Category: alternative conception

The Sun would pull more on the Earth…

Bert's understanding of the reciprocal nature of forces 

Keith S. Taber

Bert was a participant in the Understanding Science Project. A key idea in school physics is that forces occur in pairs, when two bodies exert an equal magnitude force upon each other (as required by Newton's third law). However, this seems counter intuitive to pupils, who may expect that a larger (more massive, or greater charge etc.) object would exert a greater force on a smaller body than vice versa. In physics a distinction is made between the forces (always equal) and their effects (which depend upon the force applied, and the mass of the object being acted upon). This distinction is not always made by students.

When in Y11, Bert offered an example of one of the common alternative conceptions found among students – that the larger body will exert more force:

What about the Earth going round the Sun, that's an orbit as well is it?

Yeah.

…

So why does it go round?

Why does it go round?

Yeah.

Erm because erm, well one is the gravity of it pulling and the other is, I'm not so sure what the other force is.

That's gravity of what?

The Sun.

So the gravity of the Sun pulling on the Earth?

Yeah.

Do you think the Earth pulls on the Sun?

Yeah, I guess but not strongly enough to move the Sun. Because if there's an object with a small amount of mass then it's not going to give off as much pull as something ten times bigger as it. So the Earth would pull more on the Sun, I mean the Sun would pull more on the Earth.

Whereas the physics perspective is that a force is an interaction between bodies, Bert talks as though a force is something that emanates from one body to another ("give off … pull"), a way of talking quite common among students applying their intuitive understanding of force.

Many students conflate the force acting on a body, and its effect (the acceleration produced) – so here the Sun and Earth are subject to the same force, but the earth is much less massive so will accelerate much more subject to that force than the Sun would. (The Sun's acceleration would actually depend on the net force acting on it considering the various bodies in orbit around it.)

Common experience tells us that in interactions between contrasting bodies (e.g., consider a fly on a windshield) the larger object has more effect, which may seem naturally to mean it applies more force (how much force can the tiny fly impart? – surely the car must apply more force to the fly?) So there is an intuition here, which can act as a grounded learning impediment to learning the physics formalism.


Sodium has one extra electron in its outer shell, and chlorine is minus an electron, so by force pulls they would hold together

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.

Focal figure (Fig. 5) presented to Annie

She was shown a representation of part of a lattice of ions in sodium chloride (see: Sodium and chlorine don't actually overlap or anything), but Annie identified the signified as atoms, not ions, because Annie had an idiosyncratic understanding of what was meant by charge. (Read: Na+ has an extra electron in its outer shell and Cl- is minus an electron and K-plus represents a potassium atom that has an extra electron.)

Annie was asked whether the structure made up of sodium and chlorine 'atoms' would hold together:

Do you think this thing would fall apart? Or would it hold together?

(pause, c.9s)

If you heated it, or reacted it in some way, it would hold together, and it would probably get held together by just forces.

By forces. Any idea what kind of forces would hold it together?

Probably just the attraction.

Uh hm?

The attraction from the plus to the minus because like chlorine's minus an electron and sodium is over an electron. So they could just like hold them together, but not actually combine.

Right, chlorine's, so sodium's, say that about the electrons again.

Sodium has like one extra electron, 'cause it has like an extra electron in its outer shell, and chlorine has seven electrons in its outer shell so its minus an electron so by sort of exchanging, the sodium combining with the chlorine just by force pulls they would hold together.

So Annie saw the plus (+) symbol to mean one electron over a full shell (2.8.1), and the minus (-) symbol to mean one electron short of an octet of electrons (2.8.7). For Annie these charges were not net electrical charges, but deviations from octet configurations. Yet, these 'deviation charges', for Annie, provided the basis for the attraction between the 'charged' atoms.

This was checked by asking Annie about the electron configurations.

So we looked at a sodium atom earlier, you recognised it as being a sodium atom, …

Can you tell me what the configuration is in terms of shells? How many in the first shell, how many in the second shell…

2.8.1

2.8.1?

Yeah.

So this here (indicating a cation on the figure), you are saying that this here is 2.8.1

Yes.

And this is 2.8.7 would it be?

Yeah, 2.8.7

And that is what holds them together the fact that this is one short,

yeah,

one over and one short.

One over, and that one's one short.

So the plus means one electron more than an outer, the full shell,

Yeah.

and the minus means one electron

Minus.

less than an outer shell,

Yeah.

and that's what holds them together.

Yeah.

Okay, so there is something holding them together,

right,

and it's to do with these pluses and these minuses,

Yes.

but what we don't have there is chemical bonding like we had before.

No.

Annie held an alternative conception of the nature of the charges associated with ions: that neutral atoms had 'charges' if they did not have full shells/octets of electrons. Whilst Annie's specific deviation charge conception would seem to be rather unusual, alternative conceptions relating to the significance of full shells / octets of electrons seems to be very common among chemistry students. Although Annie's thinking was idiosyncratic it reflected the common full shells explanatory principle that sees electronic configuration as a cause for chemical processes.

So Annie considered that these 'deviation' charges could actually give rise to forces between atoms (see also The force of lack of electrons pulls two hydrogen atoms together*).

Annie did not see ions, but atoms. But she thought that after a reaction, there would be attractions, 'force pulls', holding the product together, but this would not amount to chemical bonding.

Annie's notion of 'charges' on atoms (being extra or missing electrons in the outer shell), that led to her not recognising bonding in the NaCl, was an uncommon alternative conception notion. However, her notion that chemical bonding was something other than 'just forces', and that sometimes structures were held together by 'just forces' when there was no bonding, is a common alternative conception. Indeed it is part of a common 'molecular framework' for conceptualising ionic bonding, that is in turn a part of a common alternative conceptual framework for thinking about chemical bonding, stability and reactions: the octet framework.


Na+ has an extra electron in its outer shell and Cl- is minus an electron

The plus sign shows Na+ has an extra electron in its outer shell; the minus sign shows Cl– has seven electrons in its outer shell so its minus an electron

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.

Focal figure (Fig. 5) presented to Annie in interview

She was shown a representation of part of a lattice of ions in sodium chloride (see: Sodium and chlorine probably get held together by just forces*), but Annie identified the signified as atoms, not ions:

Any idea what that’s meant to be?

(pause, c.6s)

Just sodium and chlorine atoms.

As an A level student, Annie would be expected to understand the differences between atoms, ions and molecules, and to known that there were ions in NaCl, but this could have been a simple slip of the tongue. This was tested by further questioning:

Erm, so if you look at these, I mean you said they were sodium and chlorine

Yes.

because presumably you recognise the Na and the Cl,

Yeah.

but only two of them are labelled with ‘Na’ and ‘Cl’.

Yes.

What about the others – what do you think they are?

They’re probably sodium and chlorine, or else they could be, because of the signs, you’ve got plus and minus signs on them representing the charge, or else it could be similar elements going down the groups.

Okay, so you recognise that these, these things represent charges, and you probably guess it’s just me being lazy that I haven’t labelled them all, [Annie laughs] so I’ve just labelled the first couple, erm, so these are what, so you reckon this little one will be, what will that be do you reckon?

Sodium.

That will be a sodium, molecule?

Atom.

Sodium atom, what about this one here?

Chlorine atom.

That’ll be an atom. But these have got charges on?

Yeah.

So Annie recognised the symbols for positive and negative charges, and thought that the figure signified atoms, with charges. The simplest interpretation here is simply that Annie did not recall that atoms were neutral, and 'charged atoms' are called ions in chemistry.

However, Annie then told me that sodium has like one extra electron in its outer shell, and chlorine is minus an electron, so by force pulls they would hold together, and explained this in terms of her notion of charges:

…say that about the electrons again.

Sodium has like one extra electron, ‘cause it has like an extra electron in its outer shell, and chlorine has seven electrons in its outer shell so its minus an electron so by sort of exchanging, the sodium combining with the chlorine just by force pulls they would hold together.

So Annie saw the plus (+) symbol to mean one electron over a full shell (2.8.1), and the minus (-) symbol to mean one electron short of an octet of electrons (2.8.7). For Annie these charges were not net electrical charges, but deviations from octet configurations. These 'deviation charges', for Annie, provided the basis for the attraction between the 'charged' atoms.

This was checked by asking Annie about the electron configurations.

So we looked at a sodium atom earlier, you recognised it as being a sodium atom, I did not say it was, and that had an electronic configuration of…do you remember what the electron configuration was?

Eleven.

So a total of eleven electrons

Yeah.

So do you know what shells they were going to?

Sorry?

Can you tell me what the configuration is in terms of shells? How many in the first shell, how many in the second shell…

2.8.1

2.8.1?

Yeah.

So this here (indicating a cation on the figure), you are saying that this here is 2.8.1

Yes.

And this is 2.8.7 would it be?

Yeah, 2.8.7

Annie held an alternative conception of the nature of the charges associated with ions: that neutral atoms had charges if they did not have full shells/octets of electrons. That this was a general feature of her thinking became clear when she was asked about the symbols for other ions: such as K+ and F–.

Whilst Annie's specific 'deviation charge' conception (i.e., that (neutral) atoms would be charged when they did not have fill shells/octets of electrons) would seem to be rather idiosyncratic, alternative conceptions relating to the significance of full shells / octets of electrons seems to be very common among chemistry students.

Although species with Annie's deviation charges did not have actual overall electrical charge, Annie considered that these 'deviation' charges could actually give rise to forces between atoms (she thought that as sodium has one extra electron in its outer shell, and chlorine is 'minus an electron', then they would hold together; The force of lack of electrons pulls two hydrogen atoms together⚗︎).

 

Calcium and oxygen would not need to bond, they would just combine…

Calcium and oxygen would not need to bond, they would just combine, joining on to make up full shells

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. Near the end of the interview, she was asked some general questions to recap on points she had made earlier. She suggested that Ca2+ and O2- would combine, but without any chemical bonding.

Could you have a double ionic bond?

(pause, c.3s)

Can you have a double bond that's ionic?

Not really sure.

If you had say, say you had calcium, two-plus (Ca2+), and oxygen two-minus (O2-),

yeah,

could that form a double bond?

(pause, c.4s)

Are you not sure?

It wouldn't need to.

It wouldn't need to?

No.

Why's that?

Because one's lacking two electrons, and one's got two, so, they would just combine without needing to sort of worry about other, other erm elements.

Right so they…

Sort of joining on to make up full shells.

So they combine, but you wouldn't call that a chemical bond?

No.

From what Annie had reported earlier in the interview, she would see Ca2+ as a calcium atom (that's "got two" electrons in its outer shell) and O2- as a oxygen atom (that was "lacking two electrons"), as she held an alternative conception of what was meant by the symbols used to indicate electrical charge plus and minus signs represent the charges on atoms)*.

Annie here suggests that the atoms with their charges (i.e., for Annie, deviations form full shells) would combine, and join up to obtain a full shell. From her perspective, there was no need for ionic bonding. Although Annie's notion of what was signified by the charge symbols would seem to be idiosyncratic, the idea that chemical processes occur to allow atoms to obtain full shells (the 'full shells explanatory principle') is one of the most common alternative conceptions in chemistry.

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.

An element needs a certain number of electrons

An element needs a certain amount of electrons in the outer shell

Keith S. Taber

Bert was a participant in the Understanding Science project. In Y10 Bert was talking about how he had been studying electrolysis in class. Bill had described electrolysis as "where different elements are, are taken out from a compound", but it transpired that Bert thought that "a compound is just a lot of different elements put together"*. He seemed to have a tentative understanding that electrolysis could only be used to separate elements in some compounds.

if they're positive and negative then they would be able to be separated into different ones.

So some things are, some things aren't?

Yeah, it matters how many electrons that they have.

Ah. [pause, c.3s] So have you got any examples of things that you know would definitely be positive and negative?

Well I could tell you what happens.

Yeah, go on then.

Well erm, well if a, if an element gives away, electrons, then it becomes positive. But if it gains, then it becomes negative. Because the electrons are negative, so if they gain more, they just go a bit negative.

Yeah. So why would an element give away or gain some electrons? Why would it do that?

Because erm, it needs a certain amount of electrons in the outer shell. It matters on what part of the periodic table they are.

Okay, let me be really awkward. Why does it need a certain number of electrons in the outer shell?

[Pause, c.2 s]

Erm, well, I don't know. It just – 

So Bert thought that an element "needs a certain amount of electrons in the outer shell" depending upon it's position in the periodic table, but he did not seem to recall having been given any reason why this was. The use of the term 'needs' is an example of anthropomorphism, which is commonly used by students talking about atoms and molecules. Often this derives from language used by teachers to help humanise the science, and provide a way for students to make sense of the abstract ideas. If Bert comes to feel this is a sufficient explanation, then talk of what an element needs can come to stand in place of learning a more scientifically acceptable explanation, and so can act as a grounded learning impediment.

References to atoms needing a certain number of electrons is often used as an explanatory principle (the full shells explanatory principle) considered to explain why bonding occurs, why reactions occur and so forth.

Bert's final comment in the short extract above seems to reflect a sense of 'well that's just the way the world is'. It is inevitable that if we keep asking someone a sequence of 'well, why is that' question when they tell us about their understanding of the world, they eventually reach the limits of their understanding. (This tendency has been labelled 'the explanatory gestalt of essence'.) Ultimately, even science has to accept the possibility that eventually we reach answers and can not longer explain further – that's just the way the world is. Research suggests that some students seem to reach the 'it's just natural' or 'well that's just the way it is' point when teachers might hope they would be looking for further levels of explanation. This may link to when phenomena fit well with the learner's intuitive understanding of the world, or tacit knowledge.

Bert's reference to an element needing a certain amount of electrons in the outer shell also seems to confuse description at two different levels: he explicitly refer to substance (element), when he seems to mean a quanticle (atom). Element refers to the substance, at the macroscopic level of materials that can be handled in the laboratory, whilst an atom of the element (which might better be considered to gain or lose electrons) is part of the theoretical model of matter at a submicroscopic level, used by chemists as a basis for explaining much macroscopic, observed behaviour of samples of substances.


A sodium atom wants to donate its electron to another atom

Keith S. Taber

Lovesh was a participant in the Understanding Chemical Bonding Project, studying 'A level' chemistry in a further education college. He was interviewed in his second year of the two year A level course, and was presented with focal figure 1 (below). He recognised figure 1 as showing a "sodium, atom", and was asked about its stability:

Is that a stable species, do you think?

Erm (pause, c.3s) No, because it hasn't got a, a full outer – electron shell, outer electron shell hasn't got eight electrons in.

Lovesh shared the common notion that an atom without a full outer shell / octet of electrons would be unstable compared with the corresponding ion with a full outer shell / octet of electrons. When comparing isolated atoms with the corresponding ions this is seldom the case, yet this is a common alternative conception about chemical stability. A sodium ion can be considered stable in an ionic lattice, or when hydrated in solution, but does not spontaneously ionise as the outer shell electron is attracted to the atom's positive core. Ionisation only occurs when sufficient work is done to overcome this attraction.

Lovesh was demonstrating the common full shells explanatory principle alternative conception which is central to the common octet rule framework – an alternative conceptual framework reflecting very common 'misconceptions' found among learners studying chemistry.

Lovesh was asked what would happen to the atom that he considered unstable:

So if it's not stable, what would tend to happen to that, do you think?

It will wanna donate the electron to another atom.

Right, when you say 'it wants to donate' it?

Erm. (pause, c.3s) Well because that outer electron is less attracted to the nucleus, erm it is, it can easily be transferred, attracted by another atom.

Lovesh's first response here used the term 'wanna' (want to) which if take literally suggests the atom has desires and preferences. This is an example of anthropomorphism, imbuing objects with human-like traits. Using anthropomorphic explanations is a common feature of the octet rule framework which often leads to students talking as if atoms deliberately act to get full outer electron shells.

It has been suggested that such anthropomorphism may be either 'strong'- where the learner is offering an explanation they find convincing – or 'weak' if they are using language metaphorically, just as a figure of speech.

In this case, when Lovesh's use of the notion of 'wants' was queried he was able to shift to a different language register in terms of the action of physics forces – the electron being attracted elsewhere. Lovesh had clearly acquired an appropriate way of thinking about the interactions between atoms, but his spontaneous explanation was couched in anthropomorphic terms. Although in this case the anthropomorphism was of a weak form, the habitual use of this kind of language may come to stand in place of offering a scientifically acceptable account.


Electrical resistance depends upon density

Keith S. Taber

Amy was a participant in the Understanding Science project.

Amy (Y10) suggested that a circuit was "a thing containing wires and components which electricity can pass through…it has to contain a battery as well". She thought that electricity could pass through "most things".

For Amy "resistance is anything which kind of provides a barrier that, which the current has to pass through, slowing down the current in a circuit", and she thought about this in terms of the analogy with water in pipes: "we've been taught the water tank and pipe running round it… just imagine the water like flowing through a pipe, and obviously like, if the pipe becomes smaller at one point, erm, the water flow has to slow down, and that's meant to represent the resistance of something".

So for Amy, charge flow was impeded by physical barriers effectively blocking its way. She made the logical association with the density of a material, on the basis that a material with densely packed particles would have limited space for the charge to flow:

So electricity would "not very easily" pass through a wooden bench "because wood is quite a dense material and the particles in it are quite closely bonded".

In air, however, the particles were "not as dense as a solid". When asked if that meant that electricity can pass through air quite easily, Amy replied: "yeah, I think so".

Amy's connection between the density of particles and the ease with which charge could flow is a logical one, but unfortunately involves a misunderstanding of how charge flows through materials, i.e., from a canonical scientific perspective, thinking about the charge flowing through gaps between particles is unhelpful here. (So this can be considered an alternative conception.) This seems to be a creative associative learning impediment, where prior learning (here, the spacing of quanticles in different materials) is applied, but in a context beyond its range of application.


A chemical change is where two things just go together

Keith S. Taber

Morag was a participant in the Understanding Science project. In the first interview, in her first term in secondary school, Morag told me that that she was studying electricity having previously studied changing state and burning. When I asked her whether these science topics have anything in common, that made them science, we got into a conversation about chemical reactions, and chemical change:

Do they have anything in common do you think? is there anything similar about those topics?

Changing state and burning's got something in common, but I don't know about electricity.

Oh yeah? So what's, what have they got in common then?

Erm, in burning you have, you could have a chemical reaction, and in changing states you've got chemical reactions as well.

From the canonical scientific perspective, a change of state is not a chemical reaction (so this is an alternative conception), so I followed up on this.

Ah, so what's a chemical reaction?

(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.

Oh, can you give me an example of that?

{pause, c. 2 seconds}

Water mixing with sugar, but that's not a chemical reaction.

So, Morag offers a definition or at least a description of a chemical reaction, but then the example she gives of that of type of event is not something she considers to be a chemical reaction. (Dissolving is not usually considered a chemical change, although it usually involves the breaking and forming of bonds, sometimes strong bonds.)

Oh so that's something else is it, is that something different?

I don't know.

Don't know, so can you mix water with sugar?

Yeah, but you can't get the water and the sugar back together very easily.

You can't. Is there a way of doing that?

No.

No? So if I gave you a beaker with some sugar in, and a beaker with some water in, 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?

Ye-ah.

Yeah, so is that a chemical reaction?

No.

No, okay. That's not a chemical reaction.

At this point Morag suggested we look in her book as "it's in my book", but I was more interested in what she could tell me without referring to her notes.

So, have you got any examples of chemical reactions – any you think are chemical reactions?

Fireworks,

I: Fireworks, okay.

when like the gunpowder explodes, erm in the inside, and you can't get it back to the original rocket once it's has exploded.

and is that what makes it a, er, a chemical reaction, that you can't get it back?

{pause, c. 3 s}

Yeah, I suppose so.

So, now Morag has presented an example of a chemical reaction, that would be considered canonical (as chemical change) by scientists. Yet her criterion is the same as she used for the dissolving example, that she did not think was a chemical reaction.

Yeah? And then the water and the sugar, you can't get them back very easily, but we don't think that is a chemical reaction?

Yeah – that's a chemical change – {adding quietly} I think.

It's what, sorry?

Well there's, a chemical reaction and a chemical change.

Oh I see. So what's the difference between a chemical reaction and a chemical change?

Erm nothing, it's just two different ways of saying it.

Oh so they're the same thing?

Yeah, just two different ways of saying it.

So, now Morag had introduced a differentiated terminology, initially suggesting that sugar mixing with water was a chemical change, whereas a firework exploding was a chemical reaction. However, this distinction did not seem to hold up, as she believed the terms were synonyms. However, as the conversation proceeded, she seemed to change her mind on this point.

So when a firework goes off, the gunpowder, er, explodes in a firework, that's a chemical reaction?

Yeah – yeah, cause something's mixing with the gunpowder to make it blow up.

…

And So that's a chemical reaction?

Yeah.

And is that a chemical change?

{pause, c. 2 s}

Yeah.

Yeah?

(I suppose.) Yeah.

And when you mix sugar and water, you get kind of sugary water?

Yeah.

Have you got a name for that, when you mix a liquid and solid like that?

{pause, c. 1 s}

Or is that just mixing sugar and water?

{pause, c. 1 s}

There is a name for it, but I don't know it.

Ah. 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?

Yeah.

And get the sugar out and the water out?

Yeah.

So is that a chemical reaction?

{Pause, c. 3 s}

No.

No, is that a chemical change?

{Pause, c. 1 s}

Yes.

Ah, okay.

So, again, Morag was suggesting she could distinguish between a chemical reaction, and a chemical change.

So what's the difference between a chemical change and a chemical reaction?

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…

In effect we had reached a tautology: in a chemical reaction, unlike a chemical change, things react with each other. She also thought that a sugar/water and a salt/water mixtures (i.e., solutions) were different "because the sugar's so small it would evaporate with the water"*.

The idea that a chemical reactions has to involve two reactants is common, but is an alternative conception as chemists also recognise reactions where there is only one reactant which decomposes.

Morag seemed to be struggling with the distinction between a chemical and a physical change. However, that distinction is not an absolute one, and dissolving presents a problematic case. Certainly without a good appreciation of the submicroscopic models used in chemistry, it is not easy to appreciate why reactions produce a different substance, but physical changes do not. One of Morag's qualities as a learner, however, was a willingness to 'run with' ideas and try to talk her way into understanding. That did not work here, despite Morag being happy to engage in the conversation.

Morag was also here talking as though in the gunpowder example the flame was a reactant (i.e., the flame reacts to the gunpowder). Learners sometimes consider substances in a chemical reaction are reacting to heat or stirring rather than with another substance (e.g., Taber & García Franco, 2010).

Read about learners' alternative conceptions

Source cited:

Taber, K. S., & García Franco, A. (2010). Learning processes in chemistry: Drawing upon cognitive resources to learn about the particulate structure of matter. Journal of the Learning Sciences, 19(1), 99-142.