student ontology - Science-Education-Research

The states of (don't) matter?

Which state of matter is fire?

Keith S. Taber

A trick question?

Education in Chemistry recently posed the question

What state of matter is fire?

This referred to an article in a recent issue of the magazine (May 2022, and also available on line) which proposed the slightly more subtle question 'Is fire a solid, liquid, gas, plasma – or something else entirely?'

This was an interesting and fun article, and I wondered how other readers might have responded.

An invitation

No one had commented on the article on line, so I offered my own comment, reproduced below. Before reading this, I would strongly recommend visiting the web-page and reading the original article – and considering how you would respond. (Indeed, if you wish, you can offer your own response there as a comment on the article.)

A personal response – a trick question?

Ian Farrell (2022) asks: "Is fire a solid, liquid, gas, plasma – or something else entirely?" I suggest this is something of a trick question. It is 'something else', even if not 'something else entirely'.

It is perhaps not 'something else entirely' because fire involves mixtures of substances, and those substances may be describable in terms of the states of matter.

However, it is 'something else', because the classification into different states of matter strictly applies to pure samples of substances. It does not strictly apply to many mixtures: for example, honey, is mostly ('solid') sugar dissolved in ('liquid') water, but is itself neither a solid nor a liquid. Ditto jams, ketchup and so forth. Glass is in practical everyday terms a solid, obviously, but, actually, it flows and very old windows are thicker near their bottom edges. (Because glass does not have a regular molecular level structure, it does not have a definite point at which it freezes/melts.) Many plastics and waxes are not actually single substances (polymers often contain molecules of various chain lengths), so, again, do not have sharp melting points that give a clear solid-liquid boundary.

Fire, however, is not just outside the classification scheme as it involves a mixture (or even because it involves variations in mixture composition and temperature at different points in the flame), but because it is not something material, but a process.

Therefore, asking if fire is a solid, liquid, gas, or plasma could be considered an 'ontological category error' as processes are not the type of entities that the classification can be validly applied to.

You may wish to object that fire is only possible because there is material present. Yes, that is true. But, consider these questions:

Is photosynthesis a solid, liquid, gas, plasma…?
Is distillation a solid, liquid, gas, plasma…?
is the Haber process a solid, liquid, gas, plasma…?
is chromatography a solid, liquid, gas, plasma…?
Is fermentation a solid, liquid, gas, plasma… ?
Is melting a solid, liquid, gas, plasma…?

In each case the question does not make sense, as – although each involves substances, and these may individually, at least at particular points in the process, be classified by state of matter- these are processes and not samples of material.

Farrell hints at this in offering readers the clue "once the fuel or oxygen is exhausted, fire ceases to exist. But that isn't the case for solids, liquids or gases". Indeed, no, because a sample of material is not a process, and a process is not a sample of material.

I am sure I am only making a point that many readers of Education in Chemistry spotted immediately, but, unfortunately, I suspect many lay people (including probably some primary teachers charged with teaching science) would not have spotted this.

Appreciating the key distinction between material (often not able to be simply assigned to a state of matter) and individual substances (where pure samples under particular conditions can be understood in terms of solid / liquid / gas / plasma) is central to chemistry, but even the people who wrote the English National Curriculum for science seem confused on this – it incorrectly describes chocolate, butter and cream as substances.

Sometimes this becomes ridiculous – as when a BBC website to help children learn science asked them to classify a range of objects as solid, liquid or gas. Including a cat! So, Farrell's question may be a trick question, but when some educators would perfectly seriously ask learners the same question about a cat, it is well worth teachers of chemistry pausing to think why the question does not apply to fire.

Relating this to student learning difficulties

That was my response at Education in Chemistry, but I was also aware that it related to a wider issue about the nature of students' alternative conceptions.

Prof. Michelene Chi, a researcher at Arizona State University, has argued that a common factor in a wide range of student alternative conceptions relates to how they intuitively classify phenomena on 'ontological trees'.

"Ontological categories refer to the basic categories of realities or the kinds of existent in the world, such as concrete objects, events, and abstractions."
Chi, 2005, pp.163-164

We can think of all the things in the world as being classifiable on a series of branching trees. This is a very common idea in biology, where humans would appear in the animal kingdom, but more specifically among the Chordates, and more specifically still in the Mammalia class, and even more specifically still as Primates. Of course the animals branch could also be considered part of a living things tree. However, some children may think that animals and humans are inherently different types of living things – that they would be on different branches.

Some student alternative conceptions can certainly be understood in terms of getting typologies wrong. One example is how electron spin is often understood. For familiar objects, spin is a contingent property (the bicycle wheel may, or may not, be spinning – it depends…). Students commonly assume this applies to quanticles such as electrons, whereas electron spin is intrinsic – you cannot stop an electron 'spinning', as you could a cycle wheel, as spin is an inherent property of electrons. Just as you cannot take the charge away from an electron, nor can you remove its spin.

Two ways of classifying some electron properties (after Figures 8 and 9 in Taber, 2008). The top figure shows the scientific model; the bottom is a representation of a common student alternative conception.

Chi (2009) suggested three overarching (or overbranching?) distinct ontologial trees being entities, processes and mental states. These are fundamentally different types of category. The entities 'tree' encompasses a widely diverse range of things: furniture, cats, cathedrals, grains of salt, Rodin sculptures, iPads, tectonic plates, fossil shark teeth, Blue Peter badges, guitar picks, tooth picks, pick axes, large hadron colliders, galaxies, mitochondria….

Despite this diversity, all these entities are materials things, not be confused with, for example, a belief that burning is the release of phlogiston (a mental state) or the decolonisation of the curriculum (a process).

Chi suggested that often learners look to classify phenomena in science as types of material object, when they are actually processes. So, for example, children may consider heat is a substance that moves about, rather than consider heating as a process which leads to temperature changes. ¹ Similarly 'electricity' may be seen as stuff, especailly when the term is undifferentiated by younger learners (being a blanket term relating to any electrical phenomenon). Chemical bonds are often thought of as material links, rather than processes that bind structures together. So, rather than covalent bonding being seen as an interaction between entities, it is seen as an entity (often as a 'shared pair of electrons').

Of course, science teachers (or at least the vast majority) do not make these errors. But any who do think that fire should be classifiable as one of the states of matter are making a similar, if less blatant, error of confusing matter and process. Chi's research suggests this is something we can easily tend to do, so it is not shameful – and Ian Farrell has done a useful service by highlighting this issue, and asking teachers to think about the matter…or rather, not the 'matter', but the process.

Work cited:

Chi, M. T. H. (2005). Commonsense Conceptions of Emergent Processes: Why Some Misconceptions Are Robust. Journal of the Learning Sciences, 14(2), 161-199. https://doi.org/10.1207/s15327809jls1402_1
Chi, M. T. (2009). Three types of conceptual change: Belief revision, mental model transformation, and categorical shift. In International handbook of research on conceptual change (pp. 89-110). Routledge.
Farrell, I. (2022). The burning question. Is fire a solid, liquid, gas, plasma – or something elkse entirely? Education in Chemistry, 59(3), 11.
Taber, K. S. (2008) Of Models, Mermaids and Methods: The Role of Analytical Pluralism in Understanding Student Learning in Science, in Ingrid V. Eriksson (Ed.) Science Education in the 21st Century, pp.69-106. Hauppauge, NY: Nova Science Publishers.

Note:

¹ The idea that heat was a substance, known as caloric, was for a long time a respectable scientific idea.

In a molecule, the electron actually slots into spaces

Keith S. Taber

Mohammed was a participant in the Understanding Science Project. When interviewed in the first term of his upper secondary (GCSE) science course (in Y10), he told me he had been learning about ionic bonding in one of his science classes. Mohammed had quite a clear idea about ionic bonding, which he described in terms of the interactions of two atoms where "they both want to get full outer shells", leading to salt which was "like two atoms joined together":

The "two atoms joined together" sounds much like a molecule (and it is very common for students to identify molecule like ion-pairs even in representations of extensive ionic lattices), so I asked Mohammed about this:

Can I see these atoms?
No. They're really small. Because the wavelength of visible light is actually too like large to see the atoms, they just pass over them.
Okay, so I can't see them. But I can imagine them, can I?
Yeah.
So if I could imagine a sodium atom and chlorine atom, and then they form salt, what would it look like afterwards? How could I imagine it afterwards.
Oh it's like two atoms joined together.
That sounds like a molecule to me?
It's not actually, like, joined.
No?
Because I know that whenever things of opposite charge, I know two rods, when they come together, they don't actually touch, so they don't exactly touch, but they are very close, two atoms close to each other
So a molecule would be different to that in some way, would it?
Yeah, a molecule's actually bonded
So how that different?
I think in a molecule, the electron actually slots into spaces.
I see, and it doesn't do that in this case?
No.

So Mohammed thinks that the interaction between the ions will be due to their electrical charges, but, for him, this may not count as a bond, as the forces just hold the ions ("atoms") close together, and do not actually join them. Mohammed's idea of the atoms not actually touching, "they don't actually touch, so they don't exactly touch", is transferring a notion from the familiar world of macroscopic phenomena (where things touch, or they do not touch) to the submicroscopic world of quanticles that do not have definitive size/volume, and do not actually have distinct surfaces, so touching is a matter of degree. There is no more (or less) 'touching' in a covalent bond than in ionic bonding. So according to Mohammed the ions do not form a molecule, as in a molecule there would some kind of more direct joining – he suggests something like an interlocking with electrons from one atom slotting into spaces on another.

Interestingly, Mohammed bases his notion that the ions would not touch on a general principle that he considers to apply whenever considering things of opposite charge – which he justifies on his knowledge that "two [charged] rods, when they come together, they don't actually touch". He may be misremembering something here – or he may have seen a demonstration of suspended charged rods of the same material (so either both negatively or both positively changed) that when one is moved closer to the other the rods repel. Whatever the source, Mohammed seems to feel he has a valid general principle that he can apply here that act as a grounded learning impediment channelling his thinking about the case under discussion along 'the wrong lines'.

Mohammed's notion of the ionic bonding as being just due to forces rather than being a proper bond is very similar to a common alternative conceptions of ionic bonding which sees ions in a lattice only having a limited number of ionic bonds depending upon valency (the valency conjecture) but bonded with other coordination counter-ions by 'just forces' (the just forces conjecture) – although here Mohammed suspected that all ionic bonding fell short of being proper chemical bonds.

This is a very mechanical model of the covalent bond, whereas the scientific model presents bonding as more of a process than a material mechanical link. However teaching models often present bonding this way, and sometimes molecules are modelled in terms of jigsaws with atoms or radicals as pieces to be slotted together. Although such models are only meant to provide a simple analogy for the bonding they may act as learning impediments if learners take them too 'literally' as realistic representations and transfer inappropriate associations from the model to their understanding of the system being modelled.

Mohammed also uses similar language when asked about salt dissolving in water, as the charge of the water forces the sodium and chlorine ions to slot into certain places within the water molecules *.

Iron is too heavy to completely evaporate

Some molten iron would evaporate but not all of it, 'cause it's not like water and it's more heavy

Keith S. Taber

Sophia was a participant in the Understanding Science Project. In her first interview near the start of Y7, Sophia told me that she had learnt "about the particles…all the things that make – the actual thing, make them a solid, and make them a gas and make them a liquid" (i.e. the states of matter). All solids had particles, including (as examples) ice and an iron clamp stand. There would be the same particles in the ice as the iron.

"because they are a solid, but they can change , 'cause if erm they melted they would be a liquid so they would have different particles in…Well they are still the same particles but they are just changing the way they act".

Sophia's suggestion that particles in ice and the iron were the same types of particles as both were solid seems to be 'carving nature' at the wrong joints – that is in this model the particles in ice and (solid) iron would be of one type, whilst those of water and liquid iron would be of another type (that is she had an alternative ontology). Sophia quickly corrected this, so it is not clear if this reflected some intuitive idea or was just 'a slip of tongue'.

According to Sophia the ice could be melted "with something that's hot, like a candle" but for the iron "you need more heat, 'cause it's more, it's a lot more stronger…because it's got more particles pushed together".

Sophia's explanation suggested a causal path (right-hand side) quite different from a canonical causal path (left-hand side)

Strictly the difference is more about the strength of the interactions between particles, than how many were pushed together – although strong bonding forces would tend (all other factors being equal) to lead to particles being bound more tightly and being closer. We might argue here that Sophia seemed to confuse cause and effect – that a higher density of particles was an effect of strong bonding, which would also mean more energy was needed to overcome that bonding. (However, we should also be aware that when students use 'because' (which formally implies causality) they sometimes mean little more than 'is associated with'.)

If the water obtained from melting ice was heated more "it will evaporate into the sky". However, if the molten iron was heated Sophia thought that "some of it would evaporate but not all of it, 'cause it's not like water and it's more heavy". She thought only "a little" of the iron would evaporate to give iron vapour:

"No, I think that water all of it goes, but other material, other liquids some of it will go, not all of it". The rest "if it's cold enough, it will go back into a solid, but if not it really just stays as a liquid".

Sophia's idea that no matter how much liquid iron was heated it would not completely evaporate so some would remain liquid, which seemed to be linked in her mind to its density, seems to be evidence of an alternative conception. Students may not expect that something as (apparently) inherently solid as iron could evaporate (everyday experience may act as a grounded learning impediment), and so may not readily accept that the basic model of the states of matter and changes of state (i.e., a heated liquid will evaporate or boil) can apply to something like iron. Sophia seemed to have formed a hybrid conception – applying the taught model, but with a modification reflecting the counter-intuitive notion that iron could become a vapour.

Conceptual change can be a slow progress, although hybrid conceptions may be 'stepping stones' towards more scientific understandings. However, when I spoke to Sophia in Y8 she did not seem to have progressed further. [See 'Liquid iron stays a liquid when heated'.]

If you take all of the electrons off an atom, then it would not be matter

Keith S. Taber

Mohammed was a participant in the Understanding Science Project. When Mohammed was near the end of his first term of upper secondary science (in Y10) he told me that in his chemistry lessons he had been studying atoms and ionic bonding. When I asked him what an atom was, he suggested that an atom is the smallest amount of matter you can get [*] as well as being "it's the building block of all matter".

The notion that atoms are the smallest components of matter has a strong historical pedigree – but the modern idea of the atom is unlike the solid and indivisible (= atomos: uncuttable) elementary particles imagined by some Greek philosophers. Modern atoms are considered complex structures, and may be dismantled.

It is not unusual for students to suggest that atom is the smallest thing that one can get, and then go on to describe atomic structure in terms of smaller components! The idea that the atom is the smallest thing possible (a kind of motto or slogan) is commonly adopted and then retained despite learning about subatomic particles.

Mohammed, however, justified his suggestion that an atom was "the smallest amount of matter you can get" by arguing that "matter is something that is built out of protons, neutrons and electrons". So Mohammed's notion of what counted as 'matter' (an ontological question) was at odds with the scientific account

Mohammed did not suggest that matter had to have overall neutrality, and his suggestion that matter is something that is built out of protons, neutrons and electrons had to be amended when he realised it would exclude hydrogen atoms as being matter:

So what if I had a balloon full of hydrogen gas, would that, would the hydrogen be matter?
Yeah.
So would that consist of protons, neutrons and electrons?
No it wouldn't. Sorry, can I take away the neutrons
Okay, so matter's what then? What's our new definition of matter?
Protons, electrons.

Mohammed presented his responses with confidence and without hesitation, which seemed to suggest he was offering well established ideas. However, he did not seem to have fully thought through these ideas, and perhaps was constructing a rationale in situ in the interview. The logical consequences of Mohammed's new definition was that atoms and ions would be considered matter but not nuclei or electrons.

What if I had sodium. Do you think that would be matter?… if I had a lump of sodium, would that be matter?
Yeah
And why is that matter?
Because it has, it has a full atom, it has protons, neutrons, electrons, even though you can have no neutrons.
Okay, but it has to have the protons and the electrons?
Yeah.
Now what if I just had one atom of sodium, would that still be matter?
Yeah.
…so let's say I've got my atom, with my eleven protons, and my probably twelve neutrons I think usually. And I've got eleven electrons round the outside. If I take take one of the electrons off this atom, it's not an atom any more is it?
It's an ion.
Now is it still matter?
Yeah.
Because I've still got protons and electrons. What if I took a second electron off, could I take a take second electron off?
Yeah.
What have I got then, then?
You've still got matter.
What if I took a third one off?
Well if you, if you just take all of them off, then you'd stop having matter.
So if I've got eleven electrons, can I take ten of them off?
Yeah.
And I'd still have matter?
Yeah.

The idea of what counts as matter here seems a rather idiosyncratic alternative conception (rather than being a common alternative conception that is widely shared). Science teachers would probably consider that all material (sic) particles are matter, and – perhaps – that this should be obvious to students. However, the submicroscopic realm is far from everyday experience so perhaps it is not surprising that students often form their own alternative conceptions.

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')

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