learning impediments - Science-Education-Research

Single bonds are different to covalent bonds

Single bonds are different to covalent bonds or ionic bonds

Keith S. Taber

Annie was a participant in the Understanding Chemical Bonding project. She was interviewed near the start of her college 'A level' course (equivalent to Y12 of the English school system). Annie was shown, and asked about, a sequence of images representing atoms, molecules and other sub-microscopic structures of the kinds commonl y used in chemistry teaching. She was shown a representation of the resonance between three canonical forms of BF3, sometimes used as away of reflection polar bonding. She had just seen another image representing resonance in the ethanoate ion, and had suggested that it contained a double bond. She had earlier in the interview referred to covalent bonding and ionic bonding, and after introducing the ideas of double bond, suggested that a double bond is different to a covalent bond.

What about diagram 14?…
Oh.
(pause, c.13s)
Seems to be different arrangements. Of the three, or two elements.
Uh hm.
(pause, c.3s)
Which are joined by single bonds.
What, where, what single, what sorry are joined by single bonds?
All the F to the B to the F. Are single bonds they are not double like before. [i.e., a figure discussed earlier in the interview]
So are they covalent bonds? Or ionic bonds, or? Or are single bonds something different again?
Single bonds are different.

This reflected her earlier comment to the effect that a double bond is different to a covalent bond, suggesting that she did not appreciate how covalent bonds are considered to be singular or multiple.

However, as I checked what she was telling me, Annie's account seemed to shift.

They're different to double bonds?
Yeah.
And are they different to covalent bonds?
No 'cause you probably get covalent bonds which are single bonds.

So single bonds, just moments before said to different to covalent bonds, were now 'probably' capable of being covalent. As she continued to answer questions, Annie decided these were 'probably' just alternative terms.

So covalent bonds and single bonds, is that another word for the same thing?
Yeah, probably. But they can probably occur in different, things like in organic you talk about single bonds more than you talk about covalent, and then like in inorganic you talk about covalent bond, more than you talk about single bonding or double bonding.
So you think that maybe inorganic things, like sort of, >> copper iodide or something like that, that would tend to be more concerned with covalent bonds?
< Yeah. < Yeah.
But if you were doing organic things like, I don't know, erm, ethane, >> that's more likely to have single bonds in.
< Yeah. < Yeah.
So single bonds are more likely to occur in carbon compounds.
Yeah.
And covalent bonds are more likely to occur in some other type of compound?
Yeah. Sort of you've got different terminology, like you could probably use single bonds to refer to something in inorganic, but when you are talking about the structures and that, it's easier to talk about single bonds and double bonds, rather than saying that's got a covalent bond or that's got an ionic bond.

Annie's explanation did not seem to be a fully thought-out position. It was not consistent with the way she had earlier reported there being five covalent bonds and one double bond in an ethanoate ion.

It seems likely that in the context of the research interview, where being asked directly about these points, Annie was forced to make explicit the reasons she tended to label particular bonds in specific ways. The interview questions may have acted like Socratic questioning, a kind of scaffolding, leading to new insights. Only in this context did she realise that the single and double bonds her organic chemistry lecturer talked about might actually be referring to the same entities as the covalent bonds her inorganic chemistry lecturer talked about.

It would probably not have occurred to Annie's lecturers (of which, I was one) that she would not realise that single and double bonds were covalent bonds. It may well have been that if she had been taught by the same lecturer in both areas, the tendency to refer to single and multiple bonds in organic compounds (where most bonds were primarily covalent) and to focus on the covalent-ionic dissension in inorganic compounds (where degree of polarity in bonds was a main theme of teaching) would still have lead to the same confusion. Later in the interview, Annie commented that:

if I use ionic or covalent I'm talking about, sort of like a general, bond, but if I use double or single bonds, that's mainly organic, because sort of it represents, sort of the sharing, 'cause like you draw all the molecules out more.

This might be considered an example of fragmentation learning impediment, where a student does not make a link that the teacher is likely to assume is obvious.

Learning about natural selection and denying evolution

An ironic parallel

Keith S. Taber

I was checking some proofs for something I had written today* [Taber, 2017], and was struck by an ironic parallel between one of the challenges for teaching about the scientific theory of evolution by natural selection and one of the arguments put forward by those who deny the theory. The issue concerns the value of having only part of an integrated system.

The challenge of evolutionary change

One of the arguments that has long been made about the feasibility of evolution is that if it occurs by many small random events, it could not lead to progressive increases in complexity – unless it was guided by some sense of design to drive the many small changes towards some substantive new feature of ability. So, for example, birds have adaptations such as feathers that allow them to fly, even though they are thought to have evolved from creatures that could not fly. The argument goes that for a land animal to evolve into a bird there need to be a great many coordinated changes. Feathers would not appear due to a single mutation, but rather must be the result of a long series of small changes. Moreover, simply growing features would not allow an animal to fly without other coordinated changes such as evolving very light bones and changes in anatomy to support the musculature needed to power the wings.

The same argument can be made about something like the mammalian eye, which can hardly be one random mutation away from an eyeless creature. The eye requires retinal cells, linked to the optic nerve, a lens, the iris, and so on. The eye is an impressive piece of equipment which is as likely to be the result of a handful of random events, as would be – say, a pocket watch found walking on the heath (to use a famous example). A person finding a watch would not assume its mechanism was the result of a chance accumulation of parts that had somehow fallen together. Rather, the precise mechanism surely implies a designer who planned the constructions of the overall object. In 'Intelligent Design' similar arguments are made at the biochemical level, about the complex systems of proteins which only function after they have independently come into existence and become coordinated into a 'machine' such as a flagellum.

The challenge of conceptual change

The parallel concerns the nature of conceptual changes between different conceptual frameworks. Paul Thagard (e.g., 1992) has looked at historical cases and argued that such shifts depend upon judgements of 'explanatory coherence'. For example, the phlogiston theory explained a good many phenomena in chemistry, but also had well-recognised problems.

The very different conceptual framework developed by Lavoisier [the Lavoisiers? **] (before he was introduced to Madame Guillotine) saw combustion as a chemical reaction with oxygen (rather than a release of phlogiston), and with the merits of hindsight clearly makes sense of chemistry much more systematically and thoroughly. It seems hard now to understand why all other contemporary chemists did not readily switch their conceptual frameworks immediately. Thagard's argument was that those who were very familiar with phlogiston theory and had spent many years working with it genuinely found it had more explanatory coherence than the new unfamiliar oxygen theory that they had had less opportunity to work with across a wide range of examples. So chemists who history suggests were reactionary in rejecting the progressive new theory were actually acting perfectly rationally in terms of their own understanding at the time. ***

Evolution is counter-intuitive

Evolution is not an obvious idea. Our experience of the world is of very distinct types of creatures that seldom offer intermediate uncertain individuals. (That may not be true for expert naturalists, but is the common experience.) Types give rise to more of their own: young children know that pups come from dogs and grow to be adult dogs that will have pups, and not kittens, of their own. The fossil record may offer clues, but the extant biological world that children grow up in only offers a single static frame from the on-going movie of evolving life-forms. [That is, everyday 'lifeworld' knowledge can act as substantial learning impediment – we think we already know how things are.]

Natural selection is an exceptionally powerful and insightful theory – but it is not easy to grasp. Those who have become so familiar with it may forget that – but even Darwin took many years to be convinced about his theory.

Understanding natural selection means coordinating a range of different ideas about inheritance, and fitness, and random mutations, and environmental change, and geographical separation of populations, and so forth. Put it all together and the conceptual system seems elegant – perhaps even simple, and perhaps with the advantage of hindsight even obvious. It is said that when Huxley read the Origin of Species his response was "How extremely stupid not to have thought of that!" That perhaps owes as much to the pedagogic and rhetorical qualities of Darwin's writing in his "one long argument". However, Huxley had not thought of it. Alfred Russel Wallace had independently arrived at much the same scheme and it may be no coincidence that Darwin and Wallace had both spent years immersing themselves in the natural history of several continents.

Evolution is counter-intuitive, and only makes sense once we can construct a coherent theoretical structure that coordinates a range of different components. Natural selection is something like a shed that will act as a perfectly stable building once we have put it together, but which it is very difficult to hold in place whilst still under construction. Good scaffolding may be needed.

Incremental change

The response to those arguments about design in evolution is that the many generations between the land animal and the bird, or the blind animal and the mammal, get benefits from the individual mutations that will collectively, ultimately lead to the wing or mammalian eye. So a simple eye is better than no eye, and even a simple light sensitive spot may give its owner some advantage. Wings that are good enough to glide are useful even if their owners cannot actually fly. Nature is not too proud to make use of available materials that may have previously had different functions (whether at the level of proteins or anatomical structures). So perhaps features started out as useful insulation, before they were made use of for a new function. From the human scale it is hard not to see purpose – but the movie of life has an enormous number of frames and, like some art house movies, the observer might have to watch for some time to see any substantive changes.

A pedagogical suggestion – incremental teaching?

So there is the irony. Scientists counter the arguments about design by showing how parts of (what will later be recognised as) an adaptation actually function as smaller or different advantageous adaptations in their own right. Learning about natural selection presents a situation where the theory is only likely to offer greater explanatory coherence than a student's intuitive ideas about the absolute nature of species after the edifice has been fully constructed and regularly applied to a range of examples.

Perhaps we might take the parallel further. It might be worth exploring if we can scaffold learning about natural selection by finding ways to show students that each component of the theory offers some individual conceptual advantages in thinking about aspects of the natural world. That might be an idea worth exploring.

(Note. 'Representing evolution in science education: The challenge of teaching about natural selection' is published in B. Akpan (Ed.), Science Education: A Global Perspective. The International Edition is due to be published by Springer at the end of June 2016.)

Notes:

* First published 30th April 2016 at http://people.ds.cam.ac.uk/kst24/

** "as Madame Lavoisier, Marie-Anne Pierrette Paulze, was his coworker as well as his wife, and it is not clear how much credit she deserves for 'his' ideas" (Taber, 2019: 90). Due to the times in which they works it was for a long time generally assumed that Mme Lavoisier 'assisted' Antoine Lavoisier in his work, but that he was 'the' scientist. The extent of her role and contribution was very likely under-estimated and there has been some of a re-evaluation. It is known that Paulze contributed original diagrams of scientific apparatus, translated original scientific works, and after Antoine was executed by the French State she did much to ensure his work would be disseminated. It will likely never be know how much she contributed to the conceptualisation of Lavoisier's theories.

*** It has also been argued (in the work of Hasok Chang, for example) both that when the chemical revolution is considered, little weight is usually given to the less successful aspects of Lavoisier's theory, and that phlogiston theory had much greater merits and coherence than is usually now suggested.

Sources cited:

Taber, K. S. (2017). Representing evolution in science education: The challenge of teaching about natural selection. In B. Akpan (Ed.), Science Education: A Global Perspective (pp. 71-96). Switzerland: Springer International Publishing
Taber, K. S. (2019). The Nature of the Chemical Concept: Constructing chemical knowledge in teaching and learning. Cambridge: Royal Society of Chemistry.
Thagard, P. (1992). Conceptual Revolutions. Oxford: Princeton University Press.

Magnets are not much to do with electricity

Keith S. Taber

Physicists see electromagnetism as one of the fundamental forces in the universe, and physics often includes a topic or module on 'electricity and magnetism'. Magnetism can be considered an electrodynamic effect (i.e., due to the movement of charges), but this will not be obvious to students.

Sophia was a participant in the Understanding Science Project. I spoke to here in Y7 (of the English school system) when she told me about the things she had been learning in the topic of electricity. I asked her,

Anything else you've done on electricity?
The er, I don't know what, it's not that much to do with electricity but, yesterday or the day (before) we done magnets.
Oh right. So that's a new topic, is it, not to do with electricity, or?
Well, I think we're still doing electricity. I don't know if it was just something – so we know what might, er, so we know what, what electricity will flow through, and maybe it's something to do with – 'cause magnets like stick to other things, they might be – I'm not sure, I think we might just have had a break from it, I don't know, but.

So, Sophia came up with some suggestions for why magnets might be featured in the electricity topic, but she was not very convinced about this rationale, and considered it was quite possible that the teacher was just interspersing other material to give a 'break' from the main topic. So, instead, they "done magnets".

It is interesting that one of Sophia's suggestions was "what electricity will flow through". The constructivist theory of learning ( read about constructivism here) suggests that meaningful learning involves learners making sense of what they are taught by linking it to their existing ideas and wealth of past experiences. This is a creative process, and sometimes students make unhelpful associations, that can act as learning impediments. Although ceramic magnets are increasingly common, iron, a good conductor, and its alloys, are still used for bar and horseshoe magnets that children will often be familiar with – so this association has potential to be built on constructively.

Of course electricity and magnetism were at one time considered quite distinct phenomena by scientists – and James Clerk Maxwell is rightly remembered for his synthesising theoretical work showing that electricity, magnetism and light could all be understood as manifestations of a single underlying 'phenomenon' of electromagnetism. (Indeed it seems stretching then notion of phenomena to refer to electromagnetism as a single phenomenon, as no one would intuitively perceive its manifestations as being observations of the same phenomenon!) We can hardly expect students to appreciate why electricity and magnetism might be considered a unitary physics topic in school science.

To the science teacher, magnetism is an electrical effect, and electromagnetism is one of the fundamental forces in nature. The unification of electricity, magnetism, and electromagnetic radiation is seen as a major integrative step forwards in science–but our students are not going to see the connections without some help.
Taber, 2014, p.169

When I asked her to tell me what she learnt about magnets she told me that the north pole and the south poles go together because one of them is coming out and one is going in.

Light bounces off the eye so you can see

Light is actively bounced out of the eye towards objects, so we can see

Keith S. Taber

Sophia was a participant in the Understanding Science Project. Y8 pupil Sophia had been studying sound and light in her school science lessons. Her model of sight involved light entering the eye, but then reflecting out again.

Do you know how you hear and see?
Does the light come in your eye, and it reflects off so you can see. … Just reflects, does it, and bounces on.
So if I've got my eye, some light comes in, some light comes in, what does it do, it bounces where?
About.
Inside the eye?
No around … it bounces out.
And then what?
Then you can see…so you look where you want to see, so it bounces off like in that direction, …you've got to actually look over, …you've got to look that way.

One long-established historical model of sight was based on rays coming from the eyes to detect objects in the outside world. Sophia's model appears to be a hybrid of this historical model, and modern understandings. For Sophia, light does not originate form the eye, but bounds out of it towards the object of sight. The idea that something must come out of the eye for us to see seems to be an intuitive assumption some people make – perhaps because we actually turn out heads and direct out eyes at what we want to focus on. This intuition has potential to act as a grounded learning impediment to learning the scientific model for vision.

Iron turning into a gas sounds weird

Keith S. Taber

Amy was a participant in the Understanding Science Project. She was interviewed when she had just started her 'A level' (i.e. college) chemistry, and one of the topics that the course had started with was mass spectrometry – (see A dusty analogy – a visual demonstration of ionisation in a mass spectrometer). Amy seemed to be unconvinced, or at least surprised by a number of aspects of the material she had learnt about the mass spectrometer.

So, for example, she found it strange that iron could be vaporised:

So which bits of that are you not convinced about then?
(Pause, c.3 seconds)
It just all … I don't, it's not that I'm not convinced about it, it's just sound strange, because it's like…
(Pause, c.2s)
… erm, well this sounds like ridiculous but, like but before today like none of the people in out class had thought about iron being turned into a gas, and it's little things like that which sound weird.
Okay, erm so if you said to people, can you turn water into a gas, most people would say.
Yeah.
Yeah, do it in the kettle all the time, sort of thing.
Yeah.
But if you said to people can you turn iron into a gas? – do people find that a strange idea?
Yeah.
Yeah?
Well we did. (She laughs)

Although Amy and her classmates had studied the states of matter years earlier at the start of secondary school, and would have learnt that substances can commonly be converted between solid, liquid and gaseous phases, their life-world (everyday) experience of iron – the metallic material – made the idea of iron vapour seem 'weird'.

Given the prevalence of grounded learning impediments where prior learning interferes with new learning, this did not seem as "ridiculous" to the interviewer as Amy suspected it may appear.

As science teachers we have spent many years thinking in terms of substances, and the common pattern that a substance can exist as a solid, liquid or gas – yet most people (even when they refer to 'substances') usually think in terms of materials, not substances. Iron, as a material, is a strong solid material suitable for use in building structures – thinking of iron the familiar material as becoming a gas requires a lot of imagination for someone who not habitually think in terms of scientific models.

Although Amy thought her classmates had found the idea of iron as gas as weird, they had not rejected it. Yet, if it is such a counter-intuitive idea, it may not be later readily brought to mind when it might be relevant, unless it is consolidated into memory by reinforcement through being revisited and reiterated. (Indeed the research interview provides one opportunity for rehearsing the idea: research suggests that whenever a memory is activated this strengthens it.)

[Another student I interviewed told me that Iron is too heavy to completely evaporate.]

They're both attracting each other but this one's got a larger force

Iodine's got a larger force that lithium, so it will pull towards the lithium more

Keith S. Taber

When she was shown an image representing the electron cloud around an iodide ion polarised by an adjacent lithium ion Annie interpreted this as the iodine exerting a greater force on the lithium than vice versa.

What about this, any idea about this?
It's the same sort of thing again – the lithium combines with the iodine – to make a stable outer shell between the two, by sharing electrons, but the lithium has a smaller charge, or smaller pull than the iodine, so the actual shape of it goes in towards. It sort of goes inwards because its attracting the lithium, whereas if the lithium was attracting it, it would be like a reverse picture.
So, so the iodine's attracting what, sorry?
The lithium.
The iodine's attracting the lithium, and the lithium is not attracting the iodine?
Yeah, they're both attracting each other but because this one's got a larger force, then it will pull towards the lithium more.
The iodine's got a larger force,
Yeah.
so it will pull towards the lithium more?
Yeah.

Any image used to represented chemical bonding is necessarily a kind of model, and a partial representation – and there are a range of types of representations students meet. It is perhaps not surprising if students cannot always 'guess what the teacher (or textbook author or researcher) is thinking, and what they intend by a particular type of image.

Annie here demonstrates the common notion that chemical bonding can be based upon 'sharing' electrons (i.e., covalent bonding). At this point in her course Annie would not be expected to appreciate polar bonds or the polarisation of ions, but her prior learning that covalent bonding could be understood as 'sharing' of electrons could potentially act as an impediment to learning that the ionic-covalent bonding distinction should be seen as a spectrum, a continuous dimension, not a dichotomy.

The way forces are understood in physics is that they are interactions between two bodies, and that the same magnitude of force acts of both bodies (i.e., Newton's third law). However, students commonly consider that a 'larger' body (e.g., more massive, more highly charged) exerts a large force on the smaller body. Students do not clearly distinguish the force from its effect, and so this alternative conception seems to draw upon intuitions based on actual experience of the world (i.e., a grounded learning impediment) where larger sources (larger fires, bigger loudspeakers, larger lamps) often seem to have larger effects.

[Read about Newton's third law, and student learning difficulties]

In a molecule, the electron actually slots into spaces

Keith S. Taber

Mohammed was a participant in the Understanding Science Project. When interviewed in the first term of his upper secondary (GCSE) science course (in Y10), he told me he had been learning about ionic bonding in one of his science classes. Mohammed had quite a clear idea about ionic bonding, which he described in terms of the interactions of two atoms where "they both want to get full outer shells", leading to salt which was "like two atoms joined together":

The "two atoms joined together" sounds much like a molecule (and it is very common for students to identify molecule like ion-pairs even in representations of extensive ionic lattices), so I asked Mohammed about this:

Can I see these atoms?
No. They're really small. Because the wavelength of visible light is actually too like large to see the atoms, they just pass over them.
Okay, so I can't see them. But I can imagine them, can I?
Yeah.
So if I could imagine a sodium atom and chlorine atom, and then they form salt, what would it look like afterwards? How could I imagine it afterwards.
Oh it's like two atoms joined together.
That sounds like a molecule to me?
It's not actually, like, joined.
No?
Because I know that whenever things of opposite charge, I know two rods, when they come together, they don't actually touch, so they don't exactly touch, but they are very close, two atoms close to each other
So a molecule would be different to that in some way, would it?
Yeah, a molecule's actually bonded
So how that different?
I think in a molecule, the electron actually slots into spaces.
I see, and it doesn't do that in this case?
No.

So Mohammed thinks that the interaction between the ions will be due to their electrical charges, but, for him, this may not count as a bond, as the forces just hold the ions ("atoms") close together, and do not actually join them. Mohammed's idea of the atoms not actually touching, "they don't actually touch, so they don't exactly touch", is transferring a notion from the familiar world of macroscopic phenomena (where things touch, or they do not touch) to the submicroscopic world of quanticles that do not have definitive size/volume, and do not actually have distinct surfaces, so touching is a matter of degree. There is no more (or less) 'touching' in a covalent bond than in ionic bonding. So according to Mohammed the ions do not form a molecule, as in a molecule there would some kind of more direct joining – he suggests something like an interlocking with electrons from one atom slotting into spaces on another.

Interestingly, Mohammed bases his notion that the ions would not touch on a general principle that he considers to apply whenever considering things of opposite charge – which he justifies on his knowledge that "two [charged] rods, when they come together, they don't actually touch". He may be misremembering something here – or he may have seen a demonstration of suspended charged rods of the same material (so either both negatively or both positively changed) that when one is moved closer to the other the rods repel. Whatever the source, Mohammed seems to feel he has a valid general principle that he can apply here that act as a grounded learning impediment channelling his thinking about the case under discussion along 'the wrong lines'.

Mohammed's notion of the ionic bonding as being just due to forces rather than being a proper bond is very similar to a common alternative conceptions of ionic bonding which sees ions in a lattice only having a limited number of ionic bonds depending upon valency (the valency conjecture) but bonded with other coordination counter-ions by 'just forces' (the just forces conjecture) – although here Mohammed suspected that all ionic bonding fell short of being proper chemical bonds.

This is a very mechanical model of the covalent bond, whereas the scientific model presents bonding as more of a process than a material mechanical link. However teaching models often present bonding this way, and sometimes molecules are modelled in terms of jigsaws with atoms or radicals as pieces to be slotted together. Although such models are only meant to provide a simple analogy for the bonding they may act as learning impediments if learners take them too 'literally' as realistic representations and transfer inappropriate associations from the model to their understanding of the system being modelled.

Mohammed also uses similar language when asked about salt dissolving in water, as the charge of the water forces the sodium and chlorine ions to slot into certain places within the water molecules *.

In ionic bonding, they both want to get full outer shells

Keith S. Taber

And you said in chemistry you've been doing about electron arrangements [electronic configurations], and ionic bonding.
Yeah.
So what's ionic bonding, then?
Ionic bonding is when, like let's say, a sodium atom and take a chlorine atom, which make salt if they react. What happens is – the sodium atom has one electron on its outer shell, and the chlorine atom has seven, now they both want to get full outer shells, so if I er let's say move the electron from the sodium to the chlorine, then the chlorine would have a full outer shell because it would have eight, and because it's lost that shell the sodium will also have eight.

This account of ionic bonding is a common one, although it is inconsistent with the scientific model. A key problem here is that the driving force for bond formation is seen in terms of atoms wanting to complete their electron shells (the 'full shells explanatory principle'). Mohammed's explanation here uses anthropomorphism, as it treats the individual atoms as though they are alive and sentient, acting to meet their own needs – "they both want to get full outer shells".

When Mohammed was probed, he related a full outer shell to atomic stability (a central feature of the full shells explanatory principle).

Okay. How do you know they want full outer shells?
Because it makes them more stable.
Why does it make them more stable?
(pause, c.1 s)
Erm. (Why do electrons?*) (* sotto voce – apparently said to himself)
(pause, c.2s)
Er, because they don't react as much with other elements if they have a full outer shell.
I see.
They don't react.

There is an interesting contrast here between Mohammed's instant response that full shells "makes them more stable", and the long pause as he thought about why this might be so.

His response reflects something quite common in students' explanations n that a student asked why X is the case may respond by explaining why they think X is the case. (That is, as if an appropriate answer to the question "why is it raining so heavily?" would be "because I got soaked through getting here", i.e. actually responding to the question "how do you know that it is raining heavily?")

Such responses seem to be logically flawed, but of course may be a mis-perception of the question being asked (so the learner is answering the question they thought was asked), or (possibly the case here) substituting a response to a related question as a strategy adopted when aware that one cannot provide a satisfactory response to the actual question posed.

The anthropomorphic aspect of his earlier answer was probed:

How do the atoms know that they need to get a full outer shell, they want to get a full outer shell? Do they know about this stability thing?
Not really.
No?
It's just what happens.
Oh, I see, it's just what happens?
Yeah.

So although Mohammed used an anthropomorphic explanation, it seemed he did not mean this literally. (It may seem strange to suggest a 14 year old might consider atoms alive and sentient, but research suggests this is sometimes so!) This has been described as weak anthropomorphism, where the anthropomorphism is only used as a figure of speech. However, such language can act as a grounded learning impediment because if it becomes habitual it can stand in place of a scientific explanation (thus giving no reason to seek a canonical scientific understanding).

I went on to ask Mohammed about the formation of salt in the process he had described.

Liquid iron stays a liquid when heated

Keith S. Taber

Sophia was a participant in the Understanding Science Project. In Y7, Sophia had told me that if molten iron was heated "some of it would evaporate but not all of it, 'cause it's not like water and it's more heavy". She thought only "a little" of the iron would evaporate to give iron vapour: The rest "really just stays as a liquid". [See 'Iron is too heavy to completely evaporate'.]

Just over a year later (in Y8) Sophia had been studying "that different erm substances have different freezing and melting and boiling points, and some aren't like a liquid at room temperatures, some are a solid and some are a gas and things like that".

Give me an example of something else that's a solid at room temperature?
Iron.
Do you think iron would have a melting point?
Yeah.
Yeah, and if I, what would I get if I, if I heated iron to its melting point?
It would become a liquid.
And why would it do that?
Because it's got so hot that particles – they have spread out or something?
So what do you think would happen if I heated the iron liquid?
It would stay a liquid.
No matter how much I heated it?
It might, I don't know if it would become a vapour.
Can you get iron vapour?
No, I don't think so.
You don't think so?
No.

So it seems that Sophia had shifted from accepting that iron would partially evaporate (when learning about the particle model of the different states), to considering that iron (probably) can not become a vapour. The notion of iron as a gas is not something we can readily imagine, and apparently did not seem very feasible. In part this might be because we think of iron the material (a metal, which cannot exist in in the vapour phase) rather than as a substance that can take different material forms.

It seems Sophia's prior knowledge of iron the material was working against her learning about iron the substance, an examples of a grounded learning impediment where prior knowledge impedes new learning.

In Y7 Sophia had seemed to have a hybrid conception where having been taught a general model of the states of matter and changes of state, she accepted the counter-intuitive idea that iron could evaporate, but thought that (unlike in the case of water) it could not completely evaporate . This might have been a 'stepping stone' between not accepting iron could be in the gaseous state and fitting it within the general model that all substances will when progressively heated first melt and then evaporate (or boil) as long as they did not decompose first.

However, it seems that a year later Sophia was actually more resistant to the idea that iron could exist as vapour and so now she thought molten iron would remain liquid no matter how much it was heated. If anything, she had reverted to a more intuitive understanding. This is not that strange: it has been shown that apparent conceptual gains which are counter to strongly held intuitions that are brought about by teaching episodes that are not regularly reinforced can drop away as the time since teaching increases. Conceptual change does not always involve shifts towards the scientific accounts.

[Sophia was in lower secondary school when I talked to her about this: but I was also told by a much older student that the idea of iron turning into a gas sounds weird.]

Iron is too heavy to completely evaporate

Some molten iron would evaporate but not all of it, 'cause it's not like water and it's more heavy

Keith S. Taber

Sophia was a participant in the Understanding Science Project. In her first interview near the start of Y7, Sophia told me that she had learnt "about the particles…all the things that make – the actual thing, make them a solid, and make them a gas and make them a liquid" (i.e. the states of matter). All solids had particles, including (as examples) ice and an iron clamp stand. There would be the same particles in the ice as the iron.

"because they are a solid, but they can change , 'cause if erm they melted they would be a liquid so they would have different particles in…Well they are still the same particles but they are just changing the way they act".

Sophia's suggestion that particles in ice and the iron were the same types of particles as both were solid seems to be 'carving nature' at the wrong joints – that is in this model the particles in ice and (solid) iron would be of one type, whilst those of water and liquid iron would be of another type (that is she had an alternative ontology). Sophia quickly corrected this, so it is not clear if this reflected some intuitive idea or was just 'a slip of tongue'.

According to Sophia the ice could be melted "with something that's hot, like a candle" but for the iron "you need more heat, 'cause it's more, it's a lot more stronger…because it's got more particles pushed together".

Sophia's explanation suggested a causal path (right-hand side) quite different from a canonical causal path (left-hand side)

Strictly the difference is more about the strength of the interactions between particles, than how many were pushed together – although strong bonding forces would tend (all other factors being equal) to lead to particles being bound more tightly and being closer. We might argue here that Sophia seemed to confuse cause and effect – that a higher density of particles was an effect of strong bonding, which would also mean more energy was needed to overcome that bonding. (However, we should also be aware that when students use 'because' (which formally implies causality) they sometimes mean little more than 'is associated with'.)

If the water obtained from melting ice was heated more "it will evaporate into the sky". However, if the molten iron was heated Sophia thought that "some of it would evaporate but not all of it, 'cause it's not like water and it's more heavy". She thought only "a little" of the iron would evaporate to give iron vapour:

"No, I think that water all of it goes, but other material, other liquids some of it will go, not all of it". The rest "if it's cold enough, it will go back into a solid, but if not it really just stays as a liquid".

Sophia's idea that no matter how much liquid iron was heated it would not completely evaporate so some would remain liquid, which seemed to be linked in her mind to its density, seems to be evidence of an alternative conception. Students may not expect that something as (apparently) inherently solid as iron could evaporate (everyday experience may act as a grounded learning impediment), and so may not readily accept that the basic model of the states of matter and changes of state (i.e., a heated liquid will evaporate or boil) can apply to something like iron. Sophia seemed to have formed a hybrid conception – applying the taught model, but with a modification reflecting the counter-intuitive notion that iron could become a vapour.

Conceptual change can be a slow progress, although hybrid conceptions may be 'stepping stones' towards more scientific understandings. However, when I spoke to Sophia in Y8 she did not seem to have progressed further. [See 'Liquid iron stays a liquid when heated'.]

In a sponge, the particles are spread out…

In a sponge, the particles are spread out more, so it can absorb more water

Keith S. Taber

Morag was a participant in the Understanding Science Project. In her first term of secondary school, she told me that he had learnt about particles. Morag had explained, and simulated through role play for me, the arrangements of particles in the different states of matter (See: So if someone was stood here, we'd be a solid.) She had also emphasised just how tiny the particles were, "little, little-little-little things", and so how many there were in a small object: "millions and millions and millions". This suggested she had accepted and understood the gist of the scientific model of submicroscopic particles.

Yet as the conversation proceeded, Morag suggested the macroscopic behaviour of sponge in absorbing water could be explained by the arrangement of particles leaving space for the water. This is perhaps a reasonably, indeed quite imaginative, suggestion at one level, except that the material of a sponge is basically solid (where, as Morag recognised, that the particles would be very close together). A sponge as whole is more like a foam, with a great volume of space between the solid structure (where air can be displaced by liquid) and an extensive surface area.

Do you think it is important to know that everything is made of particles?
No.
It's not important?
Well it could be important, but it's not that important. Well, you see, like that [indicating the voice recorder used to record the interview] has got like lots and lots of particles pushed together this [Morag gestures]…But a sponge, the particles are like, the particles are more kind of like, they're still the same, but it's just spread out more, so it can absorb more water.
Oh I see, so are you saying that the same particles are in my little recorder, as in the sponge.
Yeah, they're the same, but there's just more of them in one than there would be in the other.

The failure here is perhaps less Morag's inappropriate explanation, than the tendency to teach about the ideals of solids, liquids and gases, which strictly apply only to single substances, where most real materials students come across in everyday life are actually mixtures or composites where the labels 'solid', liquid' and 'gas' are – at best – approximations.

Teaching has to simplify complex scientific ideas to make them accessible to students of different ages, so often teaching models are used. But sometimes simplifications can cause misunderstandings, and so the development of alternative conceptions. If 'everything is a solid, liquid or gas' is used as a kind of teaching model, or even presented as a slogan or motto for students to echo back to the teacher, when lots of things students come across in everyday life (e.g., butter, clouds, the pet cat – a bathroom sponge) do not easily fit these categories, and this is likely to lead to students overgeneralising.

Although it is often not possible to assign a single simple cause to a student's flawed thinking, this could be considered likely to be an example of a pedagogic learning impediment (a type of grounded learning impediment) in chemistry: a case where an approach to teaching can lead students' thinking in unhelpful directions.

Fuels get used-up when we burn them

Keith S. Taber

Sophia was a participant in the Understanding Science Project. Sophia (then in Y7) had been burning materials in science. She had burnt some paraffin in a small burner (a glass burner with a wick). Her understanding of the process was not in terms of a chemical reaction, but at a more 'phenomenological' level:

So what happens to paraffin when it burns then?
It keeps on burning… but you, you can put it out easily as well…. we just blew it out…
I see, but otherwise it just carried on burning, did it? Did it carry on burning for ever, if you don't blow it out?
No, 'cause it would run out.
What would it run out of?
The paraffin.
So where does the paraffin go then?
(There was a pause, of about 4 seconds. Sophia laughs, but does not offer answer.)
And what happens to the level of the paraffin in the burner?
It gets lower and lower.
So why's that, what's happened to it?
'cause you are using all of it up, when it's burning.
So it get all used up does, it – so what happens when it's all used up?
You have to refill it.

So for Sophia the burning of paraffin is not seen in terms of basic chemistry (what happens to the substance paraffin during the process of burning?), but rather she seems to interpret what she has seen in terms of everyday ideas – stuff, such as fuels, get used up – if we use it, we no longer have it.

The final question in this sequence ('what happens when it's all used up') is not treated in scientific terms (e.g., from the perspective of the conservation of matter, there is an issue of where the 'stuff' what was the paraffin has gone), but in practical terms: when we use up the fuel in the burner, we need to refill it to do more burning.

Here, understanding in 'everyday' or 'lifeworld' terms seems to dominate her thinking: the familiar idea that things get used-up obscures the scientific question of what happens to the matter in the fuel. Presumably, her teacher wanted her to focus on the scientific perspective, where burning is combustion, a type of chemical change, but it appears her life-world perspective acted as a grounded learning impediment – an existing way of thinking about a phenomenon that is taken for granted and obscures the scientific perspective.

The everyday way of understanding the world could be called the natural attitude. It seems that for Sophia it is 'just natural' that fuels get used up, and so there is nothing there to explain. Arguably, the work of a science teacher sometimes involves persuading students to seek explanations for things they had considered 'just natural', and so not in need of explanation.