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Author Topic: How do handwarmers work?  (Read 12949 times)

David McClure

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How do handwarmers work?
« on: 30/09/2011 20:01:06 »
David McClure  asked the Naked Scientists:
   
My son Sean just got back from a trip to the Snow fields in Victoria AU.

While he was there he purchased a multiple use hand warmer.  It is a thick plastic bag with some viscous liquid in it and a metal disc. 

To heat the bag up you manipulate the disc in the liquid, the liquid becomes cloudy and produces heat.  After a while the bag cools and remains very cloudy,   to re-use the bag you place it in boiling water until the liquid goes clear, you then cool the bag back to room temp.  

Once it is cool you can manipulate the disc again to produce the heat, the instructions say that the life of the product is endless - How does this work ? what reaction is going on here ? 

Regards,
David

What do you think?
« Last Edit: 30/09/2011 20:01:06 by _system »


 

Offline damocles

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How do handwarmers work?
« Reply #1 on: 01/10/2011 04:40:39 »
Greetings David from Victoria AU  ;D

The hand warmer contains a supersaturated solution of sodium acetate. Its operation is accurately described on this wikipedia page

http://en.wikipedia.org/wiki/Sodium_acetate

The metal disk is used to induce crystallization, which releases the latent heat of fusion (or of solution) associated with the liquid → solid phase change

CH3 COONa+(aq) + 3 H2O(liq) → CH3COONa.3H2O(xtal) + ~40 kJ/mol

A particular virtue of this system is that it cannot burn, because the maximum temperature it could possibly achieve is 54C -- the temperature at which the crystals melt, or, more precisely, dissolve in their own water of crystallization -- and roughly the temperature at which you would take a warm shower.
« Last Edit: 01/10/2011 04:44:41 by damocles »
 

Offline damocles

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How do handwarmers work?
« Reply #2 on: 01/10/2011 05:02:20 »
An afterthought on re-reading the original post: I believe that a viscous, water-miscible liquid has been added to the basic system in the plastic bag. Almost certainly it is glycerol.

Why has this been done? well, it would mean that the final state of the bag after it has fully discharged its heating effect would be a paste or a slurry rather than a solid. This would make for both easier handling, and less likelihood of damage to the containing bag from large sharp crystals. Glycerol is completely miscible with water, and its high viscosity would also help to limit crystal growth so that the final result was a slurry of very fine crystals -- ideal for the situation.

The glycerol modification would have absolutely no effect on the underlying physico-chemical reaction system.
 

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How do handwarmers work?
« Reply #2 on: 01/10/2011 05:02:20 »

 

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