Tag: chemical change

Dissolving salt is a chemical change as you cannot turn it back

Dissolving salt is a chemical change as you cannot turn it back as it was before

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". I asked her about burning:

Well, tell me a bit about burning then. What's burning then?
It's just when something gets set on fire, and turns into ash, or – has a chemical change, whatever.
Has a chemical change: what's a chemical change?
It means something has changed into something else and you can't turn it back.
Oh I see. So burning would be an example of that.
Yeah.

So far this seemed to fit 'target knowledge'. However, Sandra suggested that dissolving would also be a chemical change. Dissolving is not normally considered a chemical change in school science, but a physical change, the distinction is a questionable teaching model. (Chemical change is said to involve bond breaking/making, and of course dissolving a salt does involve breaking up the ionic bonding to form solvent-solute interactions.)

Are there other examples?
Erm – dissolving.
So give me an example of something you might dissolve?
Salt.
Okay, and if you dissolve salt, you can't get it back?
Not really, not as it was before.
No. Can you get it back at all?
Sort of, you can like, erm, make the, boil the water so it turns into gas, and then you have salt, salt, salt on the, left there. Sometimes.
But you think that might not be quite the same as it was before?
No.
No. Different in some way?
Yeah
How might it be different?
Be much smaller.
Oh I see, so do you think you'd have less salt than you started with?
You'd have the same, but there would just be more particles, but they'd be smaller.
Ah, so instead of having quite large grains you might have lots of small grains
Yeah.

So Sandra was clear that one could dissolve salt, and then reclaim the same amount of salt by removing the solvent (water) which from the canonical perspective would mean the change was reversible – a criterion of a physical change.

Yet Sandra also thought that although the amount of salt would be conserved, the salt would be in a different form – it would have different grain size. (Indeed, if the water was boiled off, rather than left to evaporate, it might indeed be produced as very small crystals.)

So, Sandra seemed to have a fairly good understanding of the process, but because of the way she interpreted the criterion of a chemical change, something [salt] has changed into something else [solution] and you can't turn it back [with the same granularity]. Large grains will have changed into small grains – so this would, to Sandra's mind, be a chemical change.

Science teachers deserve a great deal of public appreciation. A teacher can teach something so that a student learns it well – and yet still form an alternative conception – here because of the inherent ambiguity in the ways language is used and understood. Sandra's interpretation – if you start off with large particles and end up with smaller particles then you have not turned it back – was a reasonable interpretation of what she had learnt. (It also transpired there was ambiguity in quite what was meant by particles.)

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?…)

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 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

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.