Tag: mixtures

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.

Thank you, BBC: I'll give you 4/5

BBC corrects cruel (to cats) scientific claim on its website

Keith S. Taber

I just got 80% on a science test for primary school children

I've just scored 4/5 (80%) on an on-line KS2 science test on the BBC (the British Broadcasting Corporation) educational website. 80% sounds quite good out of context, but I am a science teacher and KS2 is meant for 7-11 year olds.

The BBC awards me 4/5 for my primary level science knowledge about the states of matter

My defence is that the question I got wrong was ambiguous (but, as Christine Keeler might have said, I would say that).

I was actually getting round to checking on something from a while back.

In 2019 I came across something on the website that I thought was very misleading – and I complained to the BBC through their website form. I had an immediate, but generic response:

"Thank you for taking the time to send us your comments. We appreciate all the feedback we receive as it plays an important role in helping to shape our decisions.

This is an automated message (sorry that we can't reply individually) to let you know that we've read your comments and will report them overnight to staff across the BBC for them to read too (after removing any personal details). This includes our programme makers, commissioning editors and senior management.

Thanks again for contacting the BBC.

BBC Audience Services.

NB: Please do not reply to this email. It includes a reference number but comes from an automated account which is not monitored."

Email: 6th Sept., 2019

This kind of response is somewhat frustating. My complaint had been recieved, and would be passed on, but it looked like I would get no specific response (as presumably if my "comments" were to be reported to relevant staff "after removing any personal details", those staff would not be in a position to let me know if they were following up, dismissing, or simply ignoring, my comments.) Indeed, I never did get any follow up.

So, my intention was to check back after a decent period had elapsed (n.b., where does all the time go?) and see if anything had been changed in response to my complaint. Strictly, if there had been a change this could be because:

  • a) I complained
  • b) someone else/some other people complained (i.e., people who's complaints were taken more seriously than mine)
  • c) I was one of number of people who complained
  • d) material had been updated compleltely independently of any compaints

That is, I could not know if I personally had had any effect, BUT if the offending material (because as a chemist I was offended professionally, even if not personally) was still there then I would know my compaint had not been heeded.

So, I intended to check back; I expected to find no change (as pointing out blatant, basic, errors in the science in the English National Curriculum to government ministers did not have any effect, so the BBC…? ); and, if so, I thought of following up with an email or an old fashioned snail-mail … ("…yours, disgusted of Cambourne"*).

Well done, BBC

So, I am happy to publicly acknowledge that the BBC has changed its materials appearing under the heading 'What are the states of matter?'

The topic comprises of a short animation (with odd anthropomorphised {"guys"} geometric shapes handling examples of the states of matter: solid, liquid and gas); a series of bullet points on each state; a sorting task; and then the set of five objective (multiple choice) questions.

There are a number of issues with the examples used here, as discussed below. But the main focus of my complaint, a cartoon cat, has now been released from the indignity of being classified as a state of matter. Yes, a cat!

Limitations of the three states of matter model

The idea that matter can exist in three states is a pretty important foundation for a good deal of other science.

However there is big problem with the generality of the model. Basically it really applies to pure samples of substances: generally substances (not materials in general, and certainly not objects) exist as solids, liquids, or gases, depending on the conditions of temperature and pressure – although at high enough temperatures plasmas are formed (and theoretically when hot enough even the atomic cores, and eventually nuclei would break down – but those conditions are pretty extreme and not found in the typical home or classroom).

Examples of substances include water, salt, calcium carbonate, iron, mercury, hydrogen, graphite, carbon dioxide, sulphur… that is, elements and compounds. Of course, many of these are seldom met in pure form in everyday life outside school science labs.

Most materials that people come across are mixtures or composites. Mixtures often exist as solutions or suspensions – as gels or foams or emulsions – not as solids, liquids or gases.

This is probably why the terms 'solids', 'liquids' and 'gases' actually have two sets of meanings – the science or technical sense, and the everyday or 'life-world' sense. So milk is a liquid(everyday) as you can pour some into your tea cup and a block of wood is a solid(everyday) as it retains its shape and integrity as you nail it to another structure. But milk and wood are not substances – and so not liquid(scientific) or solid(scientific).

Does this matter? Yes, because if we are teaching children things in science lessons, it would be good to get the science right. A solid will melt at a distinct melting temperature to give a liquid which will boil at a distinct boiling temperature. Wood, for example, does not.

Wood is a complex material. It has gas pockets. It has (variable) moisture content, and the structure contains various compounds – lignin, cellulose, and many more. The response to heating reflects that complex constitution.

The BBC's examples of solids, liquids, and gases

The BBC website suggests examples of the three states of matter to introduce primary age students to the concept.

Animation:

Solids: block of ice, football

Lquids: water, honey

Gases: none are specified – animation shows the clouds (of liquid water droplets) forming around a kettle spout, and 'gas' put into in fizzy drinks is referenced.

A football is not solid, but usually air (a mixture of gases with some other components) contained in a plastic shell. (The voiceover refers simply to a 'ball', but the animation show a large ball with a traditional football pattern being used to do 'keepy uppies' by the cartoon character.)

Honey is not a liquid(scientific) but a complex mixture of sugars in solution. There is usually much more sugar than water. (So, arguably, it is more solid than liquid – but it is better to simply not consider it as either.) This is where I dropped a mark on the terminal test:

Two of the options are NOT liquids. Only one response gets credit in this test!
Web text:

The bullet points on the site list some further examples:

"Examples of solids include ice, wood and sand." (Ice and sand are solids(scientific).)

"Examples of liquids include water, honey and milk." (Only water is liquid(scientific) here.)

"Examples of gases include steam, helium and oxygen." (3/3, well done BBC!)

Sorting task:
The BBC website task invites children to sort cards showing objects into three categories. (What is that object on the front card meant to be?)

In the sorting task, children are asked to sort a number of examples shown on cards into solid, liquid, and gas:

The examples presented are air, a feather, helium, milk, a pencil, sea, steam, syrup, wood. Of these only helium and steam strictly meet the criteria for being a solid(scientific)/liquid(scientific)/gas(scientific). Yet, as suggested above, it is difficult to find genuine examples that are both scientifically correct and familiar to young children. Perhaps sea and air (at least materials) are closer approximations than a pencil or a feather ("solids retain their shape" – would a child using the website have handled a feather, and, if so, would it have retained its shape under child-handling?)

So, I still have reservations about this material, whilst acknowledging the need to balance scientific correctness with relevant (to children) examples. Strictly, some of the examples can be seen as encouraging children to get the science wrong. These things matter if only because children are learning things on this site that later in their school career will be judged as alternative conceptions and marked as wrong.

(Read 'Are plants solid?')

None the less, I am pleased that the BBC has at least decided to amend its sorting task, and remove the poor cat:

Which pile does the cat belong in? [This example has now been removed. Bravo.]

The website had previously been quite clear that putting the cat as anything other than solid was 'wrong'. It is classed as a solid even though a cat (like any animal) is (or would be if separated out into its constituent substances – and children should not try this at home) more water than anything else.

I had real trouble seeing how that example fitted with the criteria specified on the webpage:

"[Cats] stay in one place and can be held.

[Cats] keep their shape. They do not flow like liquids.

[Cats] always take up the same amount of space. They do not spread out like gases.

[Cats] can be cut or shaped."

Characteristics of solids, but perhaps not entirely true of cats?

* cf. the idiom 'disgusted of Tunbridge Wells' – referring to a hypothetical person who writes to media complaining about matters of concern.

Images used here are screenshots, copyright of the BBC – a publicly funded public service broadcaster.

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')

A compound is just a lot of different elements put together

Keith S. Taber

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

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

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

It's erm, it's er two, er you know, it's just a lot of different elements put together – to create just one.

So if I went and got some elements, let's say I went and got a little file of carbon, a little file of sulphur, a little file of copper, er a little file of magnesium and I were to mix them into a beaker, maybe get a glass rod, give it a good stir, er, give me a compound?

Erm, so it's carbon, erm, carbon, sulphur, magne¬. Carbon, er – What's the fourth one?

Carbon, sulphur, magnesium and copper I think I said.

And copper. All right, erm. Copper, copper sulphate and – and carbon, and I think carbon and magnesium might go just as elements.

Okay, so if I ignored the carbon and magnesium,

Yeah.

if I took some copper and some sulphur,

Yeah.

and mixed them up together,

Yeah.

then I'd get copper sulphate.

Yeah.

And that's a compound now?

Yeah.

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

Read about learners' alternative conceptions