Poser for chemists/physicists?!

The bag contains a supersaturated solution of sodium acetate. When you click the metal clicker, the shock wave produced creates some bubbles, which act as centres for crystal formation. The sodium acetate crystalises out, releasing heat.

When you heat it up again, all the sodium acetate dissolves again (absorbing heat from the water), and the pack is now ready for reusing.

I /so/ want one of them :-)

where can I get one ?

tia

The heat being released is what's termed "latent heat of crystallisation". When the molecules are in solution, they are in a highly disordered state, moving around freely. This movement is energy.

When the crystals form the molecules are constrained into an ordered state, a three dimensional pattern, in which their movement is greatly reduced. Hence the crystal has less energy than the molecules in solution. As the crystals form, this energy is released as heat.

To make the crystals dissolve you need to put in more energy than was lost during crystallisation, hence the boiling water. Once enough energy has been absorbed, the molecules are able to move around freely in the solution again.

The trick is to find a molecule that can be triggered to crystallise from a saturated solution and Grunff has explained how this is done.

Thanks for the explanation, Grunff!

Tia, I'm sure you'd like my daughter's even more... the sachet's bright red and heart-shaped....

David

thanks for that.

Boots.

| . wrote: |=20 | > I /so/ want one of them :-) | >=20 | > where can I get one ? |=20 | =

Avon sells them as well, along with *many* other places.

--=20 Dave Fawthrop

As you well know, it requires energy to be added to melt things. It even requires energy to melt the things, keeping them at the same temperature

- ie. it's not enough just to "warm them up", you have to put measurable energy in to cross that phase transformation.

As you ought to remember from school, energy doesn't vanish, it just either turns from one form to another or hangs around stored somewhere. Eventually it all degrades down to heat anyway.

So if it took energy to melt the magic goop, and that energy hasn't disappeared, the the logical (if perhaps surprising) conclusion is that you can obtain heat energy by making things freeze.

Second point - why the clicky disk?

The magic goop is chosen because it's a "supercooled" liquid. It's at a temperature where it's actually a more stable state to be solid than liquid, but provided you bring it down gently from a high temperature (the pan of boiling water) then it will remain liquid - if undisturbed. Kick off a mechanical disturbance in it though, such as dropping it, clicking the disk, stirring the panful of reheated packs, or (most importantly) growing frozen crystals through it, and you can cause it to transition from liquid to solid - releasing the heat as described above.

Magic goop here is "Glauber's salt" (Google or Wiki on this) which is sodium acetate. This is chosen as it's affordable, non toxic and has the right supercooled behaviour at the right sort of cold room temperature.

The same material is also used for garden-shed sized heat stores with larger domestic solar heat or heat pump projects.

Lunartique (the tiara shop) in St Nicks Market, central Bristol

In the Christmas run-up, or around Valentine's Day, it's our best seller!

You can usually find them at any outdoor market though - someone will be selling them.

Or for the romantic:

Aldi 1.29 for two at present.

mrcheerful

Think she likes it more for its pink colour than its shape any romantic connotations, which is fine by me...

David

really ? *marvellous* I'll pop in tomorrow and have a look.

I got a couple from findmeagift.com, to replace the pair I've had about

You would be better looking at Google for a site called "howstuffworks" - I'm sure you're daughter would have been able to tell you that!

My local Texaco garage has them for a few quid if you fill up with fuel.

Dave

Brian wrote

Ruggish!

Lobster would be better informed; I wouldn't have been - but I'll pass on the pink hearty one