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?…)
