change of state - Science-Education-Research

Particles in ice and water have different characteristics

Making a link between particle identity and change of state

Keith S. Taber

Bill was a participant in the Understanding Science Project. Interviews allow learners to talk about their understanding of science topics, and so to some extent allow the researcher to gauge how well integrated or fragmented a learner's ideas are.

Occasionally there is a sense of 'seeing the cogs turn', where it appears that the interview is not just an opportunity for reporting knowledge, but a genuine site for knowledge construction (on behalf of the students, as well as the researcher) as the learner's ideas seem to change and develop in the interview itself.

One example of this occurred when Bill, a Y7 student, explained what he had learnt about particles in solids, liquids and gases. Bill seemed unsure if the particles in different states of matter were different, or just had different properties. However, when asked about a change of state Bill related heating to changes in the way particles were arranged, and seemed to realise this implied the particles themselves were the same when a substance changes state. Bill seemed to be making a link between particle identity and change of state through the process of answering the researcher's questions.

Bill introduced the idea of particles when talking about what he had learn about the states of matter

Well there's three groups, solids, liquids and gases.
So how do you know if something is a solid, a liquid or a gas?
Well, solids they stay same shape and their particles only move a tiny bit.

This point was followed up later in the interview.

So, you said that solids contain particles,
Yeah.
They don't move very much?
No.
And you've told me that ice is a solid?
Yeah.
So if I put those two things together, that tells me that ice should contain particles?
Yeah.
Yeah, and you said that liquids contain particles? Did you say they move, what did you say about the particles in liquids?
Er, they're quite, they're further apart, than the ones in erm solids, so they erm, they try and take the shape, they move away, but the volume of the water doesn't change. It just moves.
Okay. So the particles in the liquid, they seem to be doing something a bit different to particles in a solid?
Yeah.
What about the particles in the gas?
The gas, they, they're really, they're far apart and they try and expand.
Does that include steam, because you said steam was a gas?
Yeah.
Yeah?
I think.
So, we've got particles in ice?
Yeah.
And they have certain characteristics?
Yeah.
And there are particles in water?
Yeah.
That have different characteristics?
Yeah.
And particles in gas, which have different characteristics again?
Yeah.
Okay. So, are they different particles, then?
N-, I'm not sure.

There are several interesting points here. Bill reports that the particles in liquids are "further apart, than the ones in … solids". This is generally true when comparing the same substance, but not always – so ice floats in water for example. Bill uses anthropomorphic language, reporting that particles try to do things.

Of particular interest here, is that at this point in the interview Bill did not seem to have a clear idea about whether particles kept their identify across changes of state. However, the next interview question seemed to trigger a response which clarified this issue for him:

So have the solid particles, sort of gone away, when we make the liquid, and we've got liquid particles instead?
No {said firmly}, when a solid goes to a liquid, the heat gives the particles energy to spread about, and then when its a liquid, it's got even more energy to spread out into a gas.
So we're talking about the same particles, but behaving differently, in a solid to a liquid to a gas?
Yeah.
That's very clear.

It appears Bill had learnt a model of what happened to the particles when a solid melted, but had not previously appreciated the consequences of this idea for the identity of particles across the different states of matter. Being cued to bring to mind his model of the effect of heating on the particles during melting seemed to make it obvious to him that there were not different particles in the different states (for the same substance), where he had seemed quite uncertain about this a few moments earlier.

Whilst this has to remain something of a speculation, the series of questions used in research interviews can be quite similar in nature to the sequences of questions used in the method of instruction known as Socratic dialogue – a method that Plato reported being used by Socrates to lead someone towards an insight.

So, a 'eureka' moment, perhaps?

Are plants solid?

Keith S. Taber

Bill was a participant in the Understanding Science Project. Bill (a Year 7 pupil) told me Bill talked about how in his primary school he had studied "a lot about plants, and – inside them, how they produce their own food", and how "inside, it has leaves, inside it, there is chlorophyll, which stores [sic] sunlight, and then it uses that sunlight to produce its food."

Bill had been talking to me about particles, and I asked if plants had anything to do with particles:

Well in the plant, there is particles….'cause it's a solid…. inside the stem is, 'cause going up the stem there would be water, so that's a liquid. And, it also uses oxygen, which is a gas, to make its food, so. I think so.

I suspect that Bill's reference to the plant being "a solid" would seem unproblematic to many people, especially as Bill recognised the presence of water (a liquid) and oxygen (a gas) as well.

There is however a potential issue here. The model of states of matter and changes of state taught in school strictly refers to reasonably pure samples of particular substances (so water is a liquid at normal temperatures, and oxygen is a gas – although strictly speaking the air in which it is found is a mixture which is not best considered 'a gas'). A plant (like an animal) is a complex structure which cannot be considered as a solid (and indeed living things were separated out in distinct substances, water would make up much of the content).

If the scientific model of solids, liquids and gases is applied beyond the range of individual substances, this is sometimes unproblematic. To consider the air as a gas, or the sea as a liquid, is not usually a problem as it is clear what this means in everyday discourse. But of course it is not possible to find 'the' boiling point of complex mixtures such as these.

However a wooden stool is only a solid in the everyday sense, certainly not in a scientific sense, and to refer to animals or plants as solids does considerable violence to the concept. (BBC Bitesize – please note!*)

(* Read 'Thank you, BBC: I'll give you 4/5')

So if someone was stood here, we'd be a solid

Keith S. Taber

Morag was a participant in the Understanding Science Project. During her first term in secondary school, Morag told me she had studies changes of state, which was about "melting things, it's like solid, liquid and gas. Where like an ice cube melts to go to water, it evaporates to go to gas, it then condenses to go to water and then freezes to go to ice".

When I asked her about about the states of matter, Morag gave me a quite polished response. In the middle of this, she stood up and started moving about. It appeared that she had modelled the states of matter in class through a simulation, with the students acting as particles – and this association seemed to now be cued by her recalling the explanations for the different states of matter:

I: So silly question, 'cause I'm sure everybody knows really, but what's a solid, what's a liquid and what's a gas then?
Morag: A solid is an object where the particles are very close together, but still have room to move very slightly, you know like they can only move little bits, er, it has a fixed shape, it cannot be poured – and that's all I can remember.
I: That's quite a bit. And that's different to a liquid, is it?
M: Yeah, 'cause a liquid you can pour, it takes the shape of its container, the particles are spread out more evenly, but still in a, but are still spread in a – yeah they're spread evenly it can be poured, (it takes the shape of its container), the particles are still quite close, but they are further away than they were in a solid, so they can move just a bit more. If you know what I mean, like. So if someone was stood here [indicating next to her], we'd be a solid, 'cause we just move very slightly,
I: all right, yeah
M: and if we were a liquid we would be stood just a bit further away, so we can move a bit more.
I: I see, so if you had brought a friend with you,
M: Yeah, and if we were stood like that, if she was stood there, we'd be a solid, 'cause we were quite close, but we still had room to move about
I: Mm
M: if we were a liquid, we'd be a bit further, but we still, still quite close, but still had move to room, to move about, and I'm not going to tell you about gas until we get onto gas.
I: Okay. So you and your friend could be a liquid? Which means that I could pour you and you would take up the shape of your container?
M: No, I mean like we'd be the particles in liquid.
I: Ah, I see.
M: you know like
I: Moves around!
M: like, so like, like, so we'd be like that, and there would be lots of us, but we could still move about. Yeah? And if we were a liquid we would be like that, and we could still move about. And if we were a gas we'd be further apart, but and then we can, and then we can move around the room freely.

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.