What is Water? The Element that Became a Compound

November 2018 marks the 235th anniversary of the day when Antoine Lavoisier proved water to be a compound, rather than an element.

I’m a few days late at the time of writing, but November 12th 2018 was the 235th anniversary of an important discovery. It was the day, in 1783, that Antoine Lavoisier formally declared water to be a compound, not an element.

235 years seems like an awfully long time, probably so long ago that no one knew anything very much. Practically still eye of newt, tongue of bat and leeches for everyone, right? Well, not quite. In fact, there was some nifty science and engineering going on at the time. It was the year that Jean-François Pilâtre de Rozier and François Laurent made the first untethered hot air balloon flight, for example. And chemistry was moving on swiftly: lots of elements had been isolated, including oxygen (1771, by Carl Wilhelm Scheele) and hydrogen (officially by Henry Cavendish in 1766, although others had observed it before he did).

Cavendish had reported that hydrogen produced water when it reacted with oxygen (known then as inflammable air and dephlogisticated air, respectively), and others had carried out similar experiments. However, at the time most chemists favoured phlogiston theory (hence the names) and tried to interpret and explain their results accordingly. Phlogiston theory was the idea that anything which burned contained a fire-like element called phlogiston, which was then “lost” when the substance burned and became “dephlogisticated”.

Cavendish, in particular, explained the fact that inflammable air (hydrogen) left droplets of “dew” behind when it burned in “common air” (the stuff in the room) in terms of phlogiston, by suggesting that water was present in each of the two airs before ignition.

Antoine-Laurent Lavoisier proved that water was a compound. (Line engraving by Louis Jean Desire Delaistre, after a design by Julien Leopold Boilly.)

Lavoisier was very much against phlogiston theory. He carried out experiments in closed vessels with enormous precision, going to great lengths to prove that many substances actually became heavier when they burned and not, as phlogiston theory would have it, lighter. In fact, it’s Lavoisier we have to thank for the names “hydrogen” and “oxygen”. Hydrogen is Greek for “water-former”, whilst oxygen means “acid former”.

When, in June 1783, Lavoisier found out about Cavendish’s experiment he immediately reacted oxygen with hydrogen to produce “water in a very pure state” and prove that the mass of the water which formed was equal to the combined masses of the hydrogen and oxygen he started with.

He then went on to decompose water into oxygen and hydrogen by heating a mixture of water and iron filings. The oxygen that formed combined with the iron to form iron oxide, and he collected the hydrogen gas over mercury. Thanks to his careful measurements, Lavoisier was able to demonstrate that the increased mass of the iron filings plus the mass of the collected gas was, again, equal to the mass of the water he had started with.

Water is a compound of hydrogen and oxygen, with the formula H2O.

There were still arguments, of course (there always are), but phlogiston theory was essentially doomed. Water was a compound, made of two elements, and the process of combustion was nothing more mysterious than elements combining in different ways.

As an aside, Scottish chemist Elizabeth Fulhame deserves a mention at this point. Just a few years after Lavoisier she went on to demonstrate through experiment that many oxidation reactions occur only in the presence of water, but the water is regenerated at the end of the reaction. She is credited today as the chemist who invented the concept of catalysis. (Which is a pretty important concept in chemistry, and yet her name never seems to come up…)

Anyway, proving water’s composition becomes a lot simpler when you have a ready supply of electricity. The first scientist to formally demonstrate this was William Nicholson, in 1800. He discovered that when leads from a battery are placed in water, the water breaks up to form hydrogen and oxygen bubbles, which can be collected separately at the submerged ends of the wires. This is the process we now know as electrolysis.

You can easily carry out the electrolysis of water at home.

In fact, this is a really easy (and safe, I promise!) experiment to do yourself, at home. I did it myself, using an empty TicTac box, two drawing pins, a 9V battery and a bit of baking soda (sodium hydrogencarbonate) dissolved in water – you need this because water on its own is a poor conductor.

The drawing pins are pushed through the bottom of the plastic box, the box is filled with the solution, and then it’s balanced on the terminals of the battery. I’ve used some small test tubes here to collect the gases, but you’ll be able to see the bubbles without them.

Bubbles start to appear immediately. I left mine for about an hour and a half, at which point the test tube on the negative terminal (the cathode) was completely full of gas, which produced a very satisfying squeaky pop when I placed it over a flame.

The positive electrode (the anode) ended up completely covered in what I’m pretty sure is a precipitate of iron hydroxide (the drawing pins presumably being plated steel), which meant that very little oxygen was produced after the first couple of minutes. This is why in proper electrolysis experiments inert graphite or, even better, platinum, electrodes are used. If you do that, you’ll get a 1:2 ratio by volume of oxygen to hydrogen, thus proving water’s formula (H2O) as well.

So there we have it: water is a compound, and not an element. And if you’d like to amuse everyone around the Christmas dinner table, you can prove it with a 9V battery and some drawing pins. Just don’t nick the battery out of your little brother’s favourite toy, okay? (Or, if you do, don’t tell him it was my idea.)


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Why is chemistry the forgotten science?

I recently had the privilege of talking to radio DJ and author Simon Mayo and he asked me what I thought of his book, Itch.  I said I loved it, and I really do.  (I have yet to read the sequel, Itch Rocks – released at the end of February – but it’s definitely on my list.)  I like Itch for many reasons.  I liked it because the lead character is a teenage boy who’s interested in science and actually finds arty subjects rather difficult, and yet is not a nerdy stereotype.  I like it because there was lots of action and an interesting story, coupled with just the right amount of research.  I liked it because the main female character is strong-willed, principled and absolutely doesn’t get involved in any sort of love triangle (this is not, to paraphrase my favourite film, ‘a kissing book’).  And most of all, I like it because it’s science fiction about chemistry.chemistry

As a chemist, it’s long seemed to me that, when it comes to the media and fiction, it’s the forgotten science. I can think of any number of famous science fiction works that hinge around physics and astronomy.  I can think of things based on biology.  I can even recall one or two that have both, for example Christian Cantrell’s Containment, a novel about a brilliant young scientist living on Venus and working on artificial photosynthesis.  But when it comes to chemistry I’m struggling.  Poisoning turns up in quite a few murder mysteries of course, as does forensics.  I suppose you could argue that some of the medical thrillers with plots that hinge around drugs might count.   Nanotechnology, as in Prey by Michael Crichton, is often thought of as a chemical field in the real world (TM), but thrillers on the subject tend to be less about matter on the atomic scale and more about improbably aggressive tiny robots.

It’s not just fiction.  In recent years there has been a noticeable increase in the amount of science programming, particularly on the BBC.  This is fabulous, but the large majority has been focused on physics and biology.  Radio 4’s The Infinite Monkey Cage often takes great glee in ignoring, and even ridiculing, chemical disciplines (I still listen to it mind you, in the manner of someone poking at a sore tooth).  The current run of BBC’s Horizon has exactly one episode (The Truth about Taste) that might be considered to have a chemistry focus.  At the end of last year Dara O Briain’s Science Club managed a whole series of six episodes without a single one on a chemical topic.  And so on and so on.  At least the most recent Royal Institution Christmas Lectures redressed the balance a bit, even if they were tucked away on BBC Four.  And as I posted recently, the quiz show Pointless seems to be quite fond of chemistry as a topic, so that’s something.

But why the general lack of chemistry?  Especially when you consider that the A-level is not only desirable but an essential requirement for so many degrees, including medicine, veterinary science, dentistry and pharmacy.  Whereas physics and, perhaps more surprisingly, biology aren’t. Since it’s so important you’d imagine there would be a bit more enthusiasm for the subject.

Is it linked to the background of the presenters?  Dara O Briain, in a previous life, studied mathematics and theoretical physics.  Professor Brian Cox, presenter of the Infinite Monkey Cage, is of course a physicist.  The only regular presenter I can think of with anything resembling a chemistry degree (actually biochemistry) is Liz Bonnin of Bang Goes the Theory.  But surely it isn’t impossible to find a chemist capable of presenting?  Peter Wothers did a cracking job with the Royal Institution lectures for starters.  And surely, surely, there’s room for the fabulously eccentric-looking Martyn Poliakoff somewhere?  (Please go and look at The Periodic Table of Videos if you have five minutes – it’s brilliant.)

But I’m not sure that’s the problem.  I imagine presenters largely talk about what they’re told to talk about.  No, I fear it might be simply the fact that chemistry is a bit, well, hard.

Early in my teaching career an exasperated A-level student complained, “miss, I thought chemistry was all setting fire to things and explosions and stuff, but it’s mostly just numbers and symbols”.  I’m afraid there’s some truth to this, particularly by the time we get to A-level chemistry, although I do like to set fire to things wherever possible (in a controlled manner of course – I’m not an arsonist, I swear).

I often joke with students that chemists use equations because we’re lazy.  For example, take this very simple experiment that you probably do every day if you have a gas cooker – it’s what happens when you set fire to methane:

CH4 + 2O2 –> CO2 + 2H2O

Now let’s write that in words: One molecule of methane, which contains one carbon atom bonded to four hydrogen atoms, reacts with exactly two molecules of diatomic oxygen irreversibly to produce exactly one molecule of carbon dioxide, which contains one atom of carbon bonded to two oxygen atoms, and two molecules of water, which contains two atoms of hydrogen bonded to an oxygen atom. 

Phew.  You can see why chemists prefer the equation.  Imagine if we had to write something like that every time we wanted to describe a reaction?  We’d never get anywhere.  Plus, once you understand them, the equations allow you to see similarities between different reactions that could be easily missed otherwise.  The symbols are essential.  But they’re also a bit, well, impenetrable.  A TV show with lots of chemical symbols would be as impossible to understand as one presented in French for many, and rather more difficult to subtitle.

So yes, it can look a bit scary.  But it’s not impossible.  After all you need advanced mathematics to understand physics in depth, but plenty of physics programmes explain their subject matter without even hinting at the dreaded doublet of differentiation and integration.  A good chemist can make the subject accessible with a bit of creativity.

It’s not as if there’s not lots of interesting material (pun entirely intended).  Chemistry is the science behind explosives, cooking, medicines, bubbles, pigments and poisons.  It has a fascinating history, populated with characters such as Fritz Haber, the father of chemical warfare who also solved the problem of global food security, Glenn Seaborg who discovered ten (ten!) of those elements that loiter at the bottom of the periodic table, Henry Cavendish – discoverer of hydrogen and famously so shy he was unable to talk to women, Antoine Lavoisier, tax collector, traitor and the person who named both oxygen and hydrogen and let’s not forget Carl Wilhelm Scheele, discoverer of some of the most dangerous substances known to man.  There are endless stories that could be told, from the legal case of the Carbolic Smoke Ball to Kekule’s dreams of snakes eating their own tails, to bizarre medical practices such as antimony pills and the mystery of the Bradford Sweets poisoning.

If Simon Mayo can write a series of highly successful novels featuring chemistry aimed at young adults, it must surely be possible to make a few more shows on the topic.  So writers, editors and producters I beseech you not to be scared of chemistry.  Find yourself someone with a bit of knowledge in the area and get on with it.  For whatever chemistry is, it’s far from boring.

Do you know of any chemistry science fiction I’ve missed?  Have you got any favourite chemical stories that you think should be on telly?  Please tell me about them!