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  • 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.

We're talking about these funny hand

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ハンドウォーマーの働き (THERMAL IMAGING) - 動画の周期表 (How Hand Warmers Work (THERMAL IMAGING) - Periodic Table of Videos)

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    林宜悉 に公開 2021 年 01 月 14 日
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