How do we know that diamond and graphite are both made of carbon?

Most people learn fairly early on that graphite and diamond are both made up of the element carbon.  If you had a good teacher in middle school, you might remember them describing breathlessly how amazing it is that a diamond, the hardest substance on the planet, can be made of the same material as graphite, which is soft enough to use in pencil lead.  You might remember more contrasts:  diamond is transparent white, while graphite is black and shiny; graphite conducts electricity while diamond does not, etc. etc.  The teacher probably waited longingly for someone to ask how such patent contradictions could manifest themselves in a single element.  They might have even prompted your class with such classics as “Does anyone want to take a guess as to why this might be the case?” or “Isn’t this just mind-blowing?”  Of course, if you were anything like me in middle school, your most mind-blowing discovery was sitting to the left of the hot girl, because, you know, girls’ shirts button right over left and you could look over every once in a while and maybe get a glimpse of her bra.  I’d like to say that the girls in my middle school science classes probably retained more than I did, but since I believe in equality, I have to assume that teenage girls are every bit as mindless and self-absorbed as teenage guys.

The punchline that your teacher was trying to convey is that, even though diamond and graphite are both made of pure carbon, the way that carbon is structured confers wildly different properties on the two materials.  Let’s take the pictures of diamond and graphite above and zoom in about, oh I don’t know, 100 million times:

It’s pretty clear from this picture why diamond is so much stronger than graphite:  there aren’t any chemical bonds between the carbon sheets in graphite.  (Side note:  those of you who follow science news may know the individual graphite sheets as “graphene.”)  If pressed, I’ll bet most people would recall seeing a picture like the one above at some point during their education.  But, of course, asking why diamond and graphite are different isn’t the point of this website.  We want to know:  How do we know that diamond and graphite are made up of different carbon structures?  In fact, how do we know they’re both made of carbon in the first place?

 Fortunately, there’s a really easy experiment to answer the latter question.  Unfortunately, it’s just about the most expensive single experiment you can do.  The answer:  Burn them both and look at what comes out.  Now, I don’t recommend you burn your diamonds.  If you really want to get rid of them, you can give them to me.  But luckily for us, someone’s already done it!  In fact, lots of scientists in the 17th and 18th century apparently had a thing for burning priceless jewels.  In one particularly remarkable experiment, a couple of Italian chemists showed that if you heated diamonds and rubies over a roaring fire, the rubies came out intact and unharmed, while the diamonds completely disappeared.  Over the next several decades, various investigators reported diamond burning experiments.  By 1772, it was clear that diamonds were susceptible to fire, but it was still unclear whether the diamonds were simply evaporating, like a block of ice turning to steam on a hot stovetop, or whether the diamonds were actually reacting with the air or decomposing chemically.  This was a problem of some economic significance at the time, as jewelers believed that heating gems could remove imperfections.  But if diamonds burned, then heating them was the last thing you wanted to do.

A Holdover from Alchemy

During all this, it came out that one way to protect diamonds from disappearing during exposure to intense heat was to bury them in charcoal in a hermetically sealed crucible.  A brief explanation of a disproven chemical theory is in order to fully understand how the scientists of the time viewed the results of these experiments.  Some time before the diamond-burning experiments, scientists had discovered that heating charcoal (which is almost pure carbon) in a sealed vessel only consumed a fraction of the charcoal.  The rest remained unburnt, and the air that was left in the vessel was shown to extinguish candles.  From our position in the 21st century, it’s clear to modern chemists that what was going on was a reaction between the carbon in charcoal and the oxygen in the air to make carbon dioxide.  Once all of the oxygen has been used up, the charcoal can no longer burn and the air left in the reaction vessel is mostly carbon dioxide, an unreactive gas that can’t sustain a fire.  This is all basic high school chemistry now, but scientists in the 1700s had a completely different idea of what was going on.  According to them, the results of the charcoal expeirment were accounted for by the phlogiston theory.  This theory stated that all combustible materials (be they metals, charcoal, wood, etc.) were filled with a substance called phlogiston (pronounced flow-gist-on).  By burning them, the phlogiston was released into the air.  The fact that charcoal in a sealed vessel stopped burning after a while meant that the air was saturated with phlogiston; it could hold no more.  A detailed refutation of phlogiston theory will be the subject of a future HDWKI, but for now, the important point is that the scientists involved in diamond burning experiments viewed the results of their experiments through the lens of this theory, which colored their interpretation of what was going on.

The Chemical Revolution

Alright, back to our regularly scheduled episode.  In light of the observation that charcoal could protect diamonds from burning away, a few scientists put forward the argument that the diamonds were actually combusting, instead of just evaporating.  Three French chemists–Antoine Lavoisier, Pierre Macquer, and Louis Claude Cadet–got together to figure out the problem once and for all.  Looking at their names, it was probably superfluous to point out that they were French.  Anyway, these guys decided to burn one diamond in air, one diamond in a sealed tube packed full of chalk, and one diamond in a sealed tube packed full of charcoal.  They figured that if diamonds were evaporating, then it wouldn’t matter whether they were immersed in charcoal or not.  The intense heat would cause all three of them to evaporate regardless.  However, if the diamonds were reacting with the air, then the charcoal would react with the air first and saturate it with phlogiston (use up all the oxygen, in modern day chemspeak), sparing the diamond from harm.  The diamonds in chalk and in air would release their phlogiston into the air (modern translation: react with oxygen) and be consumed by the fire.  So what did they observe?  The diamonds in the air and the chalk were heated, and they lost a good portion of their mass and turned black over the course of the experiment.  The diamond in charcoal was heated.  Nothing.  It was heated some more.  Nothing.  It was heated until the entire experimental setup began to melt under the intensity of the conflagration.  As the jeweler who provided the experimental subjects dug through the ashes, he eventually came upon his diamond, unharmed except for a little ash on the surface that could easily be polished away.

Once Again, Beer Makes Everything Make Sense

So diamonds burn, rather than evaporate.  What’s next?  Well, we can ask what diamonds turn into when they burn.  At the same time the French experiments were being conducted, an English chemist named Joseph Priestley was busy on a project that would forever change the way we drink:  he was inventing soda water.  Priestley happened to live next to a brewery, and he noticed that the air that blanketed the fermentation vats could extinguish flames and kill mice.  This caught his attention, because about 20 years earlier, a Scottish physician named Joseph Black had shown that if you heated up chalk, it became lighter and gave off a gas that could extinguish flames and kill mice.  Priestley took the air over the fermentation vats and forced it into water, which absorbed it to give a fizzy, sour mixture.  He then did the same thing with the air that he got from chalk, getting the same fizzy, sour mixture.  This was pretty compelling evidence that the gas from the chalk was the same as the gas from the fermentation vat, and a great test for the presence of this gas, whatever it was.

In the next year, Lavoisier, catching wind of Priestley’s experiments, decided to set up a series of experiments to capture the gases that came off burning metals and minerals.  In order to do this, he set up a giant magnifying glass to focus sunlight onto a small sample in a closed jar.  He would then use Priestley’s test to examine the properties of the gases that were produced in the course of the experiments.  He spent several months in collaboration with Macquer burning diamonds (there were apparently many engineering problems with the experimental setup that needed to be addressed–welcome to the day-to-day drudgery of scientific inquiry) before finally showing that the gas given off in the course of burning diamonds was the same as that released when heating chalk or brewing beer.  It also happened to be the same gas released by burning charcoal.

Hard-Luck Scheele

Ok, so you can make the world’s most expensive soda water by burning diamonds and forcing the gas given off to dissolve in water.  You’re probably wondering if you can do the same thing with graphite.  Short answer: yes.  Enter Carl Wilhelm Scheele.  I feel really bad for giving him such short shrift here, but that’s pretty much his legacy.  Scheele was a prodigy:  he discovered oxygen, chlorine, tungsten, barium, manganese, molybdenum, citric acid, glycerine, cyanide gas, and a whole mess of other compounds, and he did critical experiments to disprove the phlogiston theory mentioned above.  Also, did I mention that he died at age 43, making more discoveries in half a career than most scientists could do in two?  Unfortunately, he was poorly cited and no one really paid attention to his landmark discoveries.  Isaac Asimov called him “hard-luck Scheele.”  Anyway, he took some concentrated nitric acid and reacted graphite in it.  It slowly disappeared while at the same time producing a gaseous byproduct.  He compared this with the same experiment using charcoal.  In both of these experiments, the gas given off was forced to combine with water to give a fizzy, sour mixture, implying that graphite was made of the same wonder substance as diamonds, charcoal, and the volatile part of chalk (I’m reminded of Homer Simpson, who couldn’t believe that ham, pork, and bacon all came from the same magical animal).  You have to understand:  at the time, no one really knew what graphite was.  Everyone just assumed it had to have lead metal in it (which is why, even to this day, the graphite in the center of a pencil is called “lead”).  It’s unclear why people assumed this, but it might be related to the fact that graphite looks and feels a lot like galena, which is a lead sulfide ore:

Extra Credit

So what, from a modern-day chemistry perspective, was going on?  Well, let’s first think about what was going on with the chalk.  Chalk is mostly made out of calcium carbonate, CaCO₃.  When this is heated, carbon dioxide (CO₂) is released and calcium oxide (CaO) remains:

   

Diamonds, charcoal, and graphite are all made of carbon, so burning them combines carbon and oxygen directly to make carbon dioxide in what has to be the simplest chemical equation ever:

   

What about the nitric acid?  It’s what chemists call an oxidizing agent.  In fact, it’s a pretty powerful oxidizing agent.  Basically, this means it acts like oxygen in that it reacts with things the same way oxygen normally does: it burns them.  In the case of Scheele’s experiment with graphite, it burned the carbon chemically, instead of the usual burning with heat (although the reaction generates quite a bit of heat in its own right):

   

Now, the chemists among you might say, “but wait a second, Keith. I know that is also a gas, so how did Scheele tell that apart from ?”  Alright, we’re all glad you’re so smart.  I honestly don’t know how he did it.  My German’s not that good.  But I can think of a way that we can do it:  boiling points.  It turns out that boils at just under room temperature.  So if you want to separate from , all you need to do is cool down the reaction just a little bit.  The will condense as a liquid (if I remember correctly, it’s a lovely blue color), and the will remain a gas, and you can make your soda water with it, free from nitrogen dioxide.

 Anyway, this post has stretched on for quite a bit longer than I had anticipated.  We’ve shown how Lavoisier, Priestley, and Scheele determined that diamond, graphite, and charcoal were all made of the same material.  In part 2, we’ll show how it is that people figured out that diamond and graphite actually have vastly different carbon structures, and how it revolutionized the chemistry of the 20th century as much as the work of Lavoisier and friends revolutionized the chemistry of the 18th and 19th centuries.  But just remember:  when you want to figure out a major scientific discovery involving caustic acids and priceless jewelry, beer and fire is usually the way to go.

References

These two books give a really well-written account of Lavoisier’s diamond experiments:

• Poirier, Jean Pierre.  Lavoisier: Chemist, Biologist, Economist.  1993, UPenn Press, especially chapter 3.

• Guerlac, Henry.  Lavoisier, the Crucial Year.  1961, Cornell U Press.

Scheele’s graphite experiment is described in the following.  It’s an ancient periodical and kind of hilarious:

• Acheson, E. G. “Graphite: Its Formation and Manufacture,” in The Pharmaceutical Era. Vol. 22, July 13, 1899. p. 55