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,
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.