The following vignette is reproduced with permission from Loughran, J., Milroy, P., Berry, A., Gunstone, R. and Mulhall, P. (2001), ‘Documenting science teachers' pedagogical content knowledge through PaP-eRs,’ Research in Science Education, Vol. 31, pp. 289-307.
The students were busy making models to show how they thought the particles were arranged in solids, liquids and gases. Prior to this class they had learned that the particles in a solid are packed closely together in a regular pattern, that in a liquid they are still close together but the arrangement is not regular and has spaces which enable the particles to slide over each other and that in a gas the particles are far apart with no pattern at all.
Hannah, one of the more able students raised her hand:
‘Ms Smith, you know how particles are further apart in a gas - what's in the gaps?'
Ms Smith had been waiting for a question like this. If it had not come from the students she would have raised it herself. It was only after she had been teaching for some years that she had begun to realise that the answer to this question was a difficult one for many students to imagine. Each time she taught the Particle Theory she found that even when students 'knew' from their lessons that nothing was actually between the spaces in a gas, they said or asked things which indicated they actually thought there was something in the gaps.
Ms Smith told the class, ‘Let's think about Hannah's question. Put on your magic glasses (these are used to imagine things that cannot normally be seen) and tell me what you see when you look at a gas?'
'Gas particles with big spaces between them!'
Ms Smith: 'What do you see when you look at the big spaces?'
'Lots of tiny air particles!'
As Ms Smith had often reflected, in many ways this kind of response was not surprising. If we poured all the water out of a jug, we would say there was nothing in it. Yet we would understand that it contained air. Thus in everyday life, 'nothing' was often a synonym for air.
Ms Smith then asked the class to predict what would happen to the weight of a balloon when it is blown up (many students hold the belief that air is ‘nothing’ which is a barrier to understanding the particulate nature of matter).
After the balloon was blown up and its increase in weight noted, a student raised her hand and said, 'So air particles actually have weight?'
Ms Smith: 'Yes, air particles have weight so air is actually something! Now let's think again about Hannah's question about what's in the gaps between gas particles. If you could look at the spaces between gas particles with your magic glasses, what would you see?’
One student called out, 'Other little air particles!
Hannah raised her hand: 'No I disagree. Air is something and is made up of particles so there can't be air between particles - if there were, you would never have a gas!'
Ms Smith:' Yes, so what would you see if you could look at the spaces between the particles in a gas?'
'Nothing!' a number of students chorused.