learner thinking - Science-Education-Research

A reaction is just something that happens?

Keith S. Taber

The term 'reaction' is used in at least two different technical senses in school science: in studying forces as one of the components of a interaction between two bodies such that they each experience a force ('action-reaction'), and as a chemical change which leads to a transformation of matter leading to a new substance(s).

Lomash was a participant in the Understanding Science project. Y7 student 'Lomash' reported that he had been heating materials in a Bunsen flame in his science lessons: "We were burning … coal and copper and things like that, metals."

When he heated copper "It went black…because the flame was too hot, and – it just went black , like paper." The copper stayed black after being removed form the flame, and this was because "it's something else, it's a reaction."

Lomash was using the term 'reaction' in the context of a chemical change – the copper had changed to 'something else', suggesting that he had acquired something of the technical meaning of the term as it is used in chemistry. However 'reaction' is used with a much more general meaning in everyday life, and on further questioning it seemed Lomash has not appreciated the special meaning given to the word in chemistry:

I: So what's a reaction?
L: It's like, a reaction is something that happens.
I: Okay, so if I fell off this stool, would that be a reaction?
L: Yeah.
I: And if you laughed at me falling off the stool, would that be a reaction?
L: Yeah.
I: Oh I see. So that's just another name for something that happens is it?
L: Yeah.

Where students already have meanings for words they come across in school science, they are unlikely to spontanously appreciate how the word is used in a specialised, nuanced way in this particular context. Perhaps Lomash's teacher had emphasised that in heating the copper 'something else' was produced, making the observed change a 'reaction'. Certainly Lomash happily accepted this was a reaction, but apparently only in his existing vague everyday sense of the term. His existing linguistic association for the term 'reaction' appeared to act as an associative learning impediment.

Read about learners' alternative conceptions

A molecule is a bit of a particle – or vice versa

Keith S. Taber

Tim was a participant in the Understanding Science project. When I talked to Tim during the first term of his 'A level' (college) course, he had been studying materials with one of his physics teachers. He referred to molecules in wood (suggesting the analogy that molecules are like a jigsaw)*, and referred to a molecule as "a bit of a particle",

I: So what's a molecule?
T: Erm it's like a bit of a particle, so, something that makes up something.

He then went on to refer to how malleability depended upon atoms "because it's just what they're made out of, it's different things to make it up, different atoms and stuff". His understanding of the relationship between atoms and molecules was probed:

Ah, so we've got atoms?
Yeah.
Not molecules?
(Pause, c.2s)
This is something different this time?
Yeah.
Oh, okay, tell me about atoms.
I think, I think atoms make up molecules, which make particles. Well there's them three things, but I'm not entirely sure what order they go in, and I think atoms are the smallest one.
So we've got, these three words are related, are they, atoms, molecules, particles?
Yeah.
You think there is a relationship there?
Yeah.
And, what, they are similar in some way, but not quite the same, or?
Erm, yeah I think it's like order of size.
You think atom's the smallest?
Yeah.
And bigger than an atom you might have?
A molecule. No a particle, then a molecule, I think.
Yeah, is that the same for everything do you think? Or, are some things molecules, and some things atoms, and some things particles?
(Pause, c.2s)
I think it's the same, I think it all goes – like that.

The term 'particle' is ambiguous in school science. Sometimes by particle we mean a very small, but still macroscopic objects, such as a salt grain or a dust speck. However, often, we are referring to the theoretical submicroscopic entities such as atoms, molecules, ions, neutrons etc, which are components of our theoretical models of the structure of matter. (These particles, behave in ways that are sometimes quite unlike familiar particle behaviour because of the extent to which quantum effects can dominate at their scale. The term 'quanticle' has been proposed as a collective term for these particles.) Students are expected to know which usage of 'particles' we might mean at any given time.

Tim assumes to have misunderstood how the term particle is used (as a collective term) when used to describe quantiles, and so has come to the understanding that at this level there are three different categories of quanticle based on relative size: the atoms (the smallest), and also molecules and particles which are larger than atoms, but which he is unsure how to relate.

The use of the everyday word particle to refer to theoretical submicroscopic entities by analogy with the more familiar everyday particles is very clear to scientists and science teachers, but can act as an associative learning impediment to learners who may think that quanticle particles are just like familiar particles, but perhaps quite a lot smaller. In Tim's case, however, it seems that a different 'learning bug' had occurred. Presumably he had commonly come across the use of the terms 'atom', 'molecule' and 'particle' in science lessons to describe the components of matter at the submicroscopic level, but had not realised that particle was being used as a generic term rather than describing something different to atoms and molecules.

During his years of school science Tim had constructed a different 'ontology' of the submicroscopic constituents of matter to that expected by his teachers.

Read about learners' alternative conceptions

A compound is just a lot of different elements put together

Keith S. Taber

Bert was a participant in the Understanding Science project. When interviewed in Y10 he reported that he had been studying electrolysis in chemistry:

"that's where different elements are, are taken out from a compound…there's a positive anode and a negative cathode. And what it does it attracts the positive part of the compound to the negative cathode, and the negative part goes to the positive , to, you know, so that they can erm get the different elements in the different places, so they can just have one element on its own".

To fully understand what this means from a chemical context the learner needs to appreciate the chemical distinction between elements, compounds and mixtures, so I asked Bert what he thought a compound was:

It's erm, it's er two, er you know, it's just a lot of different elements put together – to create just one.
So if I went and got some elements, let's say I went and got a little file of carbon, a little file of sulphur, a little file of copper, er a little file of magnesium and I were to mix them into a beaker, maybe get a glass rod, give it a good stir, er, give me a compound?
Erm, so it's carbon, erm, carbon, sulphur, magne¬. Carbon, er – What's the fourth one?
Carbon, sulphur, magnesium and copper I think I said.
And copper. All right, erm. Copper, copper sulphate and – and carbon, and I think carbon and magnesium might go just as elements.
Okay, so if I ignored the carbon and magnesium,
Yeah.
if I took some copper and some sulphur,
Yeah.
and mixed them up together,
Yeah.
then I'd get copper sulphate.
Yeah.
And that's a compound now?
Yeah.

In chemistry there is a crucial difference between a mixture and a compound: one which it appeared Bert had not at this point acquired. Presumably his chemistry teacher, in teaching the topic of electrolysis was assuming students in the class would apply prior learning about the difference between elements and compounds, so as to appreciate the significance of electrolysis as a technique which brings about an energetically unfavourable chemical change. This prerequisite knowledge appeared to be lacking for Bert, which provided a deficiency learning impediment when it came to understanding the teaching on electrolysis.

Read about learners' alternative conceptions

A cloud is a gas you can see

Keith S. Taber

Bill was a year 7 student who participated in the Understanding Science Project. Bill was explaining that he had been learning about the states of matter, and gave me examples of things he considered to be solids, liquids or gases. I asked him about clouds, because students commonly consider them gases:

So do you think everything, is either a solid, or a liquid or a gas?
I'm not sure? Erm, I think that, some, I think that they are mainly, fall into a group, but I'm not sure.
Not sure about that, okay. Erm, what about a cloud? You look at a nice sky, and there's a cloud? Do you think that's a solid, or a liquid or a gas?
I think that that's a gas that you can see, because it is made up of, I think it is made up of different gases, I'm not sure, though.

Gases are transparent and so generally not visible. A cloud is opaque, and is made up of many tiny droplets of liquid (water in the case of the clouds in the earth's atmosphere) that have been formed by condensation. However, because they remain in the air (until it rains!), it is understandable that students may hold the alternative conception that they are gases themselves. Liquids are much more dense than gases, so it is not immediately obvious to students how a cloud of liquid drops can remain 'floating' in the air. That Bill offered a tentative answer and was not strongly committed to the idea of clouds being gaseous, suggests he was open to revising this view given new evidence to consider.

Read about learners' alternative conceptions

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.