Let’s change the way we talk about changes

It’s nearly the end of the school year here in the U.K., traditionally a time for reflecting on what’s gone before and planning ahead for the shiny, new September coming in a mere nine weeks (sorry, teachers!). With that in mind, let’s talk about something that comes up early in most chemistry syllabuses, and which bothers me a little more each time I think about it.

Chemical reactions occur when a match burns.

It’s the concept of chemical and physical changes. For those who aren’t familiar, this is the idea that changes we observe happening to matter fall into two, broad categories: chemical changes, where new substances are made, and physical changes, where no new substances are made.

Examples of chemical changes include things like burning a match, cooking an egg, or the reaction between vinegar and baking soda. Physical changes are largely changes of state, such as melting and boiling, but also include changes such as dissolving salt in water, or grinding limestone chips to powder.

So far, so good. Except… then we start to put descriptors on these things. And that’s when the trouble starts.

multiple choice exam questionThe first problem comes with the idea that “chemical changes are irreversible.” This is often taught in early secondary science as a straight-up fact, and is so pervasive that it’s even appeared in multiple choice exam questions, like the one shown here. The student, for the record, was expected to choose option C, “the change is irreversible.”

Except. Argh. I can tell you exactly why the student has opted for D, “the change is reversible,” and it’s not because they haven’t done their revision. Quite the opposite, in fact. No, it’s because this student has learned about weak acids. And in learning about acids, this student met this symbol, ⇌, which literally indicates a reversible chemical reaction.

Yes, that’s right. Not too long after teaching students that chemical reactions are not reversible, we then explicitly teach them that they are. Indeed, this idea of chemical reversibility is such a common one, such an important concept in chemistry, that we even have a symbol for it.

Now, of course, I can explain this. When we say chemical reactions are irreversible, what we mean is “generally irreversible if they’re carried out in an open system.” In other words, when the wood in that match burns out in the open, the carbon dioxide and water vapour that form will escape to the atmosphere, never to return, and it’s impossible to recover the match to its original state.

The problem is that many chemical reactions occur in closed systems, not least a lot of reactions that happen in solution. Hence, the whole acids thing, where we talk about weak acids “partially dissociating” into ions.

Then there’s that entire topic on the Haber process…

Can I be the only one to think that this is rather a lot of nuance to expect teenagers to keep in their head? It’s nothing short of confusing. Should we really be saying one thing in one part of a course, and the literal opposite in another? To be clear, this isn’t even a GCSE vs. A level thing – these ideas appear in the same syllabus.

Melting is a change of state, in this case from (solid) ice to (liquid) water.

All right, okay, let’s move along to the idea that physical changes are reversible. That’s much more straightforward, isn’t it? If I melt some ice, I can re-freeze it again? If I boil some water, I can condense it back into the same volume of liquid… well… I can if I collect all vapour. If I do it in a closed system. The opposite of the condition we imposed on the chemical reactions. Er. Anyway…

We might just about get away with this, if it weren’t for the grinding bit. If physical changes are truly readily reversible, then we ought to be able to take that powder we made from the limestone lumps and make it back into a nice single piece again, right? Right?

See, this is the problem. What this is really all about is entropy, but that’s a fairly tricky concept and one that’s not coming up until A level chemistry.

Okay. Instead of talking about reversible and irreversible, let’s talk about bond-breaking and bond-forming. That’s fine, isn’t it? In chemical changes, bonds are broken and formed (yep) and in physical changes, they aren’t.

Except….

Let’s go back to water for a moment. Water has the formula H2O. It’s made up of molecules where one oxygen atom is chemically bonded to two hydrogen atoms. When we boil water, we don’t break any of those bonds. We don’t form hydrogen and oxygen gas when we boil water; making a hot cup of tea would be a lot more exciting if we did. So we can safely say that boiling water doesn’t involve breaking any bonds, right? We-ell…

Water molecules contain covalent bonds, but the molecules are also joined by (much weaker) hydrogen bonds.

The trouble is that water contains something called hydrogen bonds. We usually do a bit of a fudge here and describe these as “intermolecular forces,” that is, forces of attraction between molecules. This isn’t inaccurate. But the clue is in the name: hydrogen bonds are quite, well, bond-y.

When water boils, hydrogen bonds are disrupted. Although the bonds in individual H2O molecules aren’t broken, the hydrogen bonds are. Which means… bonds are broken. Sort of.

But we’re probably on safe ground if we talk about the formation of new substances. Aren’t we?

Except….

What about dissolving? If I dissolve hydrogen chloride gas, HCl, in water, that’s a physical change, right? I haven’t made anything new? Or… have I? I had molecules with a covalent bond between the hydrogen and the chlorine, and now I have… er… hydrochloric acid (note, that’s a completely different link to the one I used back there), made up of H+ and Cl- ions mingled with water molecules.

So… it’s…. a chemical change? But wait. We could (I don’t recommend it) evaporate all that water away, and we’d have gaseous HCl again. It’s reversible.

Solid iodine is silvery-grey, but iodine vapour is a brilliant violet colour.

Hm. What about the signs that a chemical change is occurring? Surely we’re all right there? Fizzing: that’s a sign of a chemical change. Except… are you sure you know the difference between boiling and fizzing? It’s basically all bubbles, after all. Vapour? But, steam is a vapour, isn’t it? Although, on the other hand, water is a product of several chemical reactions. Colour changes? Check out what happens when you heat a small amount of solid, silvery-grey iodine so that it sublimes (spoiler: there’s a colour change).

Is anyone else really confused by now?

You should be. Your students almost certainly are.

There are, in short, more exceptions to every single one of these rules that there are for that “i before e” thing you learned in English (a rule, incidently, which is particularly galling for scientists who constantly have to deal with weights and heights).

Where do we go from here? I think it’s probably time we asked ourselves why we’re even teaching this concept in the first place. Really, it’s there to get students to think about the difference between changes of state and chemical reactions.

I suspect we need to worry about this rather less than we are: most children are very good at identifying changes of state. They do it instinctively. They only start getting confused about it when we teach them a lot of rules which they then try to apply. I’m pretty sure that’s not the way teaching is supposed to work.

A complicated arrangement of chemical glassware

This could definitely be simpler.

If I had my way, I’d ditch the physical and chemical change labels altogether and, instead, just talk about changes of state and chemical reactions. There is precisely one differentiator between these two, and it is: have we made any new stuff? If the answer is no, it’s a change of state. If the answer is yes, then a chemical reaction has occurred. Job done. (And yes, this would squarely define gaseous hydrogen chloride dissolving in water to form hydrochloric acid as a chemical reaction, and I have no problem at all with that.)

I say we change the way we talk about changes: chemistry has a reputation for being tricky, and this sort of confusing, contradictory thing is part of the reason why.


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Chemical catastrophes – who were the biggest baddies of chemistry’s past?

As a big fan of chemistry I like to encourage students to believe that it will be a huge force for good in the future, providing us with solutions to problems such as sustainable energy, currently incurable diseases and new materials.  And I hope I’m right about this.  But there’s no escaping the fact that chemistry has a dark, dirty and dangerous past.  In the days before health and safety – oh we take the mickey, but trust me you wouldn’t actually want to be without it – proper regulations and rigorous testing, chemists threw dangerous chemicals around like sweeties.  Quite literally in some cases.  They tasted and smelled toxic and dangerous substances and, worse, they released them on an unsuspecting population with barely a second thought.

baddySo with that in mind, who are my top three biggest baddies of chemistry’s past?

Fritz Haber (1868 – 1934)
The German chemist Fritz Haber gets the number three slot.  In some ways, he’s a bit of double-edged sword.  He did good along with the bad, inventing – along with Carl Bosch – the Haber Bosch process for making ammonia.  No matter what your feelings about inorganic fertilisers, we have to accept that without them we wouldn’t be able to feed the population of this country, let alone the world.  There just isn’t that much pooh out there.  Some people would argue that the population rise Haber’s process facilitated has been a disaster in itself.  But this is to conveniently forget that, had he not developed it, they probably wouldn’t be here to complain about it.

Haber wasn’t entirely a misunderstood genius though.  He’s also been described as the father of chemical warfare for his work on the use of chlorine and other poisonous gases during World War I.  His work included the development of gas mask filters, but he also led teams developing deadly chlorine gas used in trench warfare.  He was even there to supervise its release.  He was a patriotic German and believed he was doing the right thing, supporting his country in the war effort.  During the second world war Haber’s skills were initially sought out by the Nazis, who offered him special funding to continue his work on weapons.  However Haber was Jewish, so in common with other scientists in a similar position he ended up leaving Germany in 1933.

Famously, Haber’s first wife disagreed vehemently with his work on chemical warfare.  In fact, perhaps unable to cope with the fact that he had personally overseen the successful use of chlorine in 1915, she committed suicide by shooting herself with his service revolver.  That same morning, Haber left again to oversee gas release against the Russians, leaving behind his grieving 13 year-old son.

Haber was awarded the Nobel prize for Chemistry in 1919 for his work on the Haber Process.  So the story goes, other scientists at the ceremony refused to shake his hand in protest at his work with chemical weapons.  A tragic story all round.

Carl Wilhelm Scheele (1742 – 1786)
We’ve seen Scheele’s name come up before of course.  In his short – thanks to his bad habit of tasting and sniffing toxic chemicals – life he made a lot of chemical discoveries, but didn’t get the recognition for many of them because he always seemed to publish after someone else.  The ones he is remembered for always seem to be the horribly dangerous ones (maybe no one else wanted the credit).  For example he discovered hydrogen cyanide (a poison beloved of many an Agatha Christie villain, hydrogen fluoride (a highly toxic gas that forms the incredibly dangerous hydrofluoric acid when dissolved in water) and hydrogen sulfide (toxic, highly flammable and stinks of rotten eggs).

But his most harmful contribution to the world was undoubtedly Scheele’s Green, the arsenic-based yellow-green dye that was used to colour fabrics, paints, candles, toys and even, most tragically of all, foodstuffs in the 1800s.  It’s impossible to count but it was undoubtably responsible, directly and indirectly, for huge numbers of deaths in the 19th century.  Essentially he invented a deadly poison that ended up in thousands of homes all around the world.  Aren’t you glad we have safety testing these days?

Thomas Midgely (1889-1944)
Who was worse, Scheele or Midgely?  It’s a tough call, but I think Midgely takes it, particularly because he had some inkling exactly how damaging at least some of his work might turn out to be.

Midgely is famously responsible for synthesising the first CFC, freon.  CFCs, or chlorofluorocarbons, are neither toxic nor flammable, so were considered much safer than other propellants and refrigerants used at the time.  In fact, he was even awarded the Perkin Medal in 1937 for his work.  This, of course, was some time before the terrible consequences of CFCs were realised.  As we now know, they turned out to very damaging to the ozone layer, and in 1989 twelve European Community nations agreed to ban their production, and they have since been phased out across the world.

Although CFCs were a disaster, Midgely could at least be defended for having no way of knowing how disastrous they would ultimately turn out to be.  Not so for his other famous invention.  Whilst working at General Motors he discovered that adding tetraethyllead, or TEL, to petrol (aka gasoline) prevented ‘knocking‘ in internal combustion engines, which is when the air/fuel mixture ignites at slightly the wrong time.  Knocking makes the engine much less efficient, and so preventing it was a big issue.  You’d think Midgely might have accepted that lead in petrol was a bad idea when he had to take a vacation to recover from severe lead poisoning, but no.  In fact he appeared to have been pretty cynical about the whole thing, pouring TEL over his hands at a press conference in 1924 to demonstrate its apparent safety (its not, and he had to take more time off afterwards to recover).

Unfortunately burning fuel with TEL in it disperses lead into the air where it’s readily inhaled by innocent bystanders, and it’s particularly harmful to children.  Lead exposure has been linked to low IQ and antisocial behaviour, and recently researchers suggested that the ban on leaded petrol across the world in the early 2000s might now be leading to a reduced crime rate.

So for knowingly poisoning people worldwide with lead, and unknowingly taking out a chunk of the ozone layer, Midgely gets my award as biggest chemical baddy.

Would you pick someone else?