My daughter was given a 'handwarmer' for Christmas, comprising a plastic
sachet about 3" square and filled with some form of gel. Also side is a
small bit of bent metal; you bend the metal piece so it 'clicks' and
bends in the opposite direction, which initiates a reaction causing the
gel to solidify over a period of several minutes, giving out substantial
heat. Later you bung the thing in a pan of boiling water for 20 mins to
melt the solid back to a gel and reverse the reaction. Not exactly
environmently friendly use of energy, but hey...

So she asked me how it worked.

"Er...."

I vaguely remember stuff from my schoolboy chemistry about seeding
crystals and latent heats and so on, but I'm lost. Anybody care to
explain it for her?!

Thanks
David

Lobster wrote:
My daughter was given a 'handwarmer' for Christmas, comprising a plastic
sachet about 3" square and filled with some form of gel. Also side is a
small bit of bent metal; you bend the metal piece so it 'clicks' and
bends in the opposite direction, which initiates a reaction causing the
gel to solidify over a period of several minutes, giving out substantial
heat. Later you bung the thing in a pan of boiling water for 20 mins to
melt the solid back to a gel and reverse the reaction. Not exactly
environmently friendly use of energy, but hey...

So she asked me how it worked.

"Er...."

I vaguely remember stuff from my schoolboy chemistry about seeding
crystals and latent heats and so on, but I'm lost. Anybody care to
explain it for her?!

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.


--
Grunff

Grunff wrote:
Lobster wrote:
My daughter was given a 'handwarmer' for Christmas, comprising a
plastic sachet about 3" square and filled with some form of gel.
Also side is a small bit of bent metal; you bend the metal piece so
it 'clicks' and bends in the opposite direction, which initiates a
reaction causing the gel to solidify over a period of several
minutes, giving out substantial heat. Later you bung the thing in a
pan of boiling water for 20 mins to melt the solid back to a gel and
reverse the reaction. Not exactly environmently friendly use of
energy, but hey...

So she asked me how it worked.

"Er...."

I vaguely remember stuff from my schoolboy chemistry about seeding
crystals and latent heats and so on, but I'm lost. Anybody care to
explain it for her?!

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

.. wrote:

I /so/ want one of them :-)

where can I get one ?

http://outdoorgear.co.uk/eshop.asp?wci=product&wce=36901801&desc=HOT+GEL+2+ PACKETS


--
Grunff

Lobster wrote:
My daughter was given a 'handwarmer' for Christmas, comprising a plastic
sachet about 3" square and filled with some form of gel. Also side is a
small bit of bent metal; you bend the metal piece so it 'clicks' and
bends in the opposite direction, which initiates a reaction causing the
gel to solidify over a period of several minutes, giving out substantial
heat. Later you bung the thing in a pan of boiling water for 20 mins to
melt the solid back to a gel and reverse the reaction. Not exactly
environmently friendly use of energy, but hey...

So she asked me how it worked.

"Er...."

I vaguely remember stuff from my schoolboy chemistry about seeding
crystals and latent heats and so on, but I'm lost. Anybody care to
explain it for her?!

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.

Grunff wrote:
. wrote:

I /so/ want one of them :-)

where can I get one ?


http://outdoorgear.co.uk/eshop.asp?wci=product&wce=36901801&desc=HOT+GEL+2+ PACKETS


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

Grunff wrote:
. wrote:

I /so/ want one of them :-)

where can I get one ?

http://outdoorgear.co.uk/eshop.asp?wci=product&wce=36901801&desc=HOT+GEL+2+ PACKETS

thanks for that.

wrote:

I /so/ want one of them :-)

where can I get one ?

Boots.

On Sun, 08 Jan 2006 13:19:56 +0000, Grunff wrote:

| . wrote:
|
| I /so/ want one of them :-)
|
| where can I get one ?
|
| http://outdoorgear.co.uk/eshop.asp?wci=product&wce=36901801&desc=HOT+GEL+2+ PACKETS

Avon sells them as well, along with *many* other places.
--
Dave Fawthrop dave hyphenologist co uk
17,000 free e-books at Project Gutenberg! http://www.gutenberg.net
For Yorkshire Dialect go to www.hyphenologist.co.uk/songs/

On Sun, 08 Jan 2006 12:06:20 GMT, Lobster
wrote:

So she asked me how it worked.

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.

On Sun, 08 Jan 2006 13:16:06 GMT, "." [email protected] wrote:

where can I get one ?

Lunartique (the tiara shop) in St Nicks Market, central Bristol
http://lunartique.com
(heart shaped ones too)

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.

.. wrote:

I /so/ want one of them :-)

where can I get one ?

Or for the romantic:

http://www.iwantoneofthose.com/searc...uctCode=HOTHEA

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

"." [email protected] wrote in message
...
Grunff wrote:
Lobster wrote:
My daughter was given a 'handwarmer' for Christmas, comprising a
plastic sachet about 3" square and filled with some form of gel.
Also side is a small bit of bent metal; you bend the metal piece so
it 'clicks' and bends in the opposite direction, which initiates a
reaction causing the gel to solidify over a period of several
minutes, giving out substantial heat. Later you bung the thing in a
pan of boiling water for 20 mins to melt the solid back to a gel and
reverse the reaction. Not exactly environmently friendly use of
energy, but hey...

So she asked me how it worked.

"Er...."

I vaguely remember stuff from my schoolboy chemistry about seeding
crystals and latent heats and so on, but I'm lost. Anybody care to
explain it for her?!

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


Aldi 1.29 for two at present.

mrcheerful

John Rumm wrote:
. wrote:

I /so/ want one of them :-)

where can I get one ?

Or for the romantic:

http://www.iwantoneofthose.com/searc...uctCode=HOTHEA

Yup, that looks like my daughter's!

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

David

mrcheerful . wrote:
"." [email protected] wrote in message
...
Grunff wrote:
Lobster wrote:
My daughter was given a 'handwarmer' for Christmas, comprising a
plastic sachet about 3" square and filled with some form of gel.
Also side is a small bit of bent metal; you bend the metal piece so
it 'clicks' and bends in the opposite direction, which initiates a
reaction causing the gel to solidify over a period of several
minutes, giving out substantial heat. Later you bung the thing in
a pan of boiling water for 20 mins to melt the solid back to a gel
and reverse the reaction. Not exactly environmently friendly use
of energy, but hey...

So she asked me how it worked.

"Er...."

I vaguely remember stuff from my schoolboy chemistry about seeding
crystals and latent heats and so on, but I'm lost. Anybody care to
explain it for her?!

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


Aldi 1.29 for two at present.

mrcheerful

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

On Sun, 8 Jan 2006 13:16:06 UTC, "." [email protected] wrote:

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 ?

I got a couple from findmeagift.com, to replace the pair I've had about
20 years!
--
The information contained in this post is copyright the
poster, and specifically may not be published in, or used by
Avenue Supplies, http://avenuesupplies.co.uk

"Lobster" wrote in message
...
My daughter was given a 'handwarmer' for Christmas, comprising a plastic
sachet about 3" square and filled with some form of gel. Also side is a
small bit of bent metal; you bend the metal piece so it 'clicks' and bends
in the opposite direction, which initiates a reaction causing the gel to
solidify over a period of several minutes, giving out substantial heat.
Later you bung the thing in a pan of boiling water for 20 mins to melt the
solid back to a gel and reverse the reaction. Not exactly environmently
friendly use of energy, but hey...

So she asked me how it worked.

"Er...."

I vaguely remember stuff from my schoolboy chemistry about seeding
crystals and latent heats and so on, but I'm lost. Anybody care to
explain it for her?!

Thanks
David

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!

.. wrote:

I /so/ want one of them :-)

where can I get one ?

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

Dave

.. wrote:
Grunff wrote:

Lobster wrote:

My daughter was given a 'handwarmer' for Christmas, comprising a
plastic sachet about 3" square and filled with some form of gel.
Also side is a small bit of bent metal; you bend the metal piece so
it 'clicks' and bends in the opposite direction, which initiates a
reaction causing the gel to solidify over a period of several
minutes, giving out substantial heat. Later you bung the thing in a
pan of boiling water for 20 mins to melt the solid back to a gel and
reverse the reaction. Not exactly environmently friendly use of
energy, but hey...
I /so/ want one of them :-)

where can I get one ?

2 for £1.99 at Aldi, I think - if not Aldi, then Lidl. These
are selling right now. 55 deg. c for "up to an hour".

Brian wrote


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!

Ruggish!

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


--

mike

mrcheerful . wrote:
"." [email protected] wrote...
Lobster wrote:
My daughter was given a 'handwarmer' for Christmas, comprising a
where can I get one ?

Aldi 1.29 for two at present.

That's the chappie. £1.99 I wrongly thought.... snort.

On Sun, 08 Jan 2006 12:06:20 GMT, Lobster
wrote:

"Er...."

Microscopic pixies.

When you click the clicker they all wake up and run around in a mad
panic thus expending lots of energy.

When you heat the sachet up again they all get sleepy and drift off
back to sleep.

It's true I tell you.

sponix

.. wrote:
Grunff wrote:

Lobster wrote:

My daughter was given a 'handwarmer' for Christmas, comprising a
plastic sachet about 3" square and filled with some form of gel.
Also side is a small bit of bent metal; you bend the metal piece so
it 'clicks' and bends in the opposite direction, which initiates a
reaction causing the gel to solidify over a period of several
minutes, giving out substantial heat. Later you bung the thing in a
pan of boiling water for 20 mins to melt the solid back to a gel and
reverse the reaction. Not exactly environmently friendly use of
energy, but hey...

So she asked me how it worked.

"Er...."

I vaguely remember stuff from my schoolboy chemistry about seeding
crystals and latent heats and so on, but I'm lost. Anybody care to
explain it for her?!

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



Got a couple from Aldi - £1.50 or thereabouts for a pair. Great fun

Grunff wrote:
Lobster wrote:

My daughter was given a 'handwarmer' for Christmas, comprising a
plastic sachet about 3" square and filled with some form of gel. Also
side is a small bit of bent metal; you bend the metal piece so it
'clicks' and bends in the opposite direction, which initiates a
reaction causing the gel to solidify over a period of several minutes,
giving out substantial heat. Later you bung the thing in a pan of
boiling water for 20 mins to melt the solid back to a gel and reverse
the reaction. Not exactly environmently friendly use of energy, but
hey...

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.

Thanks for the replies.

Well, she's fascinated by this thing and has singlehandedly trebled our
gas consumption since Xmas by continually boiling up the bloody thing.

Anyway - interestingly I note that when cold, the sachet has lots of
small bubbles of varying sizes (down to the limits of my vision) but
they don't seem to act as nuclei for crystals to form. We've duly tried
dropping the thing on the floor and bopping it smartlye with a wooden
spoon, but nothing seems to kick it off except the clicker!

David

On Mon, 09 Jan 2006 18:51:41 GMT, Lobster
wrote:


Thanks for the replies.

Well, she's fascinated by this thing and has singlehandedly trebled our
gas consumption since Xmas by continually boiling up the bloody thing.

Anyway - interestingly I note that when cold, the sachet has lots of
small bubbles of varying sizes (down to the limits of my vision) but
they don't seem to act as nuclei for crystals to form.

They are stable bubbles

We've duly tried
dropping the thing on the floor and bopping it smartlye with a wooden
spoon, but nothing seems to kick it off except the clicker!


I bet it's cavitation that does it. "Bubbles" full of vacuum form as
the wavefront passes through the material and then subsequently
collapse releasing a bit of energy but in a tiny volume

= high energy density.

DG