One of Neil's favorite experiments, and
mine, is dropping red hot charcoal into
liquid oxygen in a cauldron, a bowl.
You've seen it before but now we're
going to show it to like you've never
seen it.
We're going to show you how we do the
experiment from the very beginning and
we're going to show it to you with a
thermal-imaging camera so you can see
which bits get hot, which bits get cold
and even see some of the temperatures.
The first thing is that you have to
fetch some liquid nitrogen from the tank.
It's usually a bit of a chore but when you
look at the temperatures, it's really
quite interesting.
Once Neil did that he had to carry the
Dewars back to his lab, where we're going to
make the liquid oxygen. Now, liquid oxygen
has a higher boiling point than liquid
nitrogen and so you can condense liquid
oxygen using liquid nitrogen because
it's colder. So you begin with a cylinder
that's a high-pressure bottle of oxygen.
Now the pressure of the gas is far too
high to use for the experiment so you
have to reduce the pressure with a
so-called regulator which sits on the
top of the cylinder. Once you've got it
flowing you flow it through a coil of copper.
Copper conducts heat very well, so
if you put the coil of copper in the
liquid nitrogen and flow the oxygen
through, it will liquefy.
But because there's a flow it goes right
through and doesn't solidify in the coil.
So, the gas comes out of the cylinder at
a low pressure, but flowing quite fast,
it flows down a tube into the copper coil
which is cold, in the coil it liquefies
and when it liquefies the flowing gas
blows through probably droplets or slugs
of liquid out of the other side and then
it's fast enough so that it doesn't all
boil away when it goes through the last
piece of tube and then goes into a
Thermos flask. We didn't have much
oxygen, and because once it's in the bowl
it will boil away quite quickly, it was
important to get the hot charcoal ready
first. Because the charcoal will gently
burn in air so once it's hot
it can stay hot for quite a long time.
So, Neil heated the charcoal really like
lighting a barbecue.
Now, remember we only have one Thermos of
liquid oxygen and as soon as you pour
something cold into the bowl, which
starts at room temperature, a lot of it
will boil away. So, Neil didn't want to
waste the oxygen, so he cooled the bowl,
pre-cooled it with liquid nitrogen,
because he has far more liquid nitrogen
than oxygen.
The bowl's made of metal so it cools
quite quickly because it doesn't have a
very high heat capacity.
Once the bowl is cold, when he pours in
the liquid oxygen it will still boil a
little bit but it won't boil very
violently so we won't waste much.
So, in goes the liquid oxygen into the pre-cooled metal bowl.
Neil was ready, he had a container with
several pieces of hot charcoal, now
obviously when you drop them in, you have
charcoal which is carbon, which is going
to burn really brightly in the presence
of oxygen.
When it's dropped in, the ordinary video camera completely whites out,
looks really good, looks as if the
whole world has ended. But the
thermal-imaging camera looks very much
more selective wavelengths, so you can
actually see what's happening to the
piece of charcoal.
Not only can you see, but you can
measure its temperature, so you have two
really good bits of scientific information.
There was a really interesting thing
that happened, one of the pieces that
Neil dropped in went the usual flash of light.
Nobody knew what was happening, but Brady
who was looking through the thermal-imaging
camera said, it's broken in two, and then
when things had subsided, what has happened,
you could see in fact two pieces of
charcoal. It's important to stress, the
liquid oxygen is very cold, the charcoal
is very hot.
It's somewhere between 900 and a thousand
thousand degrees centigrade, and so the
way that these two can co-exist,
something that's very hot and something
that's very cold, is that the charcoal is
probably surrounded by a bubble of gas.
Now you can't see that hot gas because
it's probably not radiating in the same
way as the charcoal. But you do need to
remember you can't have a liquid at minus
nearly 200 degrees centigrade in contact
with the surface that's at a thousand
degrees centigrade. So, the charcoal
is surrounded by a cushion of air,
except it's round, so it's more like a bubble,
and that's important because of
course the oxygen gas gets much more
efficiently to the surface to maintain
the burning.
Eventually the reaction slows down and
the oxygen rushes in and cools down the
remaining fragments of charcoal.
So probably when Neil washed it out
he found some small pieces of charcoal at the bottom.
We thank Google's making and science
initiative for helping us make this video.
If you want more information about
the initiative, look down in the video description.