learner thinking – Page 14 – Science-Education-Research

Atoms evolved so that they could hold on to each other

Bert Suggests Chemical Bonding Evolved

Keith S. Taber

Bert was a participant in the Understanding Science Project. During one interview he reported that he had just completed a topic of alkanes and alkenes in his chemistry classes. He explained that a carbon atom has "to have four bonds", so if a carbon atom had "only got one two carbons on one side and one hydrogen then it'll make a double bond, to have four bonds". So I asked him what he understood a bond to be:

I: …what's a bond?
B: A bond is erm, it just, it's something to hold, hold two atoms together.
I: So what might you use to hold two atoms together?
B: Erm, So they can be kept, so that they're not too, I think it's just to make, so it can make big lines so it can erm, oh, so they, so not every, so because solids they have erm, I guess a lot of bonds, to keep it all, all together, I'm guessing. And erm like gas has a lot less bonds because it's a lot more free.
I: That makes sense [Bert], I'm just wondering what you would use to bond two atoms together. … I'm just wondering what kind of thing you use to bond two atoms together.
B: Erm • • I'm not sure. I guess, I guess they were just, when er, they're made with it I guess.
I: Yeah. Do you think it's made of adhesive? … is it made of a glue do you think?
B: No, I don't think so. I think it was like, I don't know, it could have been like evolution, like.
I: Ah.
B: Yeah, the atoms evolved so that they could hold on to each other.
I: Oh I love that. • • • The atoms evolved so that they could hold on to each other?
B: I guess so. That's how the world was made.

In this interview segment Bert seems not to have considered the nature of the bonds between atoms, but just to have accepted what he has learnt about valency. When asked about the nature of the bond he could offer no mechanism for bonding, but instead suggested that chemical bonds had evolved as "that's how the world was made". Here Bert is drawing upon a general explanation considered to be universal in the domain of living things, but applying his learning from biology to explain a physical phenomena.

This seems to be a creative association drawing upon prior learning, but the idea of evolution is being used outside is canonical range of application, leading to a potential associative learning impediment. Potentially, Bert's thinking about evolution as explaining how atoms can bond (a potential explanation about origins, though inappropriate if evolution is understood as natural selection) could stand in place of seeking a physical explanation for the nature of bonding.

Guessing what is produced in beta decay

Keith S. Taber

Forgetting the source?

Interestingly, in a later interview just before her GCSE examinations, Amy told me: "beta radiation is, I dunno, an electron – thing, which is emitted somehow." When I asked her where's it emitted from, she told me

"that's what I don't know, 'cause I asked about that, and I didn't get an answer because, erm, apparently the neutron is made up of other stuff, and it, that sort of decays to give other things and that's where the electron comes from, apparently, maybe, I don't know, I'm guessing now".

When asked if she knew what else was emitted, Amy suggested that "I'm guessing proton, but I don't know". It seems this had seemed so 'crazy' that Amy was not able to believe what she had been taught, and what had previously been reported as something she had been told, was now considered by her to be just a guess at what was going on. This is an interesting reminder of how human memory works, that we do not always recall the origins of our ideas, so can not always distinguish our own creative ideas from we might have been previously told at some point.

Crazy physics: radioactivity is just mad!

Some crazy thing about a neutron turning into a proton or something

Keith S. Taber

Amy was a participant in the Understanding Science Project. I talked to her after she had just started studying radioactivity in her Y11 physics class. She had been introduced to alpha, beta and gamma radiation, and the teacher had been telling the class crazy things: "he told us some crazy thing about a neutron turning into a proton or something, and losing, two electrons or some crazy thing like that."

This was crazy because "it doesn't make any sense". Amy had only just started the topic, and had not yet sorted out all the details in her own mind, but the teacher had "showed us this little equation thing they [sic] have, and when – is it the electrons that are released or something – the atom, it changes, it changes into something else". This was not something else other than an atom, but "an element will become a different element or some stupid thing like that". Amy concluded that "all of it sounds crazy, physics is crazy. I don't understand it." This seemed a rather harsh judgement.

Amy was generous enough to talk to me regularly about her understanding of the science she was taught in school. She, regularly, told me she knew or understood little about the topics she was taught, although a little probing often revealed considerable learning. However, on this occasion, Amy genuinely seemed to be genuinely mystified by what she was being told.

I spent a little time talking to Amy about radioactivity. I discussed beta decay with Amy, concluding with "an electron comes shooting out of the atom, and that's your beta radiation". Amy maintained "it's crazy". We discussed the different types of radioactivity she would study: "it's crazy". I think Amy believed what I was telling her, but she made it clear what she thought of these nuclear changes and the accompanying transmutation of elements: "it's mad"!

What I find especially interesting here, is that Amy seemed to have strong intuitive views of the way the world should work, even when the focus was something so far from our everyday experience. Atoms, nuclei, electrons, neutrons, protons, are all theoretical constructs we introduce to students that have no obvious and clear link with anything in everyday experience. They also behave in ways unlike objects of common experience. It can only have been a couple of years since Amy was taught that atoms had structure and comprised of protons, neutrons and electrons. But now she readily accepted that neutrons existed. Indeed they had become so 'real' to her, that the idea one must spontaneously change into a proton and an electron seemed quite mad. So Amy had relatively quickly applied her intuitive ideas about the way the world is to form a mental model of a neutron as a discrete, stable entity: not something that could suddenly reveal itself as potentially two other things that were supposed to be quite different. When Amy was introduced to a model of the sub-atomic components of the atom she 'assigned' (i.e., pre-consciously interpreted them to have) them certain ontological qualities (such as stability, permanence) which made sense to her at the time, but which did not capture the way scientists understand these entities. This assumption of the nature of the neutron acted as a grounded learning impediment, because when she was later taught about radioactivity it did not make sense in terms of her prior learning.

Too crazy to remember?

In a later interview, Amy was able to tell me that the neutron is made up of other stuff, and it decays to give an electron and, she thought, a proton – although (she told me) that was only a guess.

Sharing the same shell and electron makes them more joined together like one

Keith S. Taber

Umar was a participant in the Understanding Chemical Bonding project. When I spoke to him in the first term of his advanced level chemistry course he identified figure 2 (below) as representing a hydrogen molecule, with covalent bonding.

Sharing the same shell and electron makes them more joined together like one

Umar was a participant in the Understanding Chemical Bonding Project. When I spoke to him in the first term of his advanced level chemistry course he identified figure 2 (below) as representing a hydrogen molecule, with covalent bonding.

Figure 2 (Focal image – Understanding Chemical Bonding project)

Umar suggested that covalent bonding is when atoms share electrons to combine into one whole thing. That discussion took place early in the interview, before we then discussed a whole range of other images. Near the very end of the interview I returned to ask about figure 2 again.

Interviewer: And number 2 was what kind of bond?
Umar: Covalent.
I: Now, what holds the molecule together in number 2?
U: The two electrons – shared.
I: And how does that hold them together?
U: 'cause they're sharing the same – shell and electron.
I: And why does that hold them together?
U: Makes them more, together like, makes them more like joined together like one.

After we had first discussed what this image was meant to represent early in the interview, Umar discussed a wide range of other images, and in the context of some of these he discussed bonding in terms of forces and electrical charge. As he had not mentioned such notions in the context of figure 2, even after using the ideas elsewhere, I sought to see if he recognised that forces were acting in the hydrogen molecule.

I: I see. Is there any force there holding them together?
U: It's, erm could be the charges of the electrons and the charge of the nucleus.
I: Would the nucleus have some sort of interaction with the electrons – some sort of attraction or repulsion?
U: Yeah.
I: Would it be attraction or repulsion?
U: Erm, attraction.
I: So which electron does this nucleus attract.
U: Erm, it attracts both of them, and the other one attracts both of them because they are both, like, opposite charges. So that's why they are like, around there. It might be like they move around. Around that part.
I: So they might actually move about?
U: Yeah.
I: I: But you think the two nuclei attract the two electrons?
U: Yeah.
I: Do the two electrons attract the two nuclei?
(Pause, c.3s)
U: Yeah, think so, the – yeah.
I: Yeah? Do the two electrons attract each other?
U: No, they repel.
I: Do the two nuclei attract each other?
U: No they repel.

So it seemed that Umar understood the forces acting in the covalent molecule but that these ideas were not readily cued in that context even though he readily used the idea of forces between charges to explain other kinds of chemical bonding. In the context of covalent bonding however, the notion of the bond as electron 'sharing' was cued instead. Arguably the notion of the covalent bond as sharing of electrons acted as a grounded learning impediment perhaps blocking him bringing to mind alternative ways of thinking about the bond. This could be seen as an example of weak anthropomorphism: the idea that the electrons were 'shared' stood in place of a more scientific explanation of the bonding process.

Covalent bonding is when atoms share electrons to combine into one whole thing

Keith S. Taber

UCB Figure 2 (for interview-about-instances technique)

Can you tell me what you think that's meant to represent?
Er, two hy-, a hydrogen molecule, 'cause it's like they've got one electron, in the only one shell, and they're joined together, a covalent bonding, and they're sharing it.
So what is a covalent bond exactly?
When they share electrons.
When you share electrons?
Yeah.

So when Umar thought of covalent bonding he seemed to primarily associate this with the notion of 'sharing' of electrons. The idea that atoms can 'share' anything could be considered an example of anthropomorphism, but this is a common metaphor that is widely used in discussing bonding.

The 'sharing' notion is however little more than a descriptive label, and has limited explanatory power. Acceptable explanations of the bond would draw upon scientific concepts, such as electrical forces, or atomic orbital overlaps allowing the formation of lower energy molecular orbitals. I probed Umar to see how he understood the nature of the covalent bond.

Or do you think they're stuck together?
I think they're quite strong together, covalent is quite a strong bond.
So that will hold them together will it?
Yeah.

Umar certainly saw the bond as a strong linkage of some kind, but so far my questions had not revealed how he understood the bond to hold the molecule together.

Well how does it do that?
It's like, they're joined together, 'cause first of all they just had two atoms with one electron each, and now they're sharing two electrons between them. So it's quite strong.
Oh, why's that?
Because the the the actual, when they share them they're like combined into like one sort of whole thing, instead of two separate atoms.
Right, so the, so the bond, which is the sharing of two electrons, that holds them together,
Yeah.
to make one thing, which we've called a molecule.
Yeah.

So at this point in Umar's course he seemed to conceptualise the covalent bonding as electron sharing and saw the action of sharing to inherently hold the molecule together, and seemed to be satisfied with that as an explanation for the bond. This discussion took place early in the interview, before we then discussed a whole range of other images. Near the very end of the interview I returned to ask about figure 2 again (see Sharing the same shell and electron makes them more joined together like one)*.

So if someone was stood here, we'd be a solid

Keith S. Taber

Morag was a participant in the Understanding Science Project. During her first term in secondary school, Morag told me she had studies changes of state, which was about "melting things, it's like solid, liquid and gas. Where like an ice cube melts to go to water, it evaporates to go to gas, it then condenses to go to water and then freezes to go to ice".

When I asked her about about the states of matter, Morag gave me a quite polished response. In the middle of this, she stood up and started moving about. It appeared that she had modelled the states of matter in class through a simulation, with the students acting as particles – and this association seemed to now be cued by her recalling the explanations for the different states of matter:

I: So silly question, 'cause I'm sure everybody knows really, but what's a solid, what's a liquid and what's a gas then?
Morag: A solid is an object where the particles are very close together, but still have room to move very slightly, you know like they can only move little bits, er, it has a fixed shape, it cannot be poured – and that's all I can remember.
I: That's quite a bit. And that's different to a liquid, is it?
M: Yeah, 'cause a liquid you can pour, it takes the shape of its container, the particles are spread out more evenly, but still in a, but are still spread in a – yeah they're spread evenly it can be poured, (it takes the shape of its container), the particles are still quite close, but they are further away than they were in a solid, so they can move just a bit more. If you know what I mean, like. So if someone was stood here [indicating next to her], we'd be a solid, 'cause we just move very slightly,
I: all right, yeah
M: and if we were a liquid we would be stood just a bit further away, so we can move a bit more.
I: I see, so if you had brought a friend with you,
M: Yeah, and if we were stood like that, if she was stood there, we'd be a solid, 'cause we were quite close, but we still had room to move about
I: Mm
M: if we were a liquid, we'd be a bit further, but we still, still quite close, but still had move to room, to move about, and I'm not going to tell you about gas until we get onto gas.
I: Okay. So you and your friend could be a liquid? Which means that I could pour you and you would take up the shape of your container?
M: No, I mean like we'd be the particles in liquid.
I: Ah, I see.
M: you know like
I: Moves around!
M: like, so like, like, so we'd be like that, and there would be lots of us, but we could still move about. Yeah? And if we were a liquid we would be like that, and we could still move about. And if we were a gas we'd be further apart, but and then we can, and then we can move around the room freely.

Molecules are like a jigsaw

Keith S. Taber

Tim was a participant in the Understanding Science Project. When Tim was interviewed in the first term of his 'A level' (college level) physics course he had been studying the topic of materials with one of his teachers, and "at the moment we're doing about why some materials are brittle, and some aren't, and about the molecules". When Tim was asked about the molecules, he compared molecules to the pieces in a jigsaw:

Interviewer: So what's a molecule?
Tim: Erm it's like a bit of a particle, so, something that makes up something.
I: Have you got any examples?
T: Of a molecule?
I: Yeah, something that makes up something.
T: Erm, like the wood in the table is made out of wood molecules.
I: I see. So, that's one type of molecule, is it, a wood molecule? And there are other types of molecule?
T: Yes it's a bit like a jigsaw, like when you put all the, like you need to put the…, you put them all together to make – something.
I: I see, yeah. So, if I wanted to be really awkward, in what way is it like a jigsaw?
T: Erm, well they sort of fit together, like in a jigsaw some bits are sort of straight and have nice parallel, a nice parallel microstructure, and some, some jigsaws have funny bits that don't fit together quite as nicely.
I: I see. So are there some ways it's not like a jigsaw?
T: Yeah. (Tim laughs.) Well, erm, I dunno, it's like a jigsaw in the way that the bits fit together to make something, to make something, but then again, I dunno.
I: I mean, I quite like this idea of it being like a jigsaw – I was wondering whether, whether you had got that from somewhere, or that's just something you'd come up with?
T: No I just thought about it, just then.
I: Oh that's really creative.
T: It's quite random actually. (laughs)

Tim's comments about a molecules being a bit of a particle was followed up later in the interview, and it transpired he was not sure if a molecule is a bit of a particle – or vice versa.*

So when asked to explain about molecules in materials, Tim used an apparently spontaneous analogy of this being like a jigsaw, with different types of pieces that fitted together. Moreover, he also seemed to recognise that different materials had molecules that fitted together more or less readily, and materials could also be considered to have similar diversity. Tim described this as being 'random', which seems unfair as the analogy clearly has merit, but presumably saw it this way as the comparison had apparently appeared in his consciousness unexpectedly (i.e., the thought had 'popped into his mind', as a kind of insight.)

Tim seemed a little phased by being asked to explain the negative features of the analogy – and this may reflect the tendency to focus on the positive aspects of an analogy, rather than its limitations. Analogy has the potential to channel student thinking in inappropriate directions (e.g., as associative learning impediments) when not considered critically. However, analogies also have potential to help 'make the unfamiliar familiar' and so can be a powerful learning tool.

Creating an explanation for the soot from Bunsen flames

Letting the dirt out: Creating an explanation for the soot from Bunsen flames in the absence of appreciation the nature of combustion

Keith S. Taber

Jim was a participant in the Understanding Science Project. Jim, a Y7 student, had been studying burning in science. He had been using Bunsen burners, and had been taught about the different flames (i.e., the safety flame, and the 'roaring' blue flame used for heating), and the use of the valve at the base of the burner to select the frame. Not yet appreciating the nature of burning, he was not aware that the soot obtained when interrupting the safety flame was due to incomplete combustion. Rather he had developed his own interpretation of why using the burner with the hole closed off led to a dirty flame:

What is burning, then?
It usually involves a flame. Erm which can either be yellow, orangey-yellow, or …like a, bluey colour, bluey-purple.
I: Oh, so is that significant, the colour of the flame, does that mean something?
J: Well, the yellow one has a lot of …if you touch it with glass or something, …will go black, but if you use the blue flame, it won't, so if you are heating something, you should use the blue flame.
I: Why do you think it goes black, if you use the orangey-yellow flame?
J: Because with the Bunsen burners, if you are twisting the knob, open, the dirt gets out, and you get the nice clear blue flame, but to get the orange flame, you have to have it closed, don't you, and then that doesn't let the dirt out, so it doesn't kind of, when it gets out of the top it doesn't have time.
I: So what happens if the hole is open?
J: You get, a blue flame.
I: Right, and what happens if the hole is closed?
J: Get a yellow flame.
I: And why does the hole make a difference?
J: I don't know, it probably lets the dirt out, or the air get into it or something.
I: So what dirt is this, that might be let out, do you think. Dirt from where?
J: Maybe the excess gas particles that have already been burnt or something. Don't know.

Presumably no one had told Jim that the hole was to let dirt out of the Bunsen so it did not get into the flame. However the hole was presumably letting something in or out (he later suggests, the hole might let air in – perhaps something the teacher had told the class but which had not been readily recalled?) and there was dirt in the flame when it was closed, which was not there when it was open. Jim interpreted his observations in terms of prior knowledge (of what holes do, and of dirt) to construct an explanatory scheme that made some sense of the effect of closing or opening the air hole. This would seem to have potential to be an associative learning impediment of the 'creative' type.

Gas particles like to have a lot of space, so they can expand

Keith S. Taber

Derek was a participant in the Understanding Science Project. I interviewed Derek when he was in Y7 of the English school system. We had been talking about work that Derek has been doing in his science classes on burning. As part of the conversation, Derek defined a solid in particle terms:

what's a solid then, what's a solid?
Lots of particles really close together that can't move a lot.

When I followed this up, Derek explained how a liquid or gas was different to a solid:

And you say solids are made of particles. What are liquids then, they are not made of particles then?
No they are, they are just more spread out particles. And then, you get a gas, which the particles can move a lot more than solid and liquid, they can move wherever they like.
And where do they like to move?
As far away from each other as possible.
Why do you think that is?
'cause they like to have a lot of space, so they can expand.
Why do you think particles like to have a lot of space?
(Pause, c.3s)
Don't know.
Are they unfriendly lot, unsociable?
(Pause, c.2s)
No, they just, they like to have, like be as well away from each other as possible.

The question "where do they like to move" was couched in anthropomorphic terms to reflect the anthropomorphism of Derek's statement that gas particles could "move wherever they like", to see if he would reject the notion of the particles 'liking'. However Derek did not query my use of this language, and indeed suggested that the particles "like to have a lot of space".

When he was asked why, there was a pause, apparently suggesting that for Derek the notion of the particles liking to be far apart seemed to be reasonable enough for him not to have thought about any underlying reason, and his "don't now" was said in a tone suggesting this was a rather uninteresting question. Although Derek rejected the suggestion that the particles were 'unfriendly', 'unsociable' his tone did not suggest he thought this was a silly suggestion: rather it was just that the particles "like" to be as far "away from each other as possible".

The use of anthropomorphism is very common in student talk about particles. Whether or not Derek really believed these gas particles actually had 'likes' in the way that, say, he himself did, cannot be inferred from this exchange. But, in Derek's case, as in that of many other students, the anthropomorphic metaphors seem to offer a satisfactory way of thinking about particle 'behaviour' that is likely to act as a grounded learning impediment because Derek is not open to looking for a different kind (i.e., more scientifically acceptable) type of explanation. Given the common use of his language, it seems likely that it derives from the way teachers use anthropomorphic language metaphorically to communicate abstract ideas to students ('weak anthropomorphism'), but which students accept readily because thinking about particle behaviour in terms of the 'social' models makes sense to them ('strong anthropomorphism').

Do the forces from the outer shells push the protons and the neutrons together?

Keith S. Taber

Annie was a colearner (participant) in the Understanding Chemical Bonding project. In her first interview, during the first year of her two year 'A level' college course, Annie was asked about a (Bohr type) representation of a (sodium) atom. Annie did not know what held the protons and neutrons together in the atomic nucleus, but suggested it might be due to forces from the electrons "pushing":

Interviewer: Can you identify the different parts of that diagram? What's the blob in the centre?
Annie: It's the nucleus.
I: That's the nucleus. Do you know what's in the nucleus?
A: The protons and, no the electrons and the neutrons, no the protons and the neutrons. The electrons are round the outside.
I: There's protons and neutrons in the centre okay.
A: Yeah.
I: Erm, what holds them together, any idea?
A: Is it the forces from the outer ring? Outer rings or outer shells? The electronic forces?
I: What repelling them in? Holding them
A: Yeah.
I: in the centre? It could be.
A: Pushing them.
I: It's not actually, but that's a sensible suggestion. So you haven't actually done anything about what holds the nucleus together?
A: No.
…

The question of why the nucleons should be held together (given the repulsion between positive protons) is not usually considered in school chemistry lesson, and does not seem to be a question which students tend to spontaneously consider. The interview continued…

I: What holds the electrons in place?
(pause, c.4s)
A: Er (pause, c.9s) Not really sure, but I know there's a set pattern of how many can go in each shell, so if its connected with that?
I: Huh hm, do you think, do you think you need anything to hold the electrons in place, or I mean is it just the way the Universe is, or God's will, or, you know, or just aesthetic, you know nature's aesthetic,
A: Yeah.
I: and it looks pretty? I mean do you think there has to be some physical reason why the electrons are there rather than anywhere else?
A: Probably is to do with the structure of it.
I: But you are not, you're not sure why,
A: No.
I: it should be that the electrons should be in orbitals or orbits?
A: No.
I: Rather than just scattered higgledy-piggledy.
A: No, I don't know that.

In this section of the interview, Annie seems to suggest she is not aware of any forces acting on the electrons, and suggests it may be something inherent in the electronic structure which holds the electrons in place. It seems odd that Annie does not invoke a force from the nucleus, given her comment just earlier about a possible pushing from the outer electron ring/shell onto the nucleons. It seems Annie does not know about, or at least does not bring to mind, an electrical force attracting the electrons and nucleus. However, this was tested by a slightly different question…

Okay. So can you tell me why the electrons don't fall out of the atom? I mean if you imagine that this was sort of, er, an atom that's placed vertically, why don't the electron's just fall out of the bottom?
A: The forces hold them together.
I: What kind of forces are they. Do you know?
(pause, c.5s)
A: The attraction from the nucleus, from the protons.
I: So the protons in the nucleus attract the electrons?
A: Yeah.
I: So what kind of attraction is that. What kind of force is that?
A: Er (pause, c.7s) I don't know

So Annie is aware that the electrons are attracted by the nucleus, and specifically by the protons. Despite this, Annie does not suggest the interaction is electronic, or specially refer to charge. Her suggestion that the outer electron shell may push on the nucleus, holding it together, contradicts Newton's third law in that forces between bodies are either attractive or repulsive, not not a mixture of the two. So if the nucleus attracts electrons, then electrons must attract (not push) the nucleus. Annie's suggestion was also inconsistent with the way forces between charges depend upon separation (by an inverse square law): the repulsion between adjacent protons would be far larger than any force due to the more distant electrons.

Atoms within an element don't need to be bonded …

Atoms within an element don't need to be bonded because they're all the same sort

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 commonly used in chemistry teaching. Annie was shown a representation of the close packing of 'atoms' in a metal (with the iron symbol, Fe, shown).

Okay, have a look at number 6…
• • • • • • (pause, c.6 s)
They are obviously iron atoms within an element.
Iron atoms within the element?
Yeah.
Okay. Can you say anything about the arrangement of the atoms?
They're all lined together. They're all close together.
They're closely together, yes, and they're all lined together, there's some sort of regular pattern there okay?
Yeah.
So you think that's in the element, that's a lump of iron, a sort of, a magnified view of a lump of iron.
Yes.

So Annie did recognise the image as representing particles ('atoms') in solid iron. The image showed the particles close together, and Annie was asked if they would hold together – the intention being to find out what, if anything, Annie knew about metallic bonding. Annie did think the atoms would be held together, but she did not suggest this was due to a bond or even a force (cf. "Sodium and chlorine don't actually overlap or anything and would probably get held together by just forces"*).

Do you think those atoms will hold together?
Yes.
Why do you think that is?
Because they're all the same sort.
Does that make them hold together?
Yeah.

So it seemed that Annie held an alternative conception that atoms of the same sort would hold together because they were of the same type. This interpretation was tested.

Yeah? Do you think there is any kind of bonds between the atoms?
• • • • • • • • • (pause, c.9s)
No, because they're all the same and they don't need to be bonded.
Right, okay so recapping…here we've got an example of something where the atoms are all the same, and that holds them together even though there's no chemical bonds.
Yeah.

So Annie held an alternative conception of atomic coherence – that atoms of the same type did not need bonding to hold them together, as being the same kind of atom was sufficient for them to hold together.

It is unlikely that Annie had been taught this idea, and it seems quite possible it is an intuitive idea that might be acting as an example of a 'grounded learning impediment': a notion based on general experience, and inappropriately applied in the context of atomic interactions.

Because the sugar's so small it would evaporate with the water

Keith S. Taber

Morag was a participant in the Understanding Science project. In an interview in her first term of secondary school, Morag suggested that when sugar with mixed with water, it could not be separated out again. This was in the context of discussing chemical change, when she was explaining to me that a chemical change is where two things just go together:*

I: So what's a chemical reaction?
Morag: (I had to learn this) it's when two things, erm, are mixed together and can't be made to the original things easy, easily.
I: Oh, can you give me an example of that?
{pause c. 2 s }
M: Water mixing with sugar, but that's not a chemical reaction.
I: Oh so that's something else is it, is that something different?
M: I don't know.
I: Don't know, so can you mix water with sugar?
M: Yeah, but you can't get the water and the sugar back together very easily.
I: You can't. Is there a way of doing that?
M: No.
I: No? So if I gave you a beaker with some sugar in, and a beaker with some water in,
M: Mm.
I: and you mixed them together, poured them all in one beaker, and stirred them up – you would find it then difficult to get the water out or the sugar out, would you?
M: Ye-ah
I: Yeah, so is that a chemical reaction?
M: No.

The conversation went on to explore Morag's ideas about chemical reactions, and her notion that the flame reacts to the gunpowder * when a firework explodes. A little later we returned to her notions relating to mixtures of sugar and water (i.e., solutions).

I: And when you mix sugar and water, you get kind of sugary water
M: Yeah.
I: Have you got a name for that, when you mix a liquid and solid like that?
{pause c. 1 s}
I: Or is that just mixing sugar and water?
{pause c. 1 s}
M: There is a name for it,
I: Ah.
M: but I don't know it.
I: Okay, so when we mix it we get this sugar-water, whatever, and then it's harder to, it's hard to separate it is it, and get the sugar out
M: Yeah.
I: and the water out?
M: Yeah.

As I probed further, I elicited a difference that Morag perceived between water/sugar mixture (solution) and water/salt mixture (solution). At the time I was not sure what to make of this, and feeling that Morag was probably to some extent searching for answers on the spot, decided to move back to other themes. However, in retrospect, Morag seems to be saying there is a difference because in some sense the sugar is smaller, and so on evaporation can be taken away with the water – unlike the case with salt (solution). Her explanation is vague, but she refer to water:salt ratio, so appear to mean how much can dissolve rather than thinking in terms of molecular size.

I: So is that a chemical reaction?:
{pause c. 3 s}
M: No.
I: No, is that a chemical change?
{pause c. 3 s}
M: Yes.
I: Ah, okay. So what's the difference between a chemical change and a chemical reaction?
M: A reaction is where two things react with each other, like the gunpowder and flame, and a change is where two things just go together. You know like water and sugar, they go together like water and salt. Partially, they go together.
I: Mm. Partially?
M: Yeah. 'cause, erm, in water and salt you can get the salt back, whereas you can't with water and sugar.
I: Oh, so it's different, is it? Oh, I see. So if you had water and salt, how would you get them back again?
M: Erm, you'd put the water and salt by the window, and let the sun do all the evaporating of the water, and you would be left with the salt crystals.
I: So what if you took water and sugar, and put that by the window, would it evaporate the water, and leave you with the sugar?
{Pause, c. 1 s}
M: N-o.
I: That's different then, is it?
M: Yeah, cause the water's absorbed kind of like the sugar, and because they're, it's so small it would just take the sugar with it.
I: What do you mean it's so small?
{Pause, c. 1 s}
I: What if I had a big beaker of water and sugar?
{Pause, c. 2 s}
M: But there would be more water to salt ratio.
I: …Okay, so there is a difference, then, there's a difference
M: Yeah.
I: between the sugar and the salt?
M: Yeah.

This is an unsatisfactory place to leave the discussion, and in hindsight there are questions I would like to have asked. (Why did she think she could not recover sugar by leaving the water to evaporate? Was she thinking of the amount of sugar / salt needed to form what we would call a saturated solution?…)