Tag: states of matter

Where does the molecule go? A diagnostic question

Many undergraduates seem to think molecules like to hang around rather than moving on

Keith S. Taber

image showing oart of a layer of molecules in a solid
A representation of a small part of a layer of molecules in a solid substance – with one molecule highlighted by colour.
If the solid were melted, and then refrozen, where would the highlighted molecule be?

If you are a science teacher: what would your students think?

In this article I offer my own version (actually two versions, see below) of a question I saw used in a published study (Smith & Villarreal, 2015a). As I no longer have any students, I cannot easily try this out, but perhaps a reader who is currently teaching science might be tempted to see what their pupils or students might think? (If you do, I would apreciate hearing about what you find!)

The two versions of the question can be downloaded from the links below.

The question could be given to individual learners, or as the basis of small group discussion, or perhaps just projected onto the screen for a 'show of hands' for each response option. (Exploring student thinking to detect misconceptions is known as diagnostic assessment.)

Alternative conceptions abound

I am very familiar with the extensive evidence which shows that is very common for learners, at all levels, and in any topic, to hold alternative conceptions ('misconceptions') at odds with canonical science and the target knowledge set out in the science curriculum. So, I am seldom surprised when I read about a study which reports finding learners demonstrating such conceptions.

Yet one study I read which reported learners commonly holding an alternative conception did surprise me. I would have not been surprised if the respondents had been secondary levels students, and a minority of them had demonstrated this particular conception, but I would not have expected how the study found a high incidence of the alternative conception among undergraduates studying chemistry.

The research asked about what happens when a solid is either dissolved, or melted, and then returns to the solid state. It used an instrument that presented a figure representing the particles in a small section of a solid, with one particle marked out, and asked the learners to draw the equivalent images after the solid had either dissolved and then been recrystallised, or melted and then been refrozen.

I an going to limit myself to the easier context (melt, then freeze – no solvent molecules involved). According to the researchers, the results suggested that a large proportion of the undergraduates indicated that the atom that had been marked out would be found in the same position in the solid at the end of the process: the exact proportion shifted in two versions of the study (65%, 50%) but a very rough gloss was that at least half of the learners located the marked particle back at its original point.

"These results indicated that a large proportion of the students viewed the [marked] molecule as being near to the same position after melting as it was before melting, and being in the position it was originally in after the liquid froze back to the solid."

Smith & Villarreal, 2015a: 277-278

Perhaps this should not have surprised me – I have been told by very bright A level students that on homolytic bond fusion each atom would always get its own electrons back, and this seems something of a parallel notion.

Now there was some questioning of the methodology and instrument used here (Langbeheim, 2015; see also Smith & Villarreal, 2015b) – as there often is in educational research – but it seemed a substantial proportion of learners thought the solid would reform with particles in their original positions, and this suggests a rather limited understanding of the level of molecular motion in the dissolved or molten state. I would not have been so surprised if this work had been carried out with, say, twelve year olds – but such a high level of misconception among undergraduates did surprise me as it reflects a failure to imagine the nature of the molecular world, and that surely makes learning high level (e.g., degree level) chemistry very difficult.

Now there are serious challenges in representing the nanoscale (thus the questioning of the representations used in the study) simply because molecules, ions, electron, atoms – are not the kinds of things we can draw realistically – they are fuzzy objects with no surfaces that somewhat blend into their neighbours. This raises a possible defence for students in such studies

'yes, your honour, I did show the particle as having returned to the same position, but as the focal figure had been drawn unrealistically as a set of circles I did not think authenticity was being asked for!'

It seems unlikely any learner really did think that – and the researchers did ask learners about their reasoning. The most common type of explanations were (Smith & Villarreal, 2015a: 278):

  • In the molten state: The molecule doesn't move far from its original position
  • After resolidification: The molecule ends up near where it was positioned in the liquid

Representing quanticles

Molecules, ions, atoms are 'quantum objects' which do not have the properties of familiar macroscopic objects. The nanoscopic particles in a lattice or liquid are not like the particles in table salt (grains) or sugar (granules) which each have a definite volume and surface, and which cannot be made to overlap their neighbours.

The following is my representation of a section of a layer of molecules in a solid substance. I have shown them round as that is simpler. Most molecules are not round (but 'molecules' of, say, neon or argon, are.) I have tried to show them as being fuzzy rather than as if ball-bearings with definite surfaces as the 'substance' of atoms, ions and molecules is largely electric fields and electron 'clouds' (a rather appropriate metaphor) rather than anything 'solid'. (And, of course, the word solid loses its meaning for a single molecule. We might, figuratively, suggest the atom is like a tiny liquid drop surrounded by an immense volume of gas – but it is probably best to avoid using such comparisons with learners becasue of the potential for them taking the terms literally.)

Should the molecules be touching in the solid? That is a problematic question as how do we decide whether things are touching when the things concerned do not have distinct surfaces but rather fade away to infinity? (If the gas giants Jupiter and Saturn were to ever come together, how would we decide at what point they had actually physically collided?)

Often in science teaching we cheat and show molecules touching in solids when teaching about the differences between condensed and gaseous states; but then hope students have forgotten this by the time we want to teach about thermal expansion of solids.

My diagram shows a layer of the regular crystal structure, so if you think my 'molecules' should touch then you can imagine that they would once the adjacent layers were drawn in.

image showing art of a layer of molecules in a solid

The image I have used might suggest too much space between molecules…

image showing part of molecules in a solid - 2 layers

…adding another layer might help give the appearance of close packing, but if a different colour is used this may suggest some physical difference…

image showing part of molecules in a solid - 2 layers

yet making both layers the same colour makes the figure more dificult to interpret.

It is a problem of scale

The real issue for the novice learner here is one of scale. The scale of atoms is far beyond our ready grasp. My figure shows a much more extended section of material than that in the original study – but still, a tiny, tiny, tiny fraction of a solid we could readily see and manipulate. If the solid substance melted, then (e.g., around room temperature) we would expect molecular speeds of the order of hundreds of metres per second. In the gas phase that might be somewhat reflected in how far some molecules get (but diffusion is still much slowed by collisions), but in a condensed phase, so in a liquid, the molecules are not going to get very far at all before colliding with a 'neighbour' and being deflected off course.

The so-called 'random walk' of any molecule in a liquid will reflect mean speeds orders of magnitude less than the hundreds of metres per second instantaneous speed (as it is constantly being shifted to a new direction, and is just as likely to be sent back in the direction it originated).

(See an animated simulation of a random walk here)

But then, given the size of the sample represented, the distance from one end of the image to the other is of the order of maybe 0.000 000 001 metres. If a molecule with an instantaneous speed of hundreds of metres per second only has to travel of the order of perhaps 0.000 000 000 1m before colliding with the next molecule, it is going to have an awful lot of collisions each second – many billions. So, a molecule bumping around at say 300 m/s would not take very long to move 0.000 000 001 m (and so off the region of lattice shown in my figure) even with all those restrictive collisions!

Two versions of the diagnostic question for use in class

dignostic question showing particles in solid, and asking about position of molecule after melting and refreezing.
A 3-option diagnostic question testing understanding of molecular motion (Download a copy of this file)
dignostic question showing particles in solid, and asking about position of molecule after melting and refreezing.
A 4-option diagnostic question testing understanding of molecular motion (Download a copy of this file)

Even if the solid melts and is a liquid for only a few minutes (that is, a few hundred seconds), and even if we have placed the original solid in a tightly constricting container such that the liquid does not change overall shape, what are the chances of the molecule ending up in the same lattice position? Or even being in the frame when we represent such a small section of the lattice?

If we are only representing one layer of molecules, then what are the chances of the molecule even ending up in the same layer (it is likely to have moved 'up'/'down' just as much as laterally along the plane represented whilst in the liquid state).

Three random walks starting from the same origin. The molecule moves in all three dimensions.
(Image from https://commons.wikimedia.org/wiki/File:Walk3d_0.png – licensed under the Creative Commons Attribution-Share Alike 3.0 Unported licence)

So, I think this is an easy question.

😉

Each of the options (in both versions of the question) are possible outcomes.

Given that the section of the latice shown is so limited, all the positions shown are pretty much local to the starting point, so I would argue the molecule could almost equally likely end up in any of the lattice positions in the figure (so: A, C and D are, in effect, equally likely – as would be any other lattice position you selected from the image).

What about Option B?

Option B reflects all the possibilities where the molecule ends up outside the small section of lattice layer illustrated, including all the options where it has moved to a different layer. There will be billions and billions of these options, including, at least, many thousands of options close enough for the molecule to have easily moved there in the number of 'random walk' steps feasible in the time scale.

So, the answer to the question of which option is most likely (in either version of the question) is easy – option B is by far most likely.

But I wonder if most students who have been taught about particle models and states of matter would agree with me? If Smith and Villarreal's undergraduate sample is anything to go by, then I guess not.

Work cited:
  • Smith, K. C., & Villarreal, S. (2015a). Using animations in identifying general chemistry students' misconceptions and evaluating their knowledge transfer relating to particle position in physical changes [10.1039/C4RP00229F]. Chemistry Education Research and Practice, 16(2), 273-282. https://doi.org/10.1039/C4RP00229F
  • Langbeheim, E. (2015). Reinterpretation of students' ideas when reasoning about particle model illustrations. A Response to "Using Animations in Identifying General Chemistry Students' Misconceptions and Evaluating their Knowledge Transfer Relating to Particle Position in Physical Changes" [10.1039/C5RP00076A]. Chemistry Education Research and Practice, 16(3), 697-700. https://doi.org/10.1039/C5RP00076A
  • Smith, K. C., & Villarreal, S. (2015b). A Reply to "Reinterpretation of Students' Ideas when Reasoning about Particle Model Illustrations. A Response to 'Using Animations in Identifying General Chemistry Students' Misconceptions and Evaluating their Knowledge Transfer Relating to Particle Position in Physical Changes' by Smith & Villarreal (2015)" [10.1039/C5RP00095E]. Chemistry Education Research and Practice, 16, 701-703. https://doi.org/10.1039/C5RP00095E

ChemTeach – is a discussion list for sharing information and expertise among those involved in teaching chemistry. Click here to learn more about, or find out how to join, the group

The book  Student Thinking and Learning in Science: Perspectives on the Nature and Development of Learners' Ideas gives an account of the nature of learners' conceptions, and how they develop, and how teachers can plan teaching accordingly.

It includes many examples of student alternative conceptions in science topics.

Are the particles in all solids the same?

Particle intuitions may not match scientific models

Keith S. Taber

Sophia was a participant in the Understanding Science Project. I first talked to her when she was in Y7, soon after she began her secondary school course.

One of the first topics she studied in her science was 'solids, liquids and gases', where she had learnt,

that solids are really hard and they stay together more, and then liquids are close together but they move around, and gases are really free and they just go anywhere

She had studied a little about the topic in her last year of primary school (Y6), but now she was being told

about the particles…the things that make – the actual thing, make them a solid, and make them a gas and make them a liquid

Particle theory, or basic kinetic theory, is one of the most fundamental theories of modern science. In particular, much of what is taught in school chemistry is explained in terms of theories involving how the observed macroscopic properties emerge from the characteristics and interactions of conjectured sub-microscopic particles that themselves often have quite unfamiliar properties. This makes the subject very abstract, challenging, and tricky to teach (Taber, 2013a).

Read about conceptions of atoms

Particle theory is often introduced in terms of the states of matter. Strictly there are more than three states of matter (plasma and Bose-Einstein condensates are important in some areas of science) but the familiar ones, and the most important in everyday phenomena, are solid, liquid and gas.

The scientific account is, in simple terms, that

  • different substances are made up of different types of particle
  • the different states of matter of a single substance have the same particles arranged differently

These are very powerful ideas, even if there are many complications. For example,

  • the terms solid, liquid and gas only strictly apply to pure samples of a single substance, not mixtures (so not, for example, to bronze, or honey, or, milk, or ketchup, or even {if one is being very pedantic} air or sea water. And cats (please note, BBC) are completely inadmissible. )
  • common salt is an example of a pure substance, that none-the-less is considered to be made up of more than one type of particle

This reflects a common type of challenge in teaching science – the full scientific account is complex and nuanced, and not suitable for presenting in an introductory account; so we need to teach a simplified version that introduced the key ideas, and then only once this is mastered by learners are they ready to develop a more sophisticated understanding.

Yet, there is a danger that students will learn the simplified models as truths supported by the authority of science – and then later have difficulty shifting their thinking on. This is not only counter-productive, but can be frustrating and de-motivating for learners who find hard-earned knowledge is not as sound as they assumed.

One response to this is to teach science form very early in a way that is explicit about how science builds models of the natural world: models that are often simplifications which are useful but need to be refined and developed to become powerful enough to expand the range of contexts and examples where they can be applied. That is, students should learn they are being taught models that are often partial or imperfect, but that is just a reflection of how science works, developing more sophisticated understanding over time (Taber, 2017).

Sophia confirmed that the iron clamp stand near where she was sitting would have particles in it, as would a lump of ice.

Are they the same particles in the ice as the iron?

Yeah, because they are a solid, but they can change.

Ah, how can they change?

Cause if, erm, they melted they would be a liquid so they would have different particles in.

Right, so the iron is a solid, 

Uh hm.

So that's got one type of particle?

Yeah.

And ice is also a solid?

Yeah.

So that has the same sort of particles?

Yeah, but they can change.

The ones in the ice?

Mm,

To a learner just meeting particle theory for the first time, it may seem just as feasible that the same type of particle is found in one state as in one substance.

In the scientific model, we explain that different substances contain different types of particles, whereas different states of the same substance contain different arrangements of the same particles: but this may not be intuitively obvious to learners.1 It seemed Sophia was thinking that the same particles would be in different liquids, but a change of state led to different particles. This may seem a more forced model to a teacher, but then the teacher is already very familiar with the scientific account, and also has an understanding of the nature of those particles (molecules, ions, atoms – with internal structure and charges that interact with each other within and between the particles) – which are just vague, recently imagined, entities to the novice.

Sophia seemed to misunderstood or misremembered the model she had been taught, but to a novice learner these 'particles' have no more immediate referent than an elf or an ogre and would be considerably more tenuous than a will-o'-the-wisp.

Sophia seemed to have an alternative conception, that all solids have one type of particle, and all liquids another. If I had stopped probing at that point I might have considered this to be her thinking on the matter. However, when one spends time talking to students it soon becomes clear that often they have ideas that are not fully formed, or that may be hybrids of different models under consideration, and that often as they talk they can talk themselves into a position.

So, if I melted the ice – that changes the particles in the solid?

Well they are still the same particles but they are just changing the way they act…

Oh.

How do they change?

A particle in a liquid [sic, solid] is all crammed together and don't move around, but in a liquid they can move around a little but they are still close and, can, you can pour a liquid, where you can't a solid, because they can move in. 

Okay, so if I have got my ice, that's a solid, and there are particles in the ice, and they behave in a certain way, and if the ice melts, the particles behave differently?

Yeah.

Do you know why they behave differently in the liquid?

No. {giggles} So, they can, erm

• • • • • • • • • • • •  [A pause of approximately 12 s]

They've more room cause it's all spread out more1, whereas it would be in a clump

The literature on learners conceptions often suggests that students have this or that conception, or (when survey questions are used) that this percentage thinks this, and that percentage thinks that (Taber, 2013b). That this is likely to be a simplification seems obvious is we consider what thinking is – whatever thought may be, is it a dynamic process, something that moves along. Our thinking is, in part, resourced by accessing what we have represented in memory, but it is not something fixed – rather something that shifts, and that often becomes more sophisticated and nuanced as we explore a focus in greater depth.

I think Sophia did seem to have an intuition that there were different types of particles in different states of matter, and that therefore a change of state meant the particles themselves changed in some way. As I probed her, she seemed to shift to a more canonical account where change of state involved a change in the arrangement or organisation of particles rather than their identity.

This may have simply been her gradually bringing to mind what she had been taught – remembering what the teacher had said. It is also possible that the logic of the phenomenon of a solid becoming a liquid impressed on her that they must be the same particles. I suspect there was a little of both.

When interviewing students for research we inevitably change their thinking and understanding to some extent (hopefully, mostly in a beneficial way!) (If only teachers had time to engage each of their students in this way about each new topic they might both better understand their students' thinking, and help reinforce what has been taught.)

Did Sophia 'have a misconception'? 1 What did she 'really think'? That, surely, is to oversimplify.

She presented with an alternative conception, that under gentle questioning she seemed to talk /think herself out of. The extent to which her shift in position reflected further recall (so, correcting her response) or 'thinking through' (so, developing her understanding) cannot be known. Likely there was a little of both. What memory research does suggest is that being asked to engage in and think about this material will have modified and reinforced her memories of the material for the future.

Read about the role of memory in teaching and learning

Work cited:
Note

1 Actually, the particles in a liquid are not substantially spread further apart than in a solid. (Indeed, when ice melts the water molecules move closer together on average.) Understanding melting requires an appreciation of the attractions between particles, and how heating provides more energy for the particles. This idea of increased separation on melting is therefore something of an alternative conception, if one that is sometimes encouraged by the diagrams in school textbooks.

Teaching an introductory particle theory based on the arrangement of particles in different states, without reference to the attractions between particles is problematic as it offers no rational basis for why condensed states exists, and why energy is needed to disrupt them – something highlighted in the work of Philip Johnson (2012).

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

From Education in Chemistry

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.)

Article from Education in Chemistry

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:
Note:

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

A wooden table is solid…or is it?

Keith S. Taber

Wood (cork coaster captured with Veho Discovery USB microscope)

Bill was a participant in the Understanding Science Project. Bill (Y7) was explaining that he had been learning about the states of matter, and introduced the notion of there being particles:

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

So what are these particles then?

Erm, they're the bits that make it what it is, I think.

Ah. So are there any solids round here?:

Yeah, this table. [The wooden table Bill was sitting at.]

That's a solid, is it?:

Yeah

Technically the terms solid, liquid and gas refer to samples of substances and not objects. From a chemical perspective a table is not solid. A wooden table (such as those in the school laboratory where I talked to Bill) is made of a complex composite material that includes various different substances such as a lignin and cellulose in its structure.

Wood contains some water, and has air pockets, so technically wood is not a solid to a chemist. However, in everyday life we do thing of objects such as tables as being solid.

Yet if wood is heated, water can be driven off. Timber can be mostly water by weight, and is 'seasoned' to remove much of the water content before being used as a construction material. Under the microscope the complex structure of woods can be seen, including spaces containing air.

Bill also suggested that a living plant should be considered a solid.

I think teaching may be a problem here, as when the states of matter are taught it is often not made clear these distinctions only apply clearly to fairly pure samples of substances. In effect the teaching model is that materials occur as solids, liquids and gases – when a good many materials (emulsions, gels, aerosols, etc.) do not fit this model at all well.

Particles are further apart in water than ice

Keith S. Taber

Image from Pixabay 

Bill was a participant in the Understanding Science Project. Bill, a Y7 student, explained what he had learnt about particles in solids, liquids and gases. 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.

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 this is something that tends to be exaggerated in the basic teaching model often used in school science. Often figures in basic school texts show much greater spacing between the particles in a liquid than in the solid phase of the same material. This misrepresents the modest difference generally found in practice – as can be appreciated from the observations that volume increases on melting are usually modest.

Although generally the particles in a liquid are considered further apart than in the corresponding solid*, this need not always be so.

Indeed it is not so for water – so ice floats in cold water. The partial disruption of the hydrogen bonds in the solid that occurs on melting allows water molecules to be, on average, closer* in the liquid phase at 0˚C.

As ice float in water, it must have a lower density. If the density of some water is higher than that of the ice from which it was produced on melting then (as the mass will not have changed) the volume must have decreased. As the number of water molecules has not changed, then the smaller volume means the particles are on average taking up less space: that is, they seem to be closer together in the water, not further apart*.

Bill was no doubt aware that ice floats in water, but if Bill appreciated the relationship of mass and volume (i.e., density) and how relative density determines whether floatation occurs, he does not seem to have related this to his account here.

That is, he may have had the necessary elements of knowledge to appreciate that as ice floats, the particles in ice were not closer together than they were in water, but had not coordinated these discrete components to from an integrated conceptual framework.

Perhaps this is not surprising when we consider what the explanation would involve:

Coordinating concepts to understand the implication of ice floating

Not only do a range of ideas have to be coordinated, but these involve directly observable phenomena (floating), and abstract concepts (such as density), and conjectured nonobservable submicroscopic/nanoscopic level entities.

* A difficulty for teachers is that the entities being discussed as 'particles', often molecules, are not like familiar particles that have a definitive volume, and a clear surface. Rather these 'particles' (or quanticles) are fuzzy blobs of fields where the field intensity drops off gradually, and there is no surface as such.

Therefore, these quantiles do not actually have definite volumes, in the way a marble or snooker ball has a clear surface and a definite volume. These fields interact with the fields of other quanticles around them (that is, they form 'bonds' – such as dipole-dipole interactions), so in condensed phases (solids and liquids) there are really not any discrete particles with gaps between them. So, it is questionable whether we should describe the particles being further apart in a liquid, rather than just taking up a little more space.

Particles in ice and water have different characteristics

Making a link between particle identity and change of state

Keith S. Taber

Image by Colin Behrens from Pixabay 

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?

Iron turning into a gas sounds weird

Keith S. Taber

Amy was a participant in the Understanding Science Project. She was interviewed when she had just started her 'A level' (i.e. college) chemistry, and one of the topics that the course had started with was mass spectrometry – (see A dusty analogy – a visual demonstration of ionisation in a mass spectrometer). Amy seemed to be unconvinced, or at least surprised by a number of aspects of the material she had learnt about the mass spectrometer.

So, for example, she found it strange that iron could be vaporised:

So which bits of that are you not convinced about then?

(Pause, c.3 seconds)

It just all … I don't, it's not that I'm not convinced about it, it's just sound strange, because it's like…

(Pause, c.2s)

… erm, well this sounds like ridiculous but, like but before today like none of the people in out class had thought about iron being turned into a gas, and it's little things like that which sound weird.

Okay, erm so if you said to people, can you turn water into a gas, most people would say.

Yeah.

Yeah, do it in the kettle all the time, sort of thing.

Yeah.

But if you said to people can you turn iron into a gas? – do people find that a strange idea?

Yeah.

Yeah?

Well we did. (She laughs)

Although Amy and her classmates had studied the states of matter years earlier at the start of secondary school, and would have learnt that substances can commonly be converted between solid, liquid and gaseous phases, their life-world (everyday) experience of iron – the metallic material – made the idea of iron vapour seem 'weird'.

Given the prevalence of grounded learning impediments where prior learning interferes with new learning, this did not seem as "ridiculous" to the interviewer as Amy suspected it may appear.

As science teachers we have spent many years thinking in terms of substances, and the common pattern that a substance can exist as a solid, liquid or gas – yet most people (even when they refer to 'substances') usually think in terms of materials, not substances. Iron, as a material, is a strong solid material suitable for use in building structures – thinking of iron the familiar material as becoming a gas requires a lot of imagination for someone who not habitually think in terms of scientific models.

Although Amy thought her classmates had found the idea of iron as gas as weird, they had not rejected it. Yet, if it is such a counter-intuitive idea, it may not be later readily brought to mind when it might be relevant, unless it is consolidated into memory by reinforcement through being revisited and reiterated. (Indeed the research interview provides one opportunity for rehearsing the idea: research suggests that whenever a memory is activated this strengthens it.)

[Another student I interviewed told me that Iron is too heavy to completely evaporate.]

In a sponge, the particles are spread out…

In a sponge, the particles are spread out more, so it can absorb more water 

Keith S. Taber

Morag was a participant in the Understanding Science Project. In her first term of secondary school, she told me that he had learnt about particles. Morag had explained, and simulated through role play for me, the arrangements of particles in the different states of matter (See: So if someone was stood here, we'd be a solid.) She had also emphasised just how tiny the particles were, "little, little-little-little things", and so how many there were in a small object: "millions and millions and millions". This suggested she had accepted and understood the gist of the scientific model of submicroscopic particles.

Yet as the conversation proceeded, Morag suggested the macroscopic behaviour of sponge in absorbing water could be explained by the arrangement of particles leaving space for the water. This is perhaps a reasonably, indeed quite imaginative, suggestion at one level, except that the material of a sponge is basically solid (where, as Morag recognised, that the particles would be very close together). A sponge as whole is more like a foam, with a great volume of space between the solid structure (where air can be displaced by liquid) and an extensive surface area.

Do you think it is important to know that everything is made of particles?

No.

It's not important?

Well it could be important, but it's not that important. Well, you see, like that [indicating the voice recorder used to record the interview] has got like lots and lots of particles pushed together this [Morag gestures]…But a sponge, the particles are like, the particles are more kind of like, they're still the same, but it's just spread out more, so it can absorb more water.

Oh I see, so are you saying that the same particles are in my little recorder, as in the sponge.

Yeah, they're the same, but there's just more of them in one than there would be in the other.

The failure here is perhaps less Morag's inappropriate explanation, than the tendency to teach about the ideals of solids, liquids and gases, which strictly apply only to single substances, where most real materials students come across in everyday life are actually mixtures or composites where the labels 'solid', liquid' and 'gas' are – at best – approximations.

Teaching has to simplify complex scientific ideas to make them accessible to students of different ages, so often teaching models are used. But sometimes simplifications can cause misunderstandings, and so the development of alternative conceptions. If 'everything is a solid, liquid or gas' is used as a kind of teaching model, or even presented as a slogan or motto for students to echo back to the teacher, when lots of things students come across in everyday life (e.g., butter, clouds, the pet cat – a bathroom sponge) do not easily fit these categories, and this is likely to lead to students overgeneralising.

Although it is often not possible to assign a single simple cause to a student's flawed thinking, this could be considered likely to be an example of a pedagogic learning impediment (a type of grounded learning impediment) in chemistry: a case where an approach to teaching can lead students' thinking in unhelpful directions.

Are plants solid?

Keith S. Taber

Image by Martin Winkler from Pixabay 

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 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