Bondage

I’ve been part of a few Twitter discussions about bonds and energy. Several people have made insightful observations that have really pushed everyone’s thinking. It is brilliant when a bunch of science teachers get together and really wrestle with the potential conflict between accurate models in science, and how to make these conceptually difficult ideas accessible to children. We usually find that just pinning down the nitty-gritty science is hard enough, never mind teaching it to beginners.

This was how the most recent started.

Bloody good question

I’m no wiser than the next science teacher but have thought quite hard about what energy is and how to teach it, and I’ve been lucky enough to be in conversations with some very astute chemistry specialists. This is an attempt to write something down because my poor little brain otherwise tends to go round in circles.

@DrHSkelton has written a magnificent set of blog posts describing her approach to reviewing the particle model of solids, liquids, and gases, before teaching ionic, metallic and covalent bonding at GCSE. I’m in total agreement with Helen that the way to do this is to start with something concrete – the macroscopic properties of a material – and build the abstract theory from there – the sub-microscopic behaviour of the particles. I think that’s pretty much the right approach for all KS3-4 science teaching, although I’ve certainly done it the other way round many times.

Carefully considered diagram from Helen Skelton

I know @hecharden has been digging into the literature on misconceptions around bonding for the BEST teaching resources and I expect the 14-16 publication on this will be really useful, when it’s finished.

The difficulty with all this seems to me to be twofold. Firstly, the nature of these bonds between particles. Secondly, what causes those bonds to break (and how to keep track of this process). The third issue is how best to teach it.

The first point is that all ‘bonds’ – whether they are in a basic KS3 particle model, or post-16 chemistry – are just an electrostatic force. That’s good because it’s easy to think that something like a covalent bond is qualitatively different to the bonds holding the particles in a fixed position in a solid. They’re not. In some solids those bonds actually are covalent e.g. silicon dioxide, but when they are not e.g. hydrogen bonds in ice, they are still just an example of electrostatic attraction. The mechanism is different but the nature of the force is the same. That’s helpful and I sense a move towards emphasising this in teaching it. I refer you back to Helen Skelton’s magnificant blog cited earlier.

The second point is that the breaking of bonds is entirely a mechanical* process i.e. uses ideas from physics**. Bonds are not broken by energy: activation, kinetic, or otherwise. They are broken by the movement of the particles concerned. The electrostatic force between particles decreases with distance. When you warm a solid, the particles move more. At the melting point of the solid a hefty number of particles are moving so much that they move so far away from their original neighbours that they are nearer to other particles and the new attraction is bigger than the old one so they move past each other to new positions. Keep heating and the collisions become stronger and stronger until a hefty number of particles move so much that they aren’t held by the electrostatic forces at all, and you have a gas. I really like the AtomScope 2 simulation because I think it shows this really nicely but PhET is pretty good too.

So why don’t we just teach it like this? Why do we end up with children saying things like “the bonds in a solid are stronger than the bonds in a liquid” and “in a liquid the particles have enough energy to move past each other” and “to melt a solid you have to put in enough energy to break the strong bonds holding the particles in fixed positions” and “gas pressure increases when a gas is heated because the particles have more energy so move around faster”? Even BBC Bitesize says “The attractive forces (bonds) in a liquid are strong enough to keep the particles close together, but weak enough to let them move around each other.”

There is a very good reason for using energy to talk about heating things up and breaking bonds. If you want to know how much hotter a substance is, or how much faster particles are moving, or whether or not a chemical reaction will happen, then you need to do an energy calculation. As we do more science, we inevitably move from the qualitative to the quantitative, and we end up always talking about energy to do that. However, energy then creeps back down into our causal explanation and we get so used to saying “energy breaks bonds” that we forget (a) that a bond is just an electrostatic force and, (b) that energy is just a calculation tool; it doesn’t break anything.

That’s as far as I’m going to get tonight. I need to do some more thinking about whether there is a way to navigate through all this that will actually help children learning science as well as satisfy the grown-up experts. There may well not be. Life is a compromise. In the end, we all just make the universe slightly warmer.

*Well, maybe there’s some quantum mechanical stuff going on.

**At least I think that’s right but then I am a physicist and not even a particularly good one at that, so I might be wrong.

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