Density and Buoyancy Demonstration

I modified and ultimately simplified a discrepant event demonstration that Dr. Thomas Scarlett III introduced me to at the Pacific Education Conference 2011 (PEC) here on Pohnpei. Dr. Scarlett was presenting as part of a Pacific Education And Research for Leadership in Science (PEARLS) that is based out of the Hawai'i/Pacific Basin Area Health Education Center (AHEC).

Dr. Scarlett used three vials with liquids at different levels and a two fluids at different densities. I initially played around with this option, substituting clear virgin coconut oil for the second liquid. I put the graduated cylinder in the prep room and the coconut oil solidified. The unexpected complication was that the oil did not become clear upon melting - the oil remained milky.

I did decide to use fluids of equal amounts in each vial. While I fascinated by the three different levels, I decided to make the discrepant event the floating and sinking of two vials with fluids at a common level. This reduced the variables in play.

Although I did initially demo the three vials in the two fluids, I realized that there were still too many variables. For the students it was magic - nothing too surprising or mysterious. I shifted to water and the difference in performance suddenly became more puzzling for the students.

Buoyancy and density discrepant event experiment
Note that the two floaters are not actually identical and that they did hover at different levels in the problematic dual liquid cylinder on the right. Using a fluid less dense than water would allow one to use more fluid in the vial, which would make the denser than water fluid vial more reliably sink. In the above the buoyancy is very sensitive to the fluid levels in the vials. The floaters and sinkers are only just barely separated in density. A bit more fluid and both sink, a little less and both float.

If I do repeat this next term, two or more vials and a large beaker of just water is recommended. The third vial may be useful. I ask for a prediction up front. Then after the first vial is in the water, the students often predict the second vial will behave the same. If it does not, then the students are left in a quandary as to what to predict for the third vial.

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