Tag: particle theory

Are the particles in all solids the same?

Particle intuitions may not match scientific models

Keith S. Taber

Sophia was a participant in the Understanding Science Project. I first talked to her when she was in Y7, soon after she began her secondary school course.

One of the first topics she studied in her science was 'solids, liquids and gases', where she had learnt,

that solids are really hard and they stay together more, and then liquids are close together but they move around, and gases are really free and they just go anywhere

She had studied a little about the topic in her last year of primary school (Y6), but now she was being told

about the particles…the things that make – the actual thing, make them a solid, and make them a gas and make them a liquid

Particle theory, or basic kinetic theory, is one of the most fundamental theories of modern science. In particular, much of what is taught in school chemistry is explained in terms of theories involving how the observed macroscopic properties emerge from the characteristics and interactions of conjectured sub-microscopic particles that themselves often have quite unfamiliar properties. This makes the subject very abstract, challenging, and tricky to teach (Taber, 2013a).

Read about conceptions of atoms

Particle theory is often introduced in terms of the states of matter. Strictly there are more than three states of matter (plasma and Bose-Einstein condensates are important in some areas of science) but the familiar ones, and the most important in everyday phenomena, are solid, liquid and gas.

The scientific account is, in simple terms, that

  • different substances are made up of different types of particle
  • the different states of matter of a single substance have the same particles arranged differently

These are very powerful ideas, even if there are many complications. For example,

  • the terms solid, liquid and gas only strictly apply to pure samples of a single substance, not mixtures (so not, for example, to bronze, or honey, or, milk, or ketchup, or even {if one is being very pedantic} air or sea water. And cats (please note, BBC) are completely inadmissible. )
  • common salt is an example of a pure substance, that none-the-less is considered to be made up of more than one type of particle

This reflects a common type of challenge in teaching science – the full scientific account is complex and nuanced, and not suitable for presenting in an introductory account; so we need to teach a simplified version that introduced the key ideas, and then only once this is mastered by learners are they ready to develop a more sophisticated understanding.

Yet, there is a danger that students will learn the simplified models as truths supported by the authority of science – and then later have difficulty shifting their thinking on. This is not only counter-productive, but can be frustrating and de-motivating for learners who find hard-earned knowledge is not as sound as they assumed.

One response to this is to teach science form very early in a way that is explicit about how science builds models of the natural world: models that are often simplifications which are useful but need to be refined and developed to become powerful enough to expand the range of contexts and examples where they can be applied. That is, students should learn they are being taught models that are often partial or imperfect, but that is just a reflection of how science works, developing more sophisticated understanding over time (Taber, 2017).

Sophia confirmed that the iron clamp stand near where she was sitting would have particles in it, as would a lump of ice.

Are they the same particles in the ice as the iron?

Yeah, because they are a solid, but they can change.

Ah, how can they change?

Cause if, erm, they melted they would be a liquid so they would have different particles in.

Right, so the iron is a solid, 

Uh hm.

So that's got one type of particle?

Yeah.

And ice is also a solid?

Yeah.

So that has the same sort of particles?

Yeah, but they can change.

The ones in the ice?

Mm,

To a learner just meeting particle theory for the first time, it may seem just as feasible that the same type of particle is found in one state as in one substance.

In the scientific model, we explain that different substances contain different types of particles, whereas different states of the same substance contain different arrangements of the same particles: but this may not be intuitively obvious to learners.1 It seemed Sophia was thinking that the same particles would be in different liquids, but a change of state led to different particles. This may seem a more forced model to a teacher, but then the teacher is already very familiar with the scientific account, and also has an understanding of the nature of those particles (molecules, ions, atoms – with internal structure and charges that interact with each other within and between the particles) – which are just vague, recently imagined, entities to the novice.

Sophia seemed to misunderstood or misremembered the model she had been taught, but to a novice learner these 'particles' have no more immediate referent than an elf or an ogre and would be considerably more tenuous than a will-o'-the-wisp.

Sophia seemed to have an alternative conception, that all solids have one type of particle, and all liquids another. If I had stopped probing at that point I might have considered this to be her thinking on the matter. However, when one spends time talking to students it soon becomes clear that often they have ideas that are not fully formed, or that may be hybrids of different models under consideration, and that often as they talk they can talk themselves into a position.

So, if I melted the ice – that changes the particles in the solid?

Well they are still the same particles but they are just changing the way they act…

Oh.

How do they change?

A particle in a liquid [sic, solid] is all crammed together and don't move around, but in a liquid they can move around a little but they are still close and, can, you can pour a liquid, where you can't a solid, because they can move in. 

Okay, so if I have got my ice, that's a solid, and there are particles in the ice, and they behave in a certain way, and if the ice melts, the particles behave differently?

Yeah.

Do you know why they behave differently in the liquid?

No. {giggles} So, they can, erm

• • • • • • • • • • • •  [A pause of approximately 12 s]

They've more room cause it's all spread out more1, whereas it would be in a clump

The literature on learners conceptions often suggests that students have this or that conception, or (when survey questions are used) that this percentage thinks this, and that percentage thinks that (Taber, 2013b). That this is likely to be a simplification seems obvious is we consider what thinking is – whatever thought may be, is it a dynamic process, something that moves along. Our thinking is, in part, resourced by accessing what we have represented in memory, but it is not something fixed – rather something that shifts, and that often becomes more sophisticated and nuanced as we explore a focus in greater depth.

I think Sophia did seem to have an intuition that there were different types of particles in different states of matter, and that therefore a change of state meant the particles themselves changed in some way. As I probed her, she seemed to shift to a more canonical account where change of state involved a change in the arrangement or organisation of particles rather than their identity.

This may have simply been her gradually bringing to mind what she had been taught – remembering what the teacher had said. It is also possible that the logic of the phenomenon of a solid becoming a liquid impressed on her that they must be the same particles. I suspect there was a little of both.

When interviewing students for research we inevitably change their thinking and understanding to some extent (hopefully, mostly in a beneficial way!) (If only teachers had time to engage each of their students in this way about each new topic they might both better understand their students' thinking, and help reinforce what has been taught.)

Did Sophia 'have a misconception'? 1 What did she 'really think'? That, surely, is to oversimplify.

She presented with an alternative conception, that under gentle questioning she seemed to talk /think herself out of. The extent to which her shift in position reflected further recall (so, correcting her response) or 'thinking through' (so, developing her understanding) cannot be known. Likely there was a little of both. What memory research does suggest is that being asked to engage in and think about this material will have modified and reinforced her memories of the material for the future.

Read about the role of memory in teaching and learning

Work cited:
Note

1 Actually, the particles in a liquid are not substantially spread further apart than in a solid. (Indeed, when ice melts the water molecules move closer together on average.) Understanding melting requires an appreciation of the attractions between particles, and how heating provides more energy for the particles. This idea of increased separation on melting is therefore something of an alternative conception, if one that is sometimes encouraged by the diagrams in school textbooks.

Teaching an introductory particle theory based on the arrangement of particles in different states, without reference to the attractions between particles is problematic as it offers no rational basis for why condensed states exists, and why energy is needed to disrupt them – something highlighted in the work of Philip Johnson (2012).