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 gave me examples of things he considered to be solids, liquids or gases. I asked him about clouds, because students commonly consider them gases:
So do you think everything, is either a solid, or a liquid or a gas?
I’m not sure? Erm, I think that, some, I think that they are mainly, fall into a group, but I’m not sure.
Not sure about that, okay. Erm, what about a cloud? You look at a nice sky, and there’s a cloud? Do you think that’s a solid, or a liquid or a gas?
I think that that’s a gas that you can see, because it is made up of, I think it is made up of different gases, I’m not sure, though.
Gases are transparent and so generally not visible. A cloud is opaque, and is made up of many tiny droplets of liquid (water in the case clouds in the earth's atmosphere) that have been formed by condensation. However, because they remain in the air (until it rains!), it is understandable that students may think they are gases themselves. Liquids are much more dense than gases, so it is not immediately obvious to students how a cloud of liquid drops can remain 'floating' in the air. That Bill offered a tentative answer and was not strongly committed to the idea of clouds being gaseous suggests he was open to revising this view given new evidence to consider.
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Dr Keith S Taber kst24@cam.ac.uk
University of Cambridge Faculty of Education
© Keith S Taber, 2013