Tag: chemical reactions

Ambitious molecules hustle at the World Economic Forum

Keith S. Taber

Composite picture representing people from Kenya, Will.I.Am, Steve Jobs of Apple, former UK minister Rachel Maclean and financial journalist Gillian Tett with a test-tube
The World Economic Forum has been compared to a chemical reaction between disparate molecules. (A group of Kenyans in traditional dress, Apple's co-founder Steve Jobbs, former UK minister Rachel Maclean, musician and activist will.i.am, and journalist Gillian Tett – includes images accessed from Pixabay)

Analogy is a key tool in the teacher's toolbox when 'making the unfamiliar familiar'. Science teachers are often charged with presenting ideas that are abstract and unfamiliar, and sometimes it can help if the teacher can point out how in some ways a seemingly obscure notion is just like something already familiar to the learner. An analogy goes beyond a simile (which simply suggests something is a bit like something else) by offering a sense of how the structure of the 'analogue' maps onto the structure of the 'target'.

Apologies are useful well beyond the classroom. They are used by science journalists reporting on scientific developments, and by authors writing popular science books; and by scientists themselves when explaining their work to the public. But analogies have a more inherent role in science practices: not only being both used in formal scientific accounts written to explain to and persuade other scientists about new ideas, but actually as a tool in scientific discovery as a source of hypotheses.

I have on this site reported a wide range of examples of analogies I have come across for different scientific concepts and phenomena.

Sometimes, however, one comes across an analogy from a scientific concept or phenomenon to something else – rather than the other way round. The logic of using analogies is that the source analogue needs to already be familiar to a reader or listener if it is to help explain something that is novel. So, an analogy between the concept of working memory capacity and fatty acid structure might be used

  • to explain something about working memory to a chemist – but could also be used
  • to explain fatty acid structure to a psychologist who already knew about working memory.

So, the use of a scientific idea as the source analogue for some other target idea suggests the user assumes the audience is also familiar with the science. Therefore I deduce that Gillian Tett, journalist at the Financial Times presumably is confident that listeners to BBC Radio 4 will be familiar with the concept of chemical reactions.

Some chemical reactions only proceed at a viable rate on heating. However, an ice bath may be needed to cool some very vigorous reactions to limit their rate. (Image © University of Colorado at Boulder, Department of Chemistry and Biochemistry.)

A cold temperature reaction?

Tett was discussing her experience of the annual World Economic Forum meeting that has just been held in the snow of the Swiss skiing resort of Davos, and suggested that the mixing of various politicians and industry and media and lobbyists had the potential to lead to interesting outcomes – like some kind of chemistry experiment,

"I got jammed into a room with will.i.am, the rapper, who was talking about his views for A.I., and suddenly you've got these activists standing next to somebody from some of the big tech. companies, and a government minister, and a group from Kenya, all talking about whether A.I. could actually be a tool to reduce social inequality, rather than increase it. So, it is a bit like a chemistry experiment where you take all of these ambitious, self-selecting, hustling molecules from around the world, shove them into one test-tube, apply maximum pressure, and force them to collide with each other at close quarters with no sleep, and see what kind of compounds arise."

Gillian Tett talking on the BBC's 'The Week in Westminster'

An experiment (by definition) has uncertain results, and Tett used the analogue of the chemistry experiment to imply that the diverse mixes of people collected together at Davos could lead to unexpected outcomes – just like mixing a diverse range of substances might. Tett saw the way such diverse groups become 'jammed' into rooms in arbitrary combinations as they make their ways around the meeting as akin to increasing the pressure of a reaction mixture of arbitrary reagents. This reflects something of the popular media notion of dangerous 'scientific experiments', as carried out by mad scientists in their basements. Real scientific experiments are carried out in carefully controlled conditions to test specific hypothesis. The outcome is uncertain, but the composition of the reaction mixture is carefully chosen with some specific product(s) in mind.

The figure below represents the mapping between the analogue (a rather undisciplined chemistry experiment) and the reaction conditions experienced by delegates in the melting pot of Davos.

Figure showing analogy between World Economic Forum and a chemistry experiment
the World Economic Forum at Davos is like a chemical experiment because…

Inspection of my figure suggests some indiscipline in the analogy. The reaction conditions are to "apply maximum pressure, and force [the molecules] to collide with each other at close quarters with no sleep". Now this phrasing seems to shift mid-sentence,

  • from the analogue (the chemical experiment:"apply maximum pressure, and force [the molecules] to collide with each other")
  • to the target (being jammed into a room at the conference: "at close quarters with no sleep").

One explanation might be that Gillian Tett is not very good at thinking though analogies. Another might be that, as she was being interviewed for the radio, she was composing the analogy off-the-cut without time to reflect and review and revise…

Either of those options could be correct, but I suspect this shift offered some ambiguity that was deliberately introduced rhetorically to increase the impact of the analogy on a listener. Tatt ('an anthropologist by training' and Provost of King's College, Cambridge) had described the molecules anthropomorphically: just as molecules do not sleep,

  • they cannot be 'ambitious', as this is a human characteristic;
  • they are not sentient agents, so cannot be 'self-selecting'; and
  • nor can they 'hustle' as they have no control over their movements.

But the journalists, politicians, activists and industrialists can be described in these terms, reinforcing the mapping between the molecules and the Davos delegates. So, I suspect that whilst this disrupted the strict mapping of the analogy, it reinforced the metaphorical way in which Tett wanted to convey the sense that the ways in which the Davos meeting offered 'experimental' mixing of the reacting groups had the potential to produce novel syntheses.

Read about examples of different science analogies

Read about making the unfamiliar familiar

Read about anthropomorphism in learners' thinking

Read about examples of anthropomorphism in public discussion of science

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

We can't handle the scientific truth

"If the muscles and other cells of the body burn sugar instead of oxygen…"

Do they think we cannot handle the scientific truth?

I should really have gone to bed, but I was just surfing the channels in case there was some 'must watch' programme I might miss, and I came across a screening of the film 'A few good men'. This had been a very popular movie at one time, and I seem to recall watching it with my late wife. I remembered it as an engaging film, and as an example of the 'courtroom drama' genre: but beyond that I could really only remember Tom Cruise as defence advocate questioning Jack Nicholson's as a commanding officer – and the famous line from Nicholson – "You can't handle the truth!".

This became something of a meme – I suspect now there are a lot of people who 'know' and use that line, who have never even seen the film and may not know what they are quoting from.

So, I  though I might watch a bit, to remind myself what the actual case was about. In brief, a marine stationed at the U.S. Guantánamo Bay naval base and detention camp had died at the hands of two of his comrades. They had not intended to kill, but admitted mistreating him – their defence was they were simply obeying orders in subjecting a colleague who was not measuring up, and was letting the unit down, to some unpleasant, but ultimately (supposedly) harmless, punishment.

The film does not contain a lot of science, but what struck me was the failure to get some science that was invoked right.  I was so surprised at what I thought I'd heard being presented as science, that I went back and replayed a section, and I then decided to see if I  could find the script (by Aaron Sorkin*, screenplay adapted from his own theatre play) on the web, to see if what was said had actually been written into the script.

One of the witnesses is a doctor who is asked by the prosecuting counsel to explain lactic acidosis.

Burning sugar instead of oxygen?

The characters here are:

Capt. Jack Ross (played by Kevin Bacon) the prosecuting counsel,

Dr. Stone (Christopher Guest) and

 

 

 

Lt. Daniel Kaffee (Cruise's character).

On direct examination:

Ross: Dr. Stone, what's lactic acidosis?

Stone: If the muscles and other cells of the body burn sugar instead of oxygen, lactic acid is produced. That lactic acid is what caused Santiago's lungs to bleed.

Ross: How long does it take for the muscles and other cells to begin burning sugar instead of oxygen?

Stone: Twenty to thirty minutes.

Ross: And what caused Santiago's muscles and other cells to start burning sugar? [In the film, the line seems to be: And what caused this process to be speed up in Santiago's muscles?]

Stone: An ingested poison of some kind.

Later, under cross-examination

Kafee: Commander, if I had a coronary condition, and a perfectly clean rag was placed in my mouth, and the rag was accidentally pushed too far down, is it possible that my cells would continue burning sugar after the rag was taken out?

Stone: It would have to be a very serious condition.

What?

If a student suggested that lactic acid is produced when the muscles burn sugar instead of oxygen we would likely consider this an alternative conception (misconception). It is, at best, a clumsy phrasing, and is simply wrong.

Respiration

Metabolism is a set of processes under very fine controls, so whether we should refer to metabolism as burning or not, is a moot point. Combustion tends to be a vigorous process that is usually uncontrolled. But we can see it as a metaphor: carbohydrates are 'burnt' up in the sense that they undergo reactions analogous to burning.

But burning requires oxygen (well, in the lab. we might burn materials in chlorine, but, in general, and in everyday life, combustion is a reaction with oxygen), so what could burning oxygen mean?

In respiration, glucose is in effect reacted with oxygen to produce carbon dioxide and water. However, this is not a single step process, but a complex set of smaller reactions – the overall effect of which is

glucose + oxygen → carbon dioxide + water

Breaking glucose down to lactic acid also acts as an energy source, but is no where near as effective. Our muscles can undertake this ('anaerobic') process when there is insufficient oxygen supply –  for example when undertaking high stamina exercise – but this is best seen as a temporary stop-gap, as lactic acid build up causes problems (cramp for example) – even if not usually death.

Does science matter?

Now clearly the science is not central to the story of 'A few good men'. The main issues are (factual)

  • whether the accused men were acting under orders;

(ethical)

  • the nature of illegal orders,
  • when service personal should question and ignore orders (deontology) given that they seldom have the whole picture (and in this film one of the accused men is presented as something of a simpleton who viewer may suspect should not be given much responsibility for decision making),
  • whether it is acceptable to use corporal or cruel punishment on an under-performing soldier (or marine) given that the lives of many may depend upon their high levels of performance (consequentialism, or perhaps pragmatics)…

There is also a medical issue, regarding whether the torture of the soldier was the primary cause of death, or whether there was an underlying health issue which the medical officer (Stone) had missed and which might also explain the poor performance. [That is a theme which featured large in a recent very high profile real murder case.]

Otherwise the film is about the characters of, and relationships among, the legal officers. Like most good films – this is film about people, and being human in the world, and how we behave towards and relate to each other.

The nature of lactic acidosis is hardly a key point.

But if it is worth including in the script as the assumed cause of death, and its nature relevant – why not get the science right?

Perhaps, because science is complicated and needs to be simplified for the cinema-goer who, after all, wants to be entertained, not lectured?

Perhaps there is no simple account of lactic acidosis which could be included in the script without getting technical, and entering into a long and complicated explanation.

In teaching science…

But surely that is not true. In teaching we often have to employ simplifications which ignore complexity and nuance for the benefit of getting the core idea across to learners. We seek the optimal level of simplification that learners can make good sense of, but which is true to the core essence of the actual science being discussed (it is 'intellectually honest') and provides a suitable basis for later more advanced treatments.

It can be hard to find that optimum level of simplification – but I really do not think that explaining lactic acidosis as burning sugar instead of oxygen could be considered a credit-worthy attempt.

Dr. Stone, can we try again?

What about, something like:

Dr. Stone, what's lactic acidosis?

It occurs when the body tissues do not have sufficient oxygen to fully break down sugar in the usual way, and damaging lactic aid is produced instead of carbon dioxide and water.

I am sure there are lots of possible tweaks here. The point is that the script did not need to go into a long medical lecture, but by including something that was simply nonsensical, and should be obviously wrong to anyone who had studied respiration at school (which should be everyone who has been to school in the past few decades in many countries), it distracts, and so detracts, from the story.

All images from 'A few good men' (1992, Columbia Pictures)

 

 

 

 

 

 

 

 

 

 

* I see that ("acclaimed screenwriter") Aaron Sorkin is planning a new live television version of 'A Few Good Men' – so perhaps the description of lactic acidosis can be updated?

Fuels get used-up when we burn them

Keith S. Taber

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

So what happens to paraffin when it burns then?

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

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

No, 'cause it would run out.

What would it run out of?

The paraffin.

So where does the paraffin go then?

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

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

It gets lower and lower.

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

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

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

You have to refill it.

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

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

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

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

Because the sugar's so small it would evaporate with the water

Keith S. Taber

Morag was a participant in the Understanding Science project. In an interview in her first term of secondary school, Morag suggested that when sugar with mixed with water, it could not be separated out again. This was in the context of discussing chemical change, when she was explaining to me that a chemical change is where two things just go together:*

I: So what's a chemical reaction?

Morag: (I had to learn this) it's when two things, erm, are mixed together and can't be made to the original things easy, easily.

I: Oh, can you give me an example of that?

{pause c. 2 s }

M: Water mixing with sugar, but that's not a chemical reaction.

I: Oh so that's something else is it, is that something different?

M: I don't know.

I: Don't know, so can you mix water with sugar?

M: Yeah, but you can't get the water and the sugar back together very easily.

I: You can't. Is there a way of doing that?

M: No.

I: No? So if I gave you a beaker with some sugar in, and a beaker with some water in,

M: Mm.

I: and you mixed them together, poured them all in one beaker, and stirred them up – you would find it then difficult to get the water out or the sugar out, would you?

M: Ye-ah

I: Yeah, so is that a chemical reaction?

M: No.

The conversation went on to explore Morag's ideas about chemical reactions, and her notion that the flame reacts to the gunpowder * when a firework explodes. A little later we returned to her notions relating to mixtures of sugar and water (i.e., solutions).

I: And when you mix sugar and water, you get kind of sugary water

M: Yeah.

I: Have you got a name for that, when you mix a liquid and solid like that?

{pause c. 1 s}

I: Or is that just mixing sugar and water?

{pause c. 1 s}

M: There is a name for it,

I: Ah.

M: but I don't know it.

I: Okay, so when we mix it we get this sugar-water, whatever, and then it's harder to, it's hard to separate it is it, and get the sugar out

M: Yeah.

I: and the water out?

M: Yeah.

As I probed further, I elicited a difference that Morag perceived between water/sugar mixture (solution) and water/salt mixture (solution). At the time I was not sure what to make of this, and feeling that Morag was probably to some extent searching for answers on the spot, decided to move back to other themes. However, in retrospect, Morag seems to be saying there is a difference because in some sense the sugar is smaller, and so on evaporation can be taken away with the water – unlike the case with salt (solution). Her explanation is vague, but she refer to water:salt ratio, so appear to mean how much can dissolve rather than thinking in terms of molecular size.

I: So is that a chemical reaction?:

{pause c. 3 s}

M: No.

I: No, is that a chemical change?

{pause c. 3 s}

M: Yes.

I: Ah, okay. So what's the difference between a chemical change and a chemical reaction?

M: A reaction is where two things react with each other, like the gunpowder and flame, and a change is where two things just go together. You know like water and sugar, they go together like water and salt. Partially, they go together.

I: Mm. Partially?

M: Yeah. 'cause, erm, in water and salt you can get the salt back, whereas you can't with water and sugar.

I: Oh, so it's different, is it? Oh, I see. So if you had water and salt, how would you get them back again?

M: Erm, you'd put the water and salt by the window, and let the sun do all the evaporating of the water, and you would be left with the salt crystals.

I: So what if you took water and sugar, and put that by the window, would it evaporate the water, and leave you with the sugar?

{Pause, c. 1 s}

M: N-o.

I: That's different then, is it?

M: Yeah, cause the water's absorbed kind of like the sugar, and because they're, it's so small it would just take the sugar with it.

I: What do you mean it's so small?

{Pause, c. 1 s}

I: What if I had a big beaker of water and sugar?

{Pause, c. 2 s}

M: But there would be more water to salt ratio.

I: …Okay, so there is a difference, then, there's a difference

M: Yeah.

I: between the sugar and the salt?

M: Yeah.

This is an unsatisfactory place to leave the discussion, and in hindsight there are questions I would like to have asked. (Why did she think she could not recover sugar by leaving the water to evaporate? Was she thinking of the amount of sugar / salt needed to form what we would call a saturated solution?…)

Because they are laws these things have to be true

Keith S. Taber

Ralph was a participant in the Understanding Science project. When I interviewed him in Y10 he suggested that what was particular to science was that with science it will always be the same, i.e., that the nature of science was that it was universal rather than relative to a particular place. Ralph had commented that "because they're kind of like, they are laws so…these things have to be true".

I: So in say maths you have these laws that are what, universal?

R: Yeah.

I: And science you think is the same sort of thing?

R: Yeah.

So Ralph was asked about the universal nature of laws in science:

I: So what laws do you know in science then that will apply anywhere?

R: Erm, well there's kind of like the laws of gravity and things, which are always there. But they can, that is one exception, because that can be changed depending on what planet you are on, but that's kind of like very, far off so, if you went on the moon and did physics there it might be ever so slightly different, but I'm not sure because I haven't been to the moon though.

…

R: And chemistry it's so – reactions and things – but the environments can change those, but not to a large extent, so, so iron will always react with something, no matter what, and two of the same element will not react together because they're already the same and things like that.

I: Mm?

R: Erm, yep, and biology's because most is kind of like is an average so, it can be different as well, but they're kind of like, saying they're all universal laws and all actually the same is kind of a bit untrue, but if like, there are exceptions to the rule in different places, so biology you can kind of like have erm illnesses or disfigurements that change how you look at biology, and things, which is kind of complicated and you don't tend to do that in this kind of level of biology, 'cause that's more kind of like that's specialised, that's more in kind of medical biology and things.

I: So it's a kind of 'unless' law, so, you know, a dog will always have four legs,

R: Yeah.

I: unless one of them's been torn off

R: Yeah.

I: or unless it is a mutant and grown an extra one?

R: Exactly, unless there is some kind of other, erm, other … event which … changes how that will work, so … like a snail would normally have no legs, but if you put loads of radiation on it, I dunno, maybe it would grow an arm or something.

It seemed Ralph's notion of a law of gravity was not the universal law of Newtonian physics, but something linked to the local strength of the gravitational field. (Later in the interview Ralph explained that the law of gravity is that things will always move towards the centre, but it is different on the moon*). In chemistry, Ralph seemed to see ideas about which substances reacted together as laws, although he acknowledge that in some cases these patterns were dependent upon conditions. In biology, Ralph associated laws with the normal forms of organisms, which again he knew could be changed by environmental factors.

In general then, Ralph's notion of scientific laws did not match the scientific notion of a law, which would generally operate at a 'deeper' level (i.e., a higher level of abstraction from observations), but seemed more at the level of 'facts'.

A reaction is just something that happens?

Keith S. Taber

The term 'reaction' is used in at least two different technical senses in school science: in studying forces as one of the components of a interaction between two bodies such that they each experience a force ('action-reaction'), and as a chemical change which leads to a transformation of matter leading to a new substance(s).

Lomash was a participant in the Understanding Science project. Y7 student 'Lomash' reported that he had been heating materials in a Bunsen flame in his science lessons: "We were burning … coal and copper and things like that, metals."

When he heated copper "It went black…because the flame was too hot, and – it just went black , like paper." The copper stayed black after being removed form the flame, and this was because "it's something else, it's a reaction."

Lomash was using the term 'reaction' in the context of a chemical change – the copper had changed to 'something else', suggesting that he had acquired something of the technical meaning of the term as it is used in chemistry. However 'reaction' is used with a much more general meaning in everyday life, and on further questioning it seemed Lomash has not appreciated the special meaning given to the word in chemistry:

I: So what's a reaction?

L: It's like, a reaction is something that happens.

I: Okay, so if I fell off this stool, would that be a reaction?

L: Yeah.

I: And if you laughed at me falling off the stool, would that be a reaction?

L: Yeah.

I: Oh I see. So that's just another name for something that happens is it?

L: Yeah.

Where students already have meanings for words they come across in school science, they are unlikely to spontanously appreciate how the word is used in a specialised, nuanced way in this particular context. Perhaps Lomash's teacher had emphasised that in heating the copper 'something else' was produced, making the observed change a 'reaction'. Certainly Lomash happily accepted this was a reaction, but apparently only in his existing vague everyday sense of the term. His existing linguistic association for the term 'reaction' appeared to act as an associative learning impediment.

Read about learners' alternative conceptions

A chemical change is where two things just go together

Keith S. Taber

Morag was a participant in the Understanding Science project. In the first interview, in her first term in secondary school, Morag told me that that she was studying electricity having previously studied changing state and burning. When I asked her whether these science topics have anything in common, that made them science, we got into a conversation about chemical reactions, and chemical change:

Do they have anything in common do you think? is there anything similar about those topics?

Changing state and burning's got something in common, but I don't know about electricity.

Oh yeah? So what's, what have they got in common then?

Erm, in burning you have, you could have a chemical reaction, and in changing states you've got chemical reactions as well.

From the canonical scientific perspective, a change of state is not a chemical reaction (so this is an alternative conception), so I followed up on this.

Ah, so what's a chemical reaction?

(I had to learn this) it's when two things, erm, are mixed together and can't be made to the original things easy, easily.

Oh, can you give me an example of that?

{pause, c. 2 seconds}

Water mixing with sugar, but that's not a chemical reaction.

So, Morag offers a definition or at least a description of a chemical reaction, but then the example she gives of that of type of event is not something she considers to be a chemical reaction. (Dissolving is not usually considered a chemical change, although it usually involves the breaking and forming of bonds, sometimes strong bonds.)

Oh so that's something else is it, is that something different?

I don't know.

Don't know, so can you mix water with sugar?

Yeah, but you can't get the water and the sugar back together very easily.

You can't. Is there a way of doing that?

No.

No? So if I gave you a beaker with some sugar in, and a beaker with some water in, and you mixed them together, poured them all in one beaker, and stirred them up – you would find it then difficult to get the water out or the sugar out, would you?

Ye-ah.

Yeah, so is that a chemical reaction?

No.

No, okay. That's not a chemical reaction.

At this point Morag suggested we look in her book as "it's in my book", but I was more interested in what she could tell me without referring to her notes.

So, have you got any examples of chemical reactions – any you think are chemical reactions?

Fireworks,

I: Fireworks, okay.

when like the gunpowder explodes, erm in the inside, and you can't get it back to the original rocket once it's has exploded.

and is that what makes it a, er, a chemical reaction, that you can't get it back?

{pause, c. 3 s}

Yeah, I suppose so.

So, now Morag has presented an example of a chemical reaction, that would be considered canonical (as chemical change) by scientists. Yet her criterion is the same as she used for the dissolving example, that she did not think was a chemical reaction.

Yeah? And then the water and the sugar, you can't get them back very easily, but we don't think that is a chemical reaction?

Yeah – that's a chemical change – {adding quietly} I think.

It's what, sorry?

Well there's, a chemical reaction and a chemical change.

Oh I see. So what's the difference between a chemical reaction and a chemical change?

Erm nothing, it's just two different ways of saying it.

Oh so they're the same thing?

Yeah, just two different ways of saying it.

So, now Morag had introduced a differentiated terminology, initially suggesting that sugar mixing with water was a chemical change, whereas a firework exploding was a chemical reaction. However, this distinction did not seem to hold up, as she believed the terms were synonyms. However, as the conversation proceeded, she seemed to change her mind on this point.

So when a firework goes off, the gunpowder, er, explodes in a firework, that's a chemical reaction?

Yeah – yeah, cause something's mixing with the gunpowder to make it blow up.

…

And So that's a chemical reaction?

Yeah.

And is that a chemical change?

{pause, c. 2 s}

Yeah.

Yeah?

(I suppose.) Yeah.

And when you mix sugar and water, you get kind of sugary water?

Yeah.

Have you got a name for that, when you mix a liquid and solid like that?

{pause, c. 1 s}

Or is that just mixing sugar and water?

{pause, c. 1 s}

There is a name for it, but I don't know it.

Ah. Okay, so when we mix it we get this sugar-water, whatever, and then it's harder to, it's hard to separate it is it?

Yeah.

And get the sugar out and the water out?

Yeah.

So is that a chemical reaction?

{Pause, c. 3 s}

No.

No, is that a chemical change?

{Pause, c. 1 s}

Yes.

Ah, okay.

So, again, Morag was suggesting she could distinguish between a chemical reaction, and a chemical change.

So what's the difference between a chemical change and a chemical reaction?

A reaction is where two things react with each other, like the gunpowder and flame, and a change is where two things just go together. You know like water and sugar, they go together…

In effect we had reached a tautology: in a chemical reaction, unlike a chemical change, things react with each other. She also thought that a sugar/water and a salt/water mixtures (i.e., solutions) were different "because the sugar's so small it would evaporate with the water"*.

The idea that a chemical reactions has to involve two reactants is common, but is an alternative conception as chemists also recognise reactions where there is only one reactant which decomposes.

Morag seemed to be struggling with the distinction between a chemical and a physical change. However, that distinction is not an absolute one, and dissolving presents a problematic case. Certainly without a good appreciation of the submicroscopic models used in chemistry, it is not easy to appreciate why reactions produce a different substance, but physical changes do not. One of Morag's qualities as a learner, however, was a willingness to 'run with' ideas and try to talk her way into understanding. That did not work here, despite Morag being happy to engage in the conversation.

Morag was also here talking as though in the gunpowder example the flame was a reactant (i.e., the flame reacts to the gunpowder). Learners sometimes consider substances in a chemical reaction are reacting to heat or stirring rather than with another substance (e.g., Taber & García Franco, 2010).

Read about learners' alternative conceptions

Source cited:

Taber, K. S., & García Franco, A. (2010). Learning processes in chemistry: Drawing upon cognitive resources to learn about the particulate structure of matter. Journal of the Learning Sciences, 19(1), 99-142.