desktop...kind of like nitroglycerin. The clicker that initiates the crystallization in the device supplies two things...nucleation sites, and a slight mechanical vibration simultaneously. When recooling the saturated, already used once solution, there is an absence of vibration...perhaps that's the patent, too.

edit: Aha! Garnet is also piezoelectric. Outside the clicker there is no effect on cooling. A little jolt to some blasting gelatin produces quick results, though.

I once made a tiny quantity of shock sensitive explosive, and was drying it on a watch glass. A student using the pencil sharpener during a quiz, set it off with the grinding, from ~ 10 feet away. The custodian found several specks while sweeping the floor. Never made it again. It was also photosensitive. Training teacher never told me that part of the recipe.

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