Naked Science Forum

General Science => General Science => Topic started by: Seany on 09/07/2007 13:08:24

Title: How do pocket hand warmers work?
Post by: Seany on 09/07/2007 13:08:24

How do these pocket "handwarmers" work?

You put the handwarmer in boiling water, and let it boil..

Then after a while, the solid turns to a liquidy substance.

Then when you want to use it later.. All you do is find this metal thing inside, and bend it, make a click sound. Do that 3-4 times and it starts to turn solid and white!! Then the heat comes.

But how does that little metal thing obtain that energy? Shouldn't it gradually fade away? But even after a week since boiling, it still works.

Title: Re: How do pocket hand warmers work?
Post by: dentstudent on 09/07/2007 13:13:45

Hi Seany

This has been discussed a couple of times on the show. This first link is to the show where the kitchen science was about how they work.

http://www.thenakedscientists.com/HTML/podcasts/2003-2004/show/2006.11.19/

The text below is from an earlier posting....

I'm pretty sure that what they work with is sodium acetate. What you've got is a super-saturated solution of sodium acetate, and as this begins to crystallise, it gives off a constant heat of about 55 degrees centigrade, which is a nice comfortable temperature for something like a hand warmer. Now how do you get this to work? Well as you bend the metal, I think you create surfaces that are like the crystals that could be formed from the solution (these are nucleation sites). Once you've done this, you're really seeding the crystals, and so one forms and then another and another until it spreads throughout the bulk of the hand warmer. All the time it is giving off this heat. Eventually, the heat will stop because you've crystallised the whole thing. You can repeat the whole process by putting the used hand warmer in something like and oven, until the crystal is all melted again.

Title: Re: How do pocket hand warmers work?
Post by: Karen W. on 09/07/2007 13:16:03

Wow I have never seen anything like that!

Title: Re: How do pocket hand warmers work?
Post by: Seany on 09/07/2007 13:21:49

Quote from: dentstudent on 09/07/2007 13:13:45

Hi Seany

This has been discussed a couple of times on the show. This first link is to the show where the kitchen science was about how they work.

http://www.thenakedscientists.com/HTML/podcasts/2003-2004/show/2006.11.19/

The text below is from an earlier posting....

I'm pretty sure that what they work with is sodium acetate. What you've got is a super-saturated solution of sodium acetate, and as this begins to crystallise, it gives off a constant heat of about 55 degrees centigrade, which is a nice comfortable temperature for something like a hand warmer. Now how do you get this to work? Well as you bend the metal, I think you create surfaces that are like the crystals that could be formed from the solution (these are nucleation sites). Once you've done this, you're really seeding the crystals, and so one forms and then another and another until it spreads throughout the bulk of the hand warmer. All the time it is giving off this heat. Eventually, the heat will stop because you've crystallised the whole thing. You can repeat the whole process by putting the used hand warmer in something like and oven, until the crystal is all melted again.

But.. How does that little metal thing absorb that energy?

Title: Re: How do pocket hand warmers work?
Post by: Seany on 09/07/2007 13:23:29

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fimages.esellerpro.com%2F2296%2FI%2F165%2F82%2Fjackpyke_gel_handwarmers_amaz_1.jpg&hash=82e61536189ef36f8a67c0aab9316302)

Title: Re: How do pocket hand warmers work?
Post by: dentstudent on 09/07/2007 13:27:44

It doesn't absorb energy at all. You know when you have bubbles forming on the side of your drinks glass? This is usually an imperfection in the glass which forms an edge. This edge provides a place for the disolved CO2 to collect and is called a nucleation site. It's the same with the metal snapper. It forms a nucleation point around which all the (apparently) sodium acetate crystals can form, which come out of solution forming the solid, and with each crystal releasing a little bit of heat. The heat given off is the manifestation of the released energy.

Title: Re: How do pocket hand warmers work?
Post by: dentstudent on 09/07/2007 13:28:04

What a lovely pair!

Title: Re: How do pocket hand warmers work?
Post by: Seany on 09/07/2007 23:08:44

They're great!! If your hands get cold in the winter!! These are handy chums!

Title: Re: How do pocket hand warmers work?
Post by: chris on 05/08/2017 22:45:49

This question was also posed elsewhere on the forum and Lee Cronin answered it:

https://www.thenakedscientists.com/forum/index.php?topic=9619.msg119666#msg119666

It was also re-visited more recently in this thread:

https://www.thenakedscientists.com/forum/index.php?topic=41379.0