Or at least the thing that most people know, is if you breathe in helium, it makes your voice change.

Makes it go squeaky.

I've never tried this...

So, I decided that it'd be rather fun to try this time with Neil and Brady helping me.

[Brady ] Are you nervous then?

Erm. No, I'm not nervous yet.

Well, I am nervous, I've just read on Wikipedia that somebody died after breathing helium.

But I hope it won't happen now.

Make a good video if it does. *takes a breath of helium from balloon*

*higher pitched* Hello, I'm now talking with helium in my voice.

And it quickly goes away.

And the first time I did it, I didn't breathe deeply enough so my voice didn't change very much.

But the second time, it did change really quite significantly.

*takes second breath of helium from balloon*

*an even higher pitch* There are a hundred and fifteen more videos to watch.

In fact, there are a hundred and seventeen more videos to watch.

Was that different?

When you're doing it, it just feels as if you're talking normally, but somehow your voice comes out differently.

I suppose it's like when you have a very bad cold or something like that that, your voice changes as well.

I don't think that this is a good experiment for you to try yourselves.

I don't think it's dangerous if you do it just once.

But if you keep on breathing helium, it can be really quite dangerous.

And in fact, I read recently about somebody who died after breathing too much.

But if you breathe helium under controlled conditions, it's quite safe.

And in fact, it's used for deep-sea divers.

One of the problems if you're diving very deep under the sea...

Is that, you have to breathe air that's under pressure; otherwise you can't open and shut your lungs.

Under pressure, the nitrogen in the air dissolves in your blood.

Now, this doesn't matter when you're down at the bottom.

But as you come up, if you surface too quickly, the nitrogen can come out as bubbles in your blood.

And cause a very painful condition...

Which for some reason, the divers called the "bends".

And it's been found that if you replace the nitrogen in the divers breathing air, or at least most of the nitrogen with helium...

Then they don't suffer from "bends".

[AP Archive voice over] "Below 150 feet, the pressure on the body rises dramatically and divers must operate on a mixture of helium and oxygen."

But of course their voices go squeaky...

So the people on the other end of the telephone, talking to the divers, need to have some sort of electronic filter...

So that the divers' voices are transposed and they can still understand what they're saying.

The reason why your voice changes is probably due to a number of factors.

One of the factors is that the speed of sound is much greater in helium than it is in air.

The reason for this, is that the speed of sound is related to the mass of the molecules in the gas that the sound is going through.

And the helium molecule is much lighter than nitrogen or oxygen, which you find in air.

Of course if you breathed hydrogen, it would be even faster the speed of sound, but you then blow yourself up.

So this is not something that anybody should try.

The other point, is that inside the Human head,

in fact, the head of all animals, there are complicated resonances caused by the different spaces.

And that's why your voice changes when you have a cold,

because some of the tubes are blocked.

So, I had the idea...

that perhaps, if you put an iPhone playing music in helium, that the sound might change.

Though there isn't much to resonate in an iPhone.

*music from iPhone*

But Neil and Brady were quite keen to try.

And our first experiments with a small plastic bag were a bit of a failure.

So then Neil got a really big plastic bag and filled it with helium.

And when you put it over the iPhone.

Most surprisingly,

the notes didn't change, but the sound was attenuated much more.

*music continues*

So it sounded much quieter.

*music continues*

I'm not sure why this is, but it was quite a striking effect.

And Neil put the bag on and off.

*music continues*

And you could hear the music getting louder when the bag was not there and going quieter again when he put it back.

*music continues*

And we did a control experiment with a bag of air, and it wasn't the bag it definitely was the helium.

But the thing I liked best was that Neil then let go of the bag, and it went up to the ceiling and stayed there.

Even though the bottom of the bag was completely open.

So once the helium is trapped in an upturned plastic bag, it will sit there for quite a long time.

*music continues*

Because helium is an inert gas...

Because it has two electrons that are paired...

It doesn't form conventional chemical compounds in the way that say hydrogen does or sodium.

But there's a very recent paper published just a few months ago,

in this journal here, where you can see it says "Noble gas compounds: when helium met sodium".

And the people here have done a really extraordinary experiment,

where they mixed sodium metal in helium gas in a diamond anvil cell.

We have a video that explains how these work.

But in these cells, you can get really high pressures.

In their case, they went up to a pressure of nearly three million atmospheres.

Three million times higher than atmospheric pressure.

And when they did this they found, remarkably, that sodium metal...

forms a compound with helium.

Where there is helium which is still chemically neutral: not helium plus or helium minus.

And sodium and in the lattice between them, there are two electrons.

Which are sitting there. Now normally in sodium metal,

because it's a metal, the electrons are,

if you like smeared out, de-localized through the structure,

that's what gives it its metallic properties.

But in this salt: Na2He.

Two sodium atoms for each helium atom, the electrons are localized and they predicted, but they weren't able to observe...

That actually under these high pressures the sodium and helium mixture should become transparent.

So it's really quite exciting.

So if your teachers tell you that helium doesn't form compounds.

They're both right and wrong.

They're right, because it doesn't form compounds under normal conditions, but they're wrong because at three million atmospheres, Yes, you can get some.

The reason why there is helium in the Sun.

Is partly because the nuclear reactions that take place in the Sun...

are responsible for creating helium.

Hydrogen atoms, under these enormously high pressures and temperature,

the hydrogen atoms can fuse together, by a process called fusion, to generate helium.

Now and the normal circumstances, it's not possible to do this on Earth.

But if you take a nuclear bomb,

you can use an atom bomb to trigger the fusion of, usually deuterium or tritium...

these are isotopes of hydrogen and they can be made to fuse to create helium.

[Archive voice over] "On this rather unimpressive barge, floating in the Bikini Lagoon,

rests the device which carried the hopes for a large step forward in the weapons development program."

*muffled sound of a nuclear explosion*

"The distance of the camera aircraft is 50 miles. The frame size of the picture is 14 by 18 miles,

the film is running at normal speed."

There is also hope that you can do controlled fusion in reactors called Tokamaks.

Where eventually, it may be possible to use the fusion of hydrogen atoms to make helium...

as a way of generating very clean electricity.

However, experiments have been going on for most of my lifetime,

and although the results are promising, they're nowhere near being commercial production of electricity.

The most common isotope of helium is helium-4: with two protons and two neutrons.

There is a very rare isotope, helium-3, which has two protons and one neutron.

And the properties of liquid helium-3 are significantly different from the properties of liquid helium-4.

Because at the very low temperatures of these liquids, the quantum effects become very important.

In fact, I've read, and I don't know if it's true, that liquid helium-3 and liquid helium-4

are immiscible, they don't mix together, which is really quite weird?

Helium-3 can also behave as are superfluid, which means that it can creep upwards out of its container.

Whereas, normally, if we put a liquid, say tea into my cup, it stays there, but imagine if it crept up the sides and went out.

So helium is a really very exciting element.

[Brady] Alright, let me start the music...