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Setting Up for Action
Fascinate your students with a breath-taking demonstration of chemical reaction and density by arranging a performance of the Incredible Dancing Raisins. To perform this feat of seeming legerdemain, all you need are a few simple household supplies. You’ll need a clear glass or other container with tall sides and a lid, white vinegar, baking soda, raisins, a mixing spoon and optional food coloring. Measuring utensils are nice, but not entirely necessary.
Mix equal parts of vinegar and baking soda in the clear container and add some food coloring if desired.If you wish, do the mixing of these ingredients in secret and put the lid on the jar. When you bring the container out for the group to see, it will simply look like colored water. Toss in a few raisins (with the lid off), and step back to watch the action. Within a few moments, the raisins will leave their expected home on the bottom of the jar and gently waft to the surface. After bobbing a few times, they will sink back to the bottom, then rise again and again.
You can keep the action going nearly indefinitely by adding a smidge more baking soda or vinegar when things seem to be slowing down. The raisins will appear to be rising and falling of their own accord, and most children will think that quite magical indeed.
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Turning a Simple Magic Trick into Science
Now that you have the group’s attention, get them into a scientific mood by encouraging healthy skepticism. Ask them to speculate about what could be causing the raisins to move around in such a way, since high energy mental waves and telekinesis are not very likely. Build understanding of the scientific method by helping them express their theories. Be prepared to help them try experiments to figure out what is happening.
For example, ask a group to try to duplicate the demonstration. Be sure they create their own materials list from observation of your demonstration. Less experienced scientists may well request a jar, water, coloring and raisins, so let them try their idea out. More observant students may notice the bubbles clinging to each dancing raisin, and may form different theories about how the demonstration was created. The important part of the speculation is that students form hypotheses and then develop ways to check their accuracy. This is the core of true science, and will build skills that they need later in more formal science classes as well as in many technical and scientific careers.
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The Science Behind the Magic
There are a number of important science concepts that can be demonstrated with this eye-catching trick. For students studying chemistry, the gas-releasing interplay between the vinegar and baking soda is a perfect way to make the abstract concept of chemical reaction come to life. In this case, the baking soda (NaHCO3) is mixed with the vinegar (CH3COOH). Chemical bonds are broken and the atoms regroup themselves into carbon dioxide (CO2), water (H2O) and sodium acetate (CH3COONa). The carbon dioxide forms the bubbles that cause the raisins to float.
Another concept that can be pulled from this experiment is that of density. Raisins by themselves are denser than water and so they sink. When the CO2 bubbles attach themselves to the raisins, they add quite a bit of surface area but little density, and so the raisins with bubbles will float to the top of the jar. As they reach the top, though, the bubbles pop and the raisins are once again dense enough to sink.
The chemical reaction between the vinegar and the soda will eventually slow as all of the chemicals are combined. You can add more of one or the other to keep it going. It’s like adding fuel to a fire-when there are reactive chemicals available, the chemical reaction will resume.
Other experiments related to this demonstration include discovering the optimum amount of vinegar and soda to sustain the longest raisin/bubble action, trying the experiment with and without an airtight lid, using other small items besides raisins to see which can float under these circumstances (spaghetti and macaroni can work, for example), and changing the temperature of the vinegar to see how that affects the reaction.