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.
