We're talking about these funny hand
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.