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Making liquid oxygen is pretty straightforward if you happen
to have some liquid nitrogen, which I do.
So what I've got here, my cylinder of oxygen, and I'm
passing the oxygen through this copper coil here, which
is immersed in liquid nitrogen.
The oxygen has a higher boiling point than the
nitrogen, so it condenses in the coil, so by the time it
comes out the other end of the tube, it's a liquid.
So this has been going a while.
I've nearly got a full tube, so I'm just going to - Oh!
yep, that's pretty full.
So I'll just put this down over here.
OK, yeah.
So, good.
A half liter of liquid oxygen here.
I'll just show you what it looks like.
So there you go.
That's liquid oxygen.
Probably the first thing you will notice about it I would
think is that it's actually blue, which
is a bit of a surprise.
Colour quite often is just a manifestation of the way that
light interacts with the electrons in materials.
So straight away, this is saying that there might be
something a bit funny about the way the electrons are
arranged in oxygen molecules.
What that is, actually, is that there
are unpaired electrons.
Electrons usually like to pair up, but in oxygen, there are a
couple that that doesn't happen to, and we call them
unpaired electrons.
And there's actually another unusual property of oxygen
that arises from the unpaired electrons, which I want to
show you now.
All right, I've got a great big magnet here, as you can
see, just a couple of nuts between the poles just to
bring the poles slightly closer together, and I'm going
to pour the liquid oxygen between the poles like so, and
there you go.
You can see the oxygen is - until it boils away - is
sticking briefly between the poles of the magnet.
So, what's going on here, again, is because of those
unpaired electrons.
Any electron in an atom or in a molecule contributes a small
magnetic dipole moment.
Normally when those electrons are paired up, the magnetic
dipole moments cancel out, but obviously with oxygen, you
have these unpaired electrons, that doesn't happen.
So as a result, the whole molecule has a residual dipole
and acts as a tiny little magnet.
So I'll keep on pouring this in, and we can keep
on looking at it.
Now normally when the oxygen is just sitting in the flask
or in the air, all those tiny little magnets are pointing in
random directions basically, so they all cancel out.
So there's no net macroscopic magnetic field.
But when you introduce the permanent magnets to the
oxygen here, then you get those magnetic molecules all
slightly aligning a bit more than they would normally, and
that creates a net magnetic field.
So the induced magnetic field then interacts with the
magnetic field from the dirty great magnet and you get the
oxygen sticking in the poles.
Now this phenomenon is something called power
magnetism where you get an induced attractive magnetic
field in a material.
And I think it was actually Michael Faraday who discovered
the oxygen was paramagnetic right here at the Ri.
So one final thing, seeing as this is liquid oxygen and this
is the Royal Institution, I'm going to set
something on fire.
This is the classic demonstration of oxygen
relighting a glowing splint.
Lovely.