warmers in the shape of a heart.

And each one contains a liquid and a

little metal clicker, and the point is

that this can generate quite high

temperatures which will keep your hand warm.

Inside is sodium acetate that is dissolved in water.

It's a very concentrated solution.

Sodium acetate is made up of ions, positively

charged sodium, negatively charged acetate.

Positive and negative attract

each other and so you need energy to

pull them apart, and you have to imagine

a huge array of these. Millions and

billions of these,

going alternately, positive, negative and so on.

But I can't afford that amount of model.

So I press it down and there we go.

If I look at it on the screen you can

see a blob of warmth coming in the

middle of the heart. The other heart

doesn't do anything and it reaches in...

Oh, it's really getting warm and it goes

up to a temperature about 50 degrees.

It's somehow producing crystals that

come out sideways and giving out heat,

which lasts for 20 minutes or so.

What happens is when you click, a tiny

particle of something is released.

Some people say it's a bit of metal,

others say it's a tiny crystal of sodium

acetate that was trapped on the metal surface.

It doesn't matter. You release something and as soon as you have that

first tiny nucleus, the precipitation takes place.

And you can see the crystals

spreading out across the hand warmer and

the temperature rising.

So, when you drop crystals of sodium

acetate into water, the ions separate

and because they need energy,

the temperature of the water drops and you

can see quite nicely with thermal imaging.

If you drop sodium acetate crystals into water the water gets cold.

It's getting cold because some of the

thermal energy, the heat energy in the

water, is being used to pull these ions apart.

This is a physical change, so if they

come out of solution, reform their crystals,

you'll get that energy back.

So the question is how can you get that energy

back in your hand warmer. The way you've

got to do it, is to get as much sodium

acetate as possible dissolved in the water.

And with sodium acetate if you

heat the water up you can dissolve more

and more of the salt. So if you go almost to

the boiling point of water,

hundred degrees centigrade, you can dissolve

really a large amount.

What you would imagine that if you dissolve all this

stuff up by heating it, as soon as you

cooled it down you would expect it all

to come out again.

But the interesting thing is, that crystals cannot just form

in a really clean solution.

They need to have tiny particles, doesn't really

matter what of, tiny bit of metal, a tiny

crystal to form round. It's rather like

a crowd forming round a single person.

It has to have something to start it.

So if you have a really clean filtered solution,

if you cool it down, the sodium

acetate stays in solution and that's

what's inside your hand warmer.

So at least your hands will be warm when you walk

to work in the freezing conditions.

To reuse the hand warmer you have to

heat it up again. So we dropped it into

boiling water. And quite by chance we

noticed that there were all sorts of

interesting bubbles and vortices in the

surface of the boiling water.

So that's what you have to do to finish it off,

just put it in there,

leave it for a little while, you take it

out and let it relax and it gets cooled

down and remains liquid.

And if you heat it for a longish time, all the sodium

acetate redissolves and then if you

cool it slowly it will be ready to use again.

The problem is that if you don't

heat it long enough and leave just one

or two tiny crystals, it can go off

spontaneously and suddenly release its heat.

It's not dangerous but it means when you

go to it to warm your hands you find

it's fired already.

Thanks for watching this video and we'd

like to thank Google's Making and

Science team for making it possible.

If you'd like to see more of the videos

they've helped create

there's a brilliant playlist; there's links

on the screen and in the description.

Also if you'd like to see Professor

Poliakoff long-awaited appearance on my Objectivity series.

There are also links for that. And we'll

be back again soon with more videos with the thermal-imaging camera.