Morag was a participant in the Understanding Science project. Interviewed in the first term of her secondary school, she suggested that A chemical change is where two things just go together. Whilst this is not a very helpful 'definition', the concept of a chemical change is a very difficult one, especially before learners are introduced to the submicroscopic models that scientists use to make sense of substance and substance change in chemistry. Morag presented the criterion of changes that could not be readily undone, using the example of the explosion of the gunpowder in a firework:
I: So, have you got any examples of chemical reactions – any you think are chemical reactions?
Morag: Fireworks…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.
I: And is that what makes it a, er, a chemical reaction, that you can’t get it back?
{pause c. 3 s}
M: Yeah, I suppose so.
The explosion of gunpowder is a difficult reaction (or actually, set of reactions) for a novice to make sense of. Gunpowder is a mixture of substances, which will react together at a high rate provided sufficient activation energy to initiate the reaction is available. The flame, burning along the fuse, provides that initiation by making the mixture hot enough for reaction to commence. None of this is obvious to the viewing lacking the theoretical background to appreciate the chemistry. What is salient is the burning fuse.
I: So when a firework goes off, the gunpowder, er, explodes in a firework, that’s a chemical reaction?
M: Yeah - yeah, cause something’s mixing with the gunpowder to make it blow up.
I: What’s doing that? What’s mixing with the gunpowder?
M: The erm, when you light it, the flame, erm, reacts to the gunpowder, which makes it go bang.
I: Oh I see. Right, okay.
M: I think.
I: And So that’s a chemical reaction?
M: Yeah.
So for Morag 'the flame reacts to the gunpowder'. This language, that one thing reacts to another, is quite common in the talk of school students when discussing chemical phenomena, and often seem to imply that there is a more active and a more passive partner in the reaction (*). However, this may not have been implied here, as later in the interview Morag explained that "a reaction is where two things react with each other, like the gunpowder and flame", a much more symmetrical ('with each other') form of words.
It is not unusual for students to fail to appreciate the ways scientists think about substances. The flame is very obvious, and known to be important for fireworks to ignite. The flame is material, of course, but its significance here is its nature as an area of localised high temperature: a source of energy transfer to the gunpowder. For students of this age, it is not unusual for heat or light to be considered as of similar status as substances. (Technically we might say that they give energy and matter the same 'ontological status', whereas in science there is an important distinction.) Again, until students have learnt particle models to make sense of matter in the way scientists do, this confusion is understandable.
*: e.g., see: 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.
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Dr Keith S Taber kst24@cam.ac.uk
University of Cambridge Faculty of Education
© Keith S Taber