quanticles - Science-Education-Research

Particles are further apart in water than ice

Keith S. Taber

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.

A salt grain is a particle (but with more particles inside it)

Keith S. Taber

Sandra was a participant in the Understanding Science Project. When I interviewed Sandra about her science lessons in Y7 she told me "I've done changing state, burning, and we're doing electricity at the moment". She talked about burning as being a chemical change, and when asked for another example told me dissolving was a chemical change, as when salt was dissolved it was not possible to turn it back to give salt grains of the same size. She talk me that is the water was boiled off from salt solution "you'd have the same [amount of salt], but there would just be more particles, but they'd be smaller".

As Sandra had referred to had referred to the salt 'particles' being smaller,(as as she had told me she had been studying 'changing state') I wondered if she had bee taught about the particle model of matter

So the salt's got particles. The salt comes as particles, does it?
Yeah.
Do other things come as particles?
Everything has particles in it.
Everything has particles?
Yeah.
But with salt, you can get larger particles, or smaller particles?
Well, most things. Like it will have like thousands and thousands of particles inside it.
So these are other types of particles, are they?
Mm.

So although Sandra had referred to the smaller salt grains as being "smaller particles", it seemed he was aware that 'particles' could also refer to something other than the visible grains. Everything had particles in. Although salt particles (grains?) could be different sizes, it (any salt grain?) would have a great number ("like thousands and thousands") of particles (not grains – quanticles perhaps) inside it. So it seemed Sandra was aware of the possible ambiguity here, that there were small 'particles' of some materials, but all materials (or, at least, "most things") were made up of a great many 'particles' that were very much smaller.

So if you look at the salt, you can see there's tiny little grains?
Yeah.
But that's not particles then?
Well it sort of is, but you've got more particles inside that.

"It sort of is" could be taken to mean that the grains are 'a kind of particle' in a sense, but clearly not the type of particles that were inside everything. She seemed to appreciate that these were two different types of particle. However, Sandra was not entirely clear about that:

So there's two types are of particles, are there?
I don't know.
Particles within particles?
Yeah.
Something like that, is it?
Yeah.
But everything's got particles has it, even if you can't see them?
Yeah.
So if you dissolved your salt in water, would the water have particles?
Ye:ah.
'cause I've seen water, and I've never seen any particles in the water.
The part¬, you can't actually see particles.
Why not?
Because they're too small.
Things can be too small to see?
Yeah.
Oh amazing. So what can you see when you look at water, then? 'cause you see something, don't you?
You can see – what the particles make up.
Ah, I see, but not the individual particles?
No.

Sandra's understanding here seems quite strong – the particles that are inside everything (quanticles) were too small to be seen, and we could only see "what the particles make up". That is, she, to some extent at least, appreciated the emergence of new properties when very large numbers of particles that were individually too small to see were collected together.

Despite this, Sandra's learning was clearly not helped by the associations of the word 'particle'. Sandra may have been taught about submicroscopic particles outside of direct experience, but she already thought of small visible objects like salt grains as 'particles'. This seems to be quite common – science borrows a familiar term, particle, and uses it to label something unfamiliar.

We can see this as extending the usual everyday range of meaning of 'particle' to also include much smaller examples that cannot be perceived, or perhaps as a scientific metaphor – that quanticles are called particles because they are in some ways like the grains and specks that we usually think of as being very small particles. Either way, the choice of a term with an existing meaning to label something that is in some ways quite similar (small bits of matter) but in other ways very different ('particles' without definite sizes/volumes or actual edges/surfaces) can confuse students. It can act as an associative learning impediment if students transfer the properties of familiar particles to the submicroscopic entities of 'particle' theory.

Gases in bottles try to escape; liquids try to take the shape

Keith S. Taber

Bill was a participant in the Understanding Science Project. Bill, a year 7 (Y7) student, told me that:

"Gases, they try and fill whole room, they don't, like liquids, they stay at the bottom of the container, but gases go fill, do everywhere and fill, try and fill the whole thing."

When asked "Why do they try and do that?" he replied that "Erm, I'm not sure." I suggested some things that Bill might 'try' to do, and asked "so when the gas tries to fill the room, is it the same sort of thing, do we mean the same sort of thing by the word 'try'?" Bill appreciated the difference, and recanted the use of 'try':

"No, I think I phrased that wrong, I meant that it fills the whole area, 'cause it can expand."

However, it soon became clear that Bill's use of the term came easily, despite accepting that it was misleading:

Okay. So it's not, the gas does not come in and say, 'hm, I think I'll fill the whole room', and try and do it?
No, it just does it.
It just does it?
It tries to get out of everywhere, so if you put it in the bottle, it would be trying to get out.

And later:

…are there particles in other things?:
liquids, yeah there is particles in everything, but liquids the particles move quite a lot because, well they have, oh we did this this [in the most recent] lesson, erm, they have energy to move, so they try and move away, but their particles are quite close together.
What about the gases?
The gases, their particles try to stay as far away from each other as possible.
Why is that? Don't they like each other?
No, it's because they are trying to spread out into the whole room.

And later:

…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.
What about the particles in the gas?
The gas, they're really, they're far apart and they try and expand.

Bill had only learnt about particles recently in science, but seemed to have already developed a habitual way of talking about them with anthropomorphism: as if they were conscious agents that strived to fill rooms, escape bottles, and take up the shape of containers.

To some extent this is surely a lack of familiarity with objects that can have inherent motion without having an external cause (like a projectile) or internal purposes (like animals) and/or having a suitable language for talking about the world of molecular level particles ('quanticles'). Such habits may be harmless, but it is a concern if such habitual ways of talking and thinking later come to stand for more scientific descriptions and explanations of natural processes (what has been called strong anthropomorphism).

Bill's lack of a suitable language for talking about particle actions could act as a learning impediment (a deficiency learning impediment), impeding desired learning.

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