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.
