Journey to the centre of the Earth – creating liquid carbon dioxide

by Rosie Coates

In this blogpost we’ll look at what we know about the structure of the Earth, how we know about it and show you how you can make your very own liquid carbon dioxide

Amazing!  Edouard Riou's illustration from Jules Verne's Voyage au centre de la Terre (1864: Public Domain)

Amazing! Edouard Riou’s illustration from Jules Verne’s Voyage au centre de la Terre (1864: Public Domain)

I’d like to take you on a journey: to see amazing, unexpected things and to discover what lies at the centre of the Earth. I’d like to. I really would. But unlike the intrepid adventurers in Jules Verne’s 1864 classic ‘Voyages au centre de la Terre’ I can’t.  We just can’t get there.

The centre of the Earth is 6370 km down and the deepest drilled hole we currently have (the Kola superdeep borehole in Russia) goes down a puny 12.262 km. We’re not going to get there anytime soon. Well, I guess that’s it for this post then.

Or maybe not, because one of the great things about science is that it allows us to explore distant or difficult environments from the comfort of the lab.

What do we know?

We are able to go quite far down into the crust.  The Kola superdeep borehole takes us almost a third of the way down into the continental crust, the outer surface of the Earth, on average 45km thick.  Kola isn’t the only big hole in the crust.  There are also drills in the ocean floor.

Don't look down!  The Mir Mine in Yakutia Photo: Vladimir Artukhov (CC-BY-3.0)

Don’t look down! The Mir Mine in Yakutia Photo: Vladimir Artukhov (CC-BY-3.0)

Add to those the immense mines in the USA, Africa and Russia, such as the Mir open pit diamond mine in Yakutia, Siberia at 525m deep and we have lots of opportunities to study the surface of our planet.

But about a third of the way through the crust is as far as we can go.  Despite this you may have seen diagrams like the one below showing the different layers that make up the earth.

First is the mantle, the layer from the crust down to 2900km below the surface. This is followed by the outer core down to 5150km, and beneath this the inner core down to the centre at 6370km below the surface.

The Earth Image: Kelvinsong (CC-BY_SA 3.0)

The Earth
Image: Kelvinsong (CC-BY_SA 3.0)

We also know things about these layers, whether they are solid or liquid and the materials they are made of. How on Earth do we know that if we can’t take a sample?

How do we know it?

Although we can’t get very far down into the earth, sometimes things do come up which shed light on what may be down there. Diamonds are formed deep in the earth and occasionally other minerals also surface which help us understand what the Earth is made of.

Click on the image to read the article on the BBC website

Click on the image to read the article on the BBC website

Even these don’t tell us about the very centre though. How do we know about that?

Let’s take a look at a glass of water with a straw in it. Looking straight through the glass from the side on, the straw looks bent between the surface of the water.  This is due to refraction. As light moves from the air to the water it is bent. The amount of refraction depends on the material the light wave travels through. A similar process occurs in seismic waves (during earthquakes). Monitoring where the waves are felt and how long it takes for them to travel tells us about materials they have moved through. In particular we can find out the density and the state (whether they are solid, liquid or gas) of the material.  Then we can get on with some really interesting chemistry.

New worlds at the centre of our world

I’ve said it before and I’ll say it again, chemists are lazy!  We don’t want to bother with khaki shorts and crampons, instead of setting off for the centre of the Earth we’d much rather create the centre of the Earth in our cozy labs.  Using a handy piece of kit and a couple of diamonds we can do just that.  A diamond anvil cell presses material between the points of two diamonds.  Diamonds are very, very tough so they can stand the pressures created.  Add a bit of heat and we can recreate the conditions at the very centre of the Earth.

This isn’t just an interesting way to find out more about the structure of our planet. The technique can be used to create new materials which need high pressure and temperatures to be formed, but can then exist comfortably at room temperature and pressure. New materials are always being developed, some have superconducting properties, some are very, very hard, and they all offer material chemists new possibilities.

Under pressure- Diamond Anvil Cell Photo: Emmanuel Soignard, UCL/Arizona State University (By Permission)

Under pressure- Diamond Anvil Cell
Photo: Emmanuel Soignard, UCL/Arizona State University (By Permission)

Try it yourself – Let’s make liquid Carbon Dioxide!

Ok, you may not have a diamond anvil cell at your disposal but you can still use pressure to change the phase (solid, liquid or gas) of a substance in the school lab and see something most people will never see.  You may have seen solid carbon dioxide (dry ice), you breathe out carbon dioxide as a gas, but have you ever seen liquid carbon dioxide?  No, I hear you cry, solid carbon dioxide sublimes (turns directly from a solid to a gas) to carbon dioxide gas, it doesn’t form a liquid!

That’s true at room temperature and room pressure, but if we increase the pressure we can melt solid carbon dioxide to give us a liquid.

You will need

  • Safety glasses
  • Pellet of dry ice (solid carbon dioxide)
  • Plastic beaker
  • Warm water
  • Tongs
  • Centrifuge tube (This is important – DO NOT use an alternative vessel as they will not be able to withstand the high pressure and you may injure yourself).

What to do?

1. Put the safety googles on.

2. Use the tongs to place pop a piece of dry ice into the centrifuge tube and screw on the lid.

3. Put the tube into the plastic beaker filled with warm water.

4. Watch the solid carbon dioxide melt to form liquid carbon dioxide.

This makes a lovely classroom demonstration and only a few will ever see liquid carbon dioxide!

To find out more about why we love demos and for some of our favourites take a look here.

Curriculum Links


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Posted in Activity, Chemistry, Geology
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