PRESENTING THE CLOUD IN A BOX
I framed my demo as a series of experiments leading up to cloud formations, in which the students learned a new requirement of the cloud-making process with every "failed" experiment. I began with an re-introduction to the different states of matter, then asked the students which one they believed composed a cloud. The students responded "gas", leading to the first demo. I asked them, given water and the box, how they thought we could make a cloud. With a review of evaporation, the first hypothesis was made: If we heat the water and it evaporates, a cloud will form. I boiled the water in the kettle and poured a small amount of it into the cloud box, then asked the students if they thought a cloud had formed. They all agreed that there was no cloud - no one was confused by the slight condensation on the walls. I then asked them what they thought might be missing, or why a cloud hadn't formed. When nothing came up, I asked them where clouds formed (high in the sky), and what happened there that could affect cloud formation. We discussed how the air gets colder at high altitudes, reviewed condensation, and finally concluded that clouds were not in a gaseous state, but rather, were composed of small drops of liquid! We came up with a second hypothesis: If we heat the water and it evaporates, then cool it so it condenses, a cloud will form. While the water boiled in the kettle, I placed a few ice cubes on the top of the cloud box. When I poured a small amount of water into it, the students again agreed that there was no cloud. I asked them again what might be missing - what else happens high in the sky - and then introduced them to pressure changes at different levels in the atmosphere. I explained that the higher up you go, the less atmosphere there is to push down on you, so clouds are formed at heights where there is less pressure. We came up with a third hypothesis: If we heat the water and it evaporates, then cool it so it condenses, and lower the pressure in the box, a cloud will form. I repeated the steps from the failed experiments above, then sealed the cloud box and reduced the pressure with the reverse-flow air pump. For a third time, the students did not see a cloud. At this point, I introduced the most difficult concept: surface tension. Since this concept is difficult to explain to even some university students, I did not try to justify it to the 4th graders scientifically; I simply told them that when drops of water are very small, their outsides try very hard to keep them small. I then explained that the last required component to cloud creation was a contaminate like dust, smoke, or very small particles of dirt. The tiny drops of water can stick to the contaminate and thus overcome surface tension by gathering all around it and making themselves bigger. The fourth and final hypothesis, then, was: If we heat the water and it evaporates, then introduce a contaminate (smoke), cool it so it condenses, and lower the pressure in the box, a cloud will form. Watch the video (link in top right) to see the steps I followed for this final portion of my demo. The students finally got to see a cloud form, and we reviewed the four necessary elements: evaporation, condensation, pressure change and an atmospheric contaminate.

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