Bill was a a year 7 student who participated in the Understanding Science Project. Bill was explaining that he had been learning about the states of matter, and introduced the notion of there being particles:
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
So what are these particles then?
Erm, they’re the bits that make it what it is, I think.
Ah. So are there any solids round here?:
Yeah, this table.
That’s a solid, is it?:
Yeah
Technically the terms solid, liquid and gas refer to samples of substances and not objects. From a chemical perspective a table is not solid. A wooden table (such as those in the school laboratroy where I talked to Bill) is made of a complex composite material that includes substances such as a lignin and cellulose in its structure. Wood contains some water, and has air pockets, so technically is not a solid to a chemist. However, in everyday life we do thing of objects such as tables as being solid.
I continued, accepting Bills suggestion of a table being solid as a reasonable example.
Okay. So is that made of particles?
Yeah. You can’t see them.
No I can’t!
‘cause they’re very, very tiny.
So if I got a magnifying glass?
No.
No?
No.
What about a microscope?
Yeah.
Yeah?
Probably
Possibly?
Yeah, I haven’t tried it.
You haven’t tried that yet?
No.
But they are very, very tiny are they?
Yeah.
Bill knew that the particles in a solid were very tiny. He seemed to be convinced of their existence, despite not being able to see them. He considered they were too small to be seem with a magnifying glass, but large enough to probably be seen with a magnifying glass. Bill, like a good scientist, qualified this answer as he had not actually undertaken the necessary observation to confirm this: but his intuition what that these particles could not be so small that they would not be visible through a microscope.
later in the interview,Bill used the term microscopic to describe the particles in a solid, where a scientist would describe them as 'submicroscopic':
Tell me the bit about the solids again? Tell me what you said about the particles in the solids?
They move a very tiny amount, but we can’t see that … because they are microscopic.
The term 'particle' used in introductory science classes is often used generically to cover atoms, molecules and ion. These entities are usually much too small to be see with an optical light scope (although other instruments such as scanning tunnelling 'microscopes' provide images showing electric potential profiles that can be interpreted as indicating individual atoms).
Students have no real basis on which to understand the scale of atoms and molecules (e.g. There are about a thousand particles in one grain of salt, if not more), and often assume they are particles much like the specks and grains that can just be seen (see Confusing the particles that everything contains and very small grains of material). Bill did not make this error, as later in the interview he told me that "the kind of specks of dust, has lots of particles in it, to make up the shape of it".
This becomes important later because much of chemsitry supposes that many of the characteristics of substances as observed in the lab. are emergent properties that results from enormous numbers of molecule-scale particles that themselves have quite different behaviour individually. Learners however may assume that the properties of the bulk materials are due to the particles having those properties - so students may suggest that, for example, that Some particles are softer than others or that In a sponge, the particles are spread out more, so it can absorb more water.
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Dr Keith S Taber kst24@cam.ac.uk
University of Cambridge Faculty of Education
© Keith S Taber, 2013