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.
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.
The 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.
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.
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…
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?
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.
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.
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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How would you describe dissolving of ionic compounds or non-acidic covalent compounds such as sucrose or ethanol? In an ionic compound, ionic bonds are being broken, but the fundamental chemical properties are unchanged. With the covalent compound, the intermolecular forces that hold the molecules in the solid are broken (much like melting) and no changes in chemical properties occur. These are the specific subtleties that make it difficult for me to teach simple change of state vs. everything else.
Dissolving isn’t a change of state: when you dissolve substances, there’s an associated energy change (enthalpy of solution) due to solvent-solute, and solute-solute, interactions. At the end, we have a different substance with different physical properties. It’s not even necessarily simple to reverse: if you mix two different salts with water so the ions mingle, can you get them back as separate substances? If I had to force ‘dissolving’ into a box, I’d call it a chemical change (contrary to most courses and text books!). But that’s why I don’t like these labels.
You can’t use enthalpy change as the determinant for a chemical reaction, though, since there are specific enthalpies for all phase changes. A solution is not a pure substance, so of course there are different physical properties; its formation is also relatively reversible, if you keep the system simple–a single salt in water. Chemical properties are also unchanged, as I demonstrate by reacting Al with aqueous CuCl2 and with solid CuCl2, the latter simply proceeding more slowly due to decreased surface area. I’m pushing on this not because I’m trying to split hairs–we’ve been discussing exactly this issue lately and really want to come up with a simple explanation that can’t be picked apart by a determined student.
A simple explanation for what? That a chemical reaction has occurred? I think the problem is trying to force all processes into one box or another. I’m going to go one further than I did in my post and say stop doing that. Explain the process that’s happening. Provide an equation. Move on. There’s no NEED to make students label things as chemical reactions or not. Debating whether dissolving an ionic compound is a chemical change achieves precisely nothing. It certainly doesn’t help students to understand the process. Simple phase changes are worth identifying, I think — otherwise, let’s just stop worrying about it 🙂
I’ve debated this topic in my head over and over… my personal convictions are that, firstly, the classification of physical and chemical changes is only useful at a very early age, when kids are learning what the identity of a chemical means – there really is no benefit to the classification beyond that point. Secondly, dissolution and precipitation can be treated as physical changes (for substances whose chemical structures remain in tact, e.g., glucose) or as chemical changes (for substances that exhibit significant structural and/or energetic changes, e.g., ionic substances). Thirdly, I agree that we’d be much better off if we taught students specific processes, and encouraged them to compare and contrast them, rather than taught poorly defined categories and expected students to make binary assignments.
I agree, except that I’d avoid categorising dissolution and precipitation at all early on — perhaps just saying that they fall somewhere in between, we’ll come back to them later, and leaving it there.
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