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Using BB's to demonstrate Molecular Structure

Overhead Demonstrations 

This overhead demonstration uses BBs and petri dishes to show the difference among solids, liquids, and gases. Prepare and seal three plastic petri dishes. The first has almost enough BBs to cover the entire surface; the second has about one-third as many BBs; and the last one has only a few BBs. These three dishes can be used to show many of the microscopic differences among gases, liquids, and solids.

Put the first dish (with only a few BBs) on the overhead and gently shake it back and forth. The BBs move randomly over the enclosed area, rather like gas molecules. Now try the liquid model (about 1/3 full of BBs). If your overhead is on a slant, which most are, the BBs will accumulate on the lower edge. When you agitate this dish, only a few of the BBs (atoms) will be bounced loose from the general mass. That is, evaporation occurs. If you look at the bulk of the BBs, you can usually point out small sets of BBs in orderly arrays to compare with other BBs that look totally disorganized. Finally, when you get to the solid, the same amount of agitation doesn't produce much obvious change, but the individual BBs are still moving slightly. I like this because it helps to counteract the vision that particles in solids are rigid and unmoving. Usually, you can point out various packing defects in the BBs - ranging from glide planes to lattice vacancies.

I know that this is a pretty simple-minded way to show the differences, and there are some differences that aren't clearly shown in this way, but it's easy and it does vastly improve the students understanding of what goes on in the various states of matter. It's also easy to do, even in classrooms with not scientitic facilities.

I'll only add that this is one of many useful tips I've picked up from attending the Woodrow Wilson Institutes held on my campus.

                | Harry E. Pence           BITNET: PENCEHE@SNYONEVA |
                | Professor of Chemistry   PHONE:  607-436-3179     |
                | SUNY Oneonta             OFFICE: 607-436-3193     |
                | Oneonta, NY 13820        FAX:    607-436-2107     |
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