Tag: changes of state

Iron turning into a gas sounds weird

Keith S. Taber

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

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

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

(Pause, c.3 seconds)

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

(Pause, c.2s)

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

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

Yeah.

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

Yeah.

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

Yeah.

Yeah?

Well we did. (She laughs)

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

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

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

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

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

Liquid iron stays a liquid when heated

Keith S. Taber

Sophia was a participant in the Understanding Science Project. In Y7, Sophia had told me that if molten iron was heated "some of it would evaporate but not all of it, 'cause it's not like water and it's more heavy". She thought only "a little" of the iron would evaporate to give iron vapour: The rest "really just stays as a liquid". [See 'Iron is too heavy to completely evaporate'.]

Just over a year later (in Y8) Sophia had been studying "that different erm substances have different freezing and melting and boiling points, and some aren't like a liquid at room temperatures, some are a solid and some are a gas and things like that".

Give me an example of something else that's a solid at room temperature?

Iron.

Do you think iron would have a melting point?

Yeah.

Yeah, and if I, what would I get if I, if I heated iron to its melting point?

It would become a liquid.

And why would it do that?

Because it's got so hot that particles – they have spread out or something?

So what do you think would happen if I heated the iron liquid?

It would stay a liquid.

No matter how much I heated it?

It might, I don't know if it would become a vapour.

Can you get iron vapour?

No, I don't think so.

You don't think so?

No.

So it seems that Sophia had shifted from accepting that iron would partially evaporate (when learning about the particle model of the different states), to considering that iron (probably) can not become a vapour. The notion of iron as a gas is not something we can readily imagine, and apparently did not seem very feasible. In part this might be because we think of iron the material (a metal, which cannot exist in in the vapour phase) rather than as a substance that can take different material forms.

It seems Sophia's prior knowledge of iron the material was working against her learning about iron the substance, an examples of a grounded learning impediment where prior knowledge impedes new learning.

In Y7 Sophia had seemed to have a hybrid conception where having been taught a general model of the states of matter and changes of state, she accepted the counter-intuitive idea that iron could evaporate, but thought that (unlike in the case of water) it could not completely evaporate . This might have been a 'stepping stone' between not accepting iron could be in the gaseous state and fitting it within the general model that all substances will when progressively heated first melt and then evaporate (or boil) as long as they did not decompose first.

However, it seems that a year later Sophia was actually more resistant to the idea that iron could exist as vapour and so now she thought molten iron would remain liquid no matter how much it was heated. If anything, she had reverted to a more intuitive understanding. This is not that strange: it has been shown that apparent conceptual gains which are counter to strongly held intuitions that are brought about by teaching episodes that are not regularly reinforced can drop away as the time since teaching increases. Conceptual change does not always involve shifts towards the scientific accounts.

[Sophia was in lower secondary school when I talked to her about this: but I was also told by a much older student that the idea of iron turning into a gas sounds weird.]