Category: grounded learning impediment

Fuels get used-up when we burn them

Keith S. Taber

Sophia was a participant in the Understanding Science Project. Sophia (then in Y7) had been burning materials in science. She had burnt some paraffin in a small burner (a glass burner with a wick). Her understanding of the process was not in terms of a chemical reaction, but at a more 'phenomenological' level:

So what happens to paraffin when it burns then?

It keeps on burning… but you, you can put it out easily as well…. we just blew it out…

I see, but otherwise it just carried on burning, did it? Did it carry on burning for ever, if you don't blow it out?

No, 'cause it would run out.

What would it run out of?

The paraffin.

So where does the paraffin go then?

(There was a pause, of about 4 seconds. Sophia laughs, but does not offer answer.)

And what happens to the level of the paraffin in the burner?

It gets lower and lower.

So why's that, what's happened to it?

'cause you are using all of it up, when it's burning.

So it get all used up does, it – so what happens when it's all used up?

You have to refill it.

So for Sophia the burning of paraffin is not seen in terms of basic chemistry (what happens to the substance paraffin during the process of burning?), but rather she seems to interpret what she has seen in terms of everyday ideas – stuff, such as fuels, get used up – if we use it, we no longer have it.

The final question in this sequence ('what happens when it's all used up') is not treated in scientific terms (e.g., from the perspective of the conservation of matter, there is an issue of where the 'stuff' what was the paraffin has gone), but in practical terms: when we use up the fuel in the burner, we need to refill it to do more burning.

Here, understanding in 'everyday' or 'lifeworld' terms seems to dominate her thinking: the familiar idea that things get used-up obscures the scientific question of what happens to the matter in the fuel. Presumably, her teacher wanted her to focus on the scientific perspective, where burning is combustion, a type of chemical change, but it appears her life-world perspective acted as a grounded learning impediment – an existing way of thinking about a phenomenon that is taken for granted and obscures the scientific perspective.

The everyday way of understanding the world could be called the natural attitude. It seems that for Sophia it is 'just natural' that fuels get used up, and so there is nothing there to explain. Arguably, the work of a science teacher sometimes involves persuading students to seek explanations for things they had considered 'just natural', and so not in need of explanation.

Current only slows down at the resistor

Current only slows down at the resistor – by analogy with water flow 

Keith S. Taber

Students commonly think that resistance in a circuit has local effects, and in part that is because forming a mental model of what is going on in circuits is very difficult. Often models and analogies can be useful. However when an analogy is used in teaching there is also the potential for it to mislead.

Amy was a participant in the Understanding Science Project. Amy (when in Y10) told me she had been taught to use a water flow analogy for electric current. However, because her visualisation of what happens in water circuits was incorrect, she used the analogy to inform an alternative conception about circuits:

Do you have any kind of imagined sort of idea, any little mental models, about what (the flow of electricity round the circuit) might look like? Do you have a way of imagining that?

Erm, yeah, we've been taught the water tank and pipe running round it. … just imagine the water like flowing through a pipe, and obviously like, if the pipe becomes smaller a one point, erm, the water flow has to slow down, and that's meant to represent the resistance of something.

So, so if I had my water, er, tank and I had a series of pipes, they'd be water flowing through the pipes, and if I had a narrower pipe at one point, what happens then?

The water would have to slow down.

So would it slow down just as it goes through the narrow pipe, or would it slow down all the way round?

Erm – just through that part.

(Amy does not appreciate the implications of conservation of mass {that is, the continuity principle} here – at steady state there cannot be a greater mass flow at different points in the circuit).

…

And so how do you imagine that's got to do with resistance, how does that help you understand resistance?

…well resistance, it slows the current down, but then erm, once it passes a resistor or something it, the current is free to flow through the wire again

Analogies can be very useful teaching tools, but when using them it is important to check that the students already understand the features of the analogue that are meant to be helpful. It is also important to ensure that they understand which features are meant to be mapped onto the target system they are learning about, and which are not relevant.

Analogies are only useful when the learner has a good understand of the analogue. In this case, as Amy did not appreciate that the water flow throughout the system would be limited by the constriction, she could not use that as a useful analogy for why a resistor influences current flow at all points in a series circuit. This is an example of where a teaching model meant to support learning, which actually misleads the learner. That is, for Amy, with her flawed understanding of fluid flow, the teaching model acted as a pedagogic learning impediment – a type of grounded learning impediment.

The Sun would pull more on the Earth…

Bert's understanding of the reciprocal nature of forces 

Keith S. Taber

Bert was a participant in the Understanding Science Project. A key idea in school physics is that forces occur in pairs, when two bodies exert an equal magnitude force upon each other (as required by Newton's third law). However, this seems counter intuitive to pupils, who may expect that a larger (more massive, or greater charge etc.) object would exert a greater force on a smaller body than vice versa. In physics a distinction is made between the forces (always equal) and their effects (which depend upon the force applied, and the mass of the object being acted upon). This distinction is not always made by students.

When in Y11, Bert offered an example of one of the common alternative conceptions found among students – that the larger body will exert more force:

What about the Earth going round the Sun, that's an orbit as well is it?

Yeah.

…

So why does it go round?

Why does it go round?

Yeah.

Erm because erm, well one is the gravity of it pulling and the other is, I'm not so sure what the other force is.

That's gravity of what?

The Sun.

So the gravity of the Sun pulling on the Earth?

Yeah.

Do you think the Earth pulls on the Sun?

Yeah, I guess but not strongly enough to move the Sun. Because if there's an object with a small amount of mass then it's not going to give off as much pull as something ten times bigger as it. So the Earth would pull more on the Sun, I mean the Sun would pull more on the Earth.

Whereas the physics perspective is that a force is an interaction between bodies, Bert talks as though a force is something that emanates from one body to another ("give off … pull"), a way of talking quite common among students applying their intuitive understanding of force.

Many students conflate the force acting on a body, and its effect (the acceleration produced) – so here the Sun and Earth are subject to the same force, but the earth is much less massive so will accelerate much more subject to that force than the Sun would. (The Sun's acceleration would actually depend on the net force acting on it considering the various bodies in orbit around it.)

Common experience tells us that in interactions between contrasting bodies (e.g., consider a fly on a windshield) the larger object has more effect, which may seem naturally to mean it applies more force (how much force can the tiny fly impart? – surely the car must apply more force to the fly?) So there is an intuition here, which can act as a grounded learning impediment to learning the physics formalism.


Gas particles like to have a lot of space, so they can expand

Keith S. Taber

Derek was a participant in the Understanding Science Project. I interviewed Derek when he was in Y7 of the English school system. We had been talking about work that Derek has been doing in his science classes on burning. As part of the conversation, Derek defined a solid in particle terms:

what's a solid then, what's a solid?

Lots of particles really close together that can't move a lot.

When I followed this up, Derek explained how a liquid or gas was different to a solid:

And you say solids are made of particles. What are liquids then, they are not made of particles then?

No they are, they are just more spread out particles. And then, you get a gas, which the particles can move a lot more than solid and liquid, they can move wherever they like.

And where do they like to move?

As far away from each other as possible.

Why do you think that is?

'cause they like to have a lot of space, so they can expand.

Why do you think particles like to have a lot of space?

(Pause, c.3s)

Don't know.

Are they unfriendly lot, unsociable?

(Pause, c.2s)

No, they just, they like to have, like be as well away from each other as possible.

The question "where do they like to move" was couched in anthropomorphic terms to reflect the anthropomorphism of Derek's statement that gas particles could "move wherever they like", to see if he would reject the notion of the particles 'liking'. However Derek did not query my use of this language, and indeed suggested that the particles "like to have a lot of space".

When he was asked why, there was a pause, apparently suggesting that for Derek the notion of the particles liking to be far apart seemed to be reasonable enough for him not to have thought about any underlying reason, and his "don't now" was said in a tone suggesting this was a rather uninteresting question. Although Derek rejected the suggestion that the particles were 'unfriendly', 'unsociable' his tone did not suggest he thought this was a silly suggestion: rather it was just that the particles "like" to be as far "away from each other as possible".

The use of anthropomorphism is very common in student talk about particles. Whether or not Derek really believed these gas particles actually had 'likes' in the way that, say, he himself did, cannot be inferred from this exchange. But, in Derek's case, as in that of many other students, the anthropomorphic metaphors seem to offer a satisfactory way of thinking about particle 'behaviour' that is likely to act as a grounded learning impediment because Derek is not open to looking for a different kind (i.e., more scientifically acceptable) type of explanation. Given the common use of his language, it seems likely that it derives from the way teachers use anthropomorphic language metaphorically to communicate abstract ideas to students ('weak anthropomorphism'), but which students accept readily because thinking about particle behaviour in terms of the 'social' models makes sense to them ('strong anthropomorphism').

Atoms within an element don't need to be bonded …

Atoms within an element don't need to be bonded because they're all the same sort

Keith S. Taber

Annie was a participant in the Understanding Chemical Bonding project. She was interviewed near the start of her college 'A level' course (equivalent to Y12 of the English school system). Annie was shown, and asked about, a sequence of images representing atoms, molecules and other sub-microscopic structures of the kinds commonly used in chemistry teaching. Annie was shown a representation of the close packing of 'atoms' in a metal (with the iron symbol, Fe, shown).

Okay, have a look at number 6…

• • • • • • (pause, c.6 s)

They are obviously iron atoms within an element.

Iron atoms within the element?

Yeah.

Okay. Can you say anything about the arrangement of the atoms?

They're all lined together. They're all close together.

They're closely together, yes, and they're all lined together, there's some sort of regular pattern there okay?

Yeah.

So you think that's in the element, that's a lump of iron, a sort of, a magnified view of a lump of iron.

Yes.

So Annie did recognise the image as representing particles ('atoms') in solid iron. The image showed the particles close together, and Annie was asked if they would hold together – the intention being to find out what, if anything, Annie knew about metallic bonding. Annie did think the atoms would be held together, but she did not suggest this was due to a bond or even a force (cf. "Sodium and chlorine don't actually overlap or anything and would probably get held together by just forces"*).

Do you think those atoms will hold together?

Yes.

Why do you think that is?

Because they're all the same sort.

Does that make them hold together?

Yeah.

So it seemed that Annie held an alternative conception that atoms of the same sort would hold together because they were of the same type. This interpretation was tested.

Yeah? Do you think there is any kind of bonds between the atoms?

• • • • • • • • • (pause, c.9s)

No, because they're all the same and they don't need to be bonded.

Right, okay so recapping…here we've got an example of something where the atoms are all the same, and that holds them together even though there's no chemical bonds.

Yeah.

So Annie held an alternative conception of atomic coherence – that atoms of the same type did not need bonding to hold them together, as being the same kind of atom was sufficient for them to hold together.

It is unlikely that Annie had been taught this idea, and it seems quite possible it is an intuitive idea that might be acting as an example of a 'grounded learning impediment': a notion based on general experience, and inappropriately applied in the context of atomic interactions.

An element needs a certain number of electrons

An element needs a certain amount of electrons in the outer shell

Keith S. Taber

Bert was a participant in the Understanding Science project. In Y10 Bert was talking about how he had been studying electrolysis in class. Bill had described electrolysis as "where different elements are, are taken out from a compound", but it transpired that Bert thought that "a compound is just a lot of different elements put together"*. He seemed to have a tentative understanding that electrolysis could only be used to separate elements in some compounds.

if they're positive and negative then they would be able to be separated into different ones.

So some things are, some things aren't?

Yeah, it matters how many electrons that they have.

Ah. [pause, c.3s] So have you got any examples of things that you know would definitely be positive and negative?

Well I could tell you what happens.

Yeah, go on then.

Well erm, well if a, if an element gives away, electrons, then it becomes positive. But if it gains, then it becomes negative. Because the electrons are negative, so if they gain more, they just go a bit negative.

Yeah. So why would an element give away or gain some electrons? Why would it do that?

Because erm, it needs a certain amount of electrons in the outer shell. It matters on what part of the periodic table they are.

Okay, let me be really awkward. Why does it need a certain number of electrons in the outer shell?

[Pause, c.2 s]

Erm, well, I don't know. It just – 

So Bert thought that an element "needs a certain amount of electrons in the outer shell" depending upon it's position in the periodic table, but he did not seem to recall having been given any reason why this was. The use of the term 'needs' is an example of anthropomorphism, which is commonly used by students talking about atoms and molecules. Often this derives from language used by teachers to help humanise the science, and provide a way for students to make sense of the abstract ideas. If Bert comes to feel this is a sufficient explanation, then talk of what an element needs can come to stand in place of learning a more scientifically acceptable explanation, and so can act as a grounded learning impediment.

References to atoms needing a certain number of electrons is often used as an explanatory principle (the full shells explanatory principle) considered to explain why bonding occurs, why reactions occur and so forth.

Bert's final comment in the short extract above seems to reflect a sense of 'well that's just the way the world is'. It is inevitable that if we keep asking someone a sequence of 'well, why is that' question when they tell us about their understanding of the world, they eventually reach the limits of their understanding. (This tendency has been labelled 'the explanatory gestalt of essence'.) Ultimately, even science has to accept the possibility that eventually we reach answers and can not longer explain further – that's just the way the world is. Research suggests that some students seem to reach the 'it's just natural' or 'well that's just the way it is' point when teachers might hope they would be looking for further levels of explanation. This may link to when phenomena fit well with the learner's intuitive understanding of the world, or tacit knowledge.

Bert's reference to an element needing a certain amount of electrons in the outer shell also seems to confuse description at two different levels: he explicitly refer to substance (element), when he seems to mean a quanticle (atom). Element refers to the substance, at the macroscopic level of materials that can be handled in the laboratory, whilst an atom of the element (which might better be considered to gain or lose electrons) is part of the theoretical model of matter at a submicroscopic level, used by chemists as a basis for explaining much macroscopic, observed behaviour of samples of substances.