Why is there no liquid form of carbon?

Mark - Well, there is a liquid form of carbon, but it's extremely unusual. When you think about carbon, you normally think of things like graphite - the lead in your pencil, which we were talking about before in the context of graphene. Or you think about diamonds. You might even think about buckminsterfullerene - these are cages of 60 carbon atoms that look like footballs.

All of these are held together by very, very strong chemical covalent bonds and that means that you need to heat up elemental carbon a huge amount before you can actually start to separate those atoms. In fact, once you heat it up to about 3,600 degrees C, you finally get to pull those atoms apart and it turns into a gas: it sublimes; so it normally goes from a solid to a gas. If you want it to be a liquid, you have to put it under incredible pressure while you're actually heating it up that much. So, it's only under very, very extreme circumstances that you can force carbon to be a liquid.

Chris - Aren't people using carbon compounds - like carbon dioxide - as liquids though? This is a whole area of what we call green chemistry. People have discovered, instead of buying expensive solvents to dissolve things - and even make decaffeinated coffee - you can pressurise CO2 (carbon dioxide) and it becomes liquid: "supercritical CO2". It's got the same properties as many industrial solvents except that it's pretty harmless?

Mark - That's right. Carbon dioxide is used to decaffeinate coffee because if you squeeze carbon dioxide, unlike carbon it behaves as a different chemical. If you squeeze it enough then it turns into a liquid, like you said, a supercritical liquid because it only exists past that critical point when you've got it under extreme pressure. And as a green solvent it's certainly a lot better to work with than what they used before which was a chlorinated solvent. It might have been dichloromethane, or chloroform, or something like that; but filthy stuff! A real pain to dispose of, and if you're exposed to it as a worker, it's going to make a right mess of your lungs. So, carbon dioxide is just a lot easy to work with because, of course, when you're finish with it, in theory, you can just release it to the atmosphere.