Why is there no liquid form of carbon?

07 January 2014


Simulation of graphene



Just wondering why there is no liquid form of carbon? I can't think of any other element which doesn't have a liquid form. Is that the case?

Cheers, and thanks for your wonderful podcasts, Peter.


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.


You vary gut

So what exactly is AutoGlym's LifeShine car paintwork coating?
Description from their product brochure below:
"Carbon Shield Technology fuses liquid carbon to vehicle paintwork to create an impenetrable barrier against harsh contaminants and the elements."

With the little info I can glean from their marketing, I would guess that it is more of a carbon suspension in solution, that dries out leaving the carbon behind. (Marketing buzz-words) It is like (I suppose) how some additives in washer fluid works, by depositing a thin layer of a soap to your windshield to help bead up water. This seems to kind of fill in the surface imperfections, leaving a thin layer of some sort of dissolved carbon-based chemical that resists interactions with water and other soluble contaminants. It seems to do something similar to paste wax, but stays on the surface longer (maybe? I haven't tested it, though I have heard that these solutions do last longer than standard waxes)

Is it a liquid carbon?

It isn't correct to call CS2 a liquid carbon. Sure, it is liquid at STP, but it is a completely different molecule with completely different properties compared to elemental carbon. Like how Sodium is highly reactive in air, and Chlorine will kill you within a moments notice, but salt (NaCl) is pretty inert, and our oceans aren't on the verge of bursting into flames any time soon. Once you begin to bond elements to make a molecule, their properties change significantly.

Yay so there is carbonite

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