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MOVIE 1:   8 minutes 45 seconds

Modeling the Chemical Reaction

The last segment of work introduces students to a paper-based model of the reaction that they have been studying, which will help them predict the optimal ratio of ingredients and which chemical would be left over from the reaction. In MOVIE 1, Nancy introduces this model that uses differently sized pieces of oaktag paper to represent the acetic acid and sodium bicarbonate molecules, and a paper clip to represent carbon dioxide. Students use a scale to choose the ratio of ingredients (paper and clips) whose total weigh is 50 grams. They then do a molecule-for-molecule pairing up of paper slips that involves removing the paper clip from the large molecule and attaching it to the small one. This represents a chemical reaction that releases the carbon dioxide.

The model predicts two things. The first is the volume of gas produced, which students see as they lay out the small gas molecule along a meter stick. The length along the stick corresponds to the height that the bubbles would travel up the clear plastic column (from the previous day's investigation). Residue (and aftertaste) is represented as unused molecules in the model. Students plot predicted results on the same chart where previously they placed observations their experimental observations.

One question about the Baking Soda activity is whether, in the end, it can be considered a design task at all. One trademark feature of What Makes for a Good Design Challenge, is that there are many viable solutions -- they do not convergent and there is no one single right answer to the challenge. Such is not the case with Baking Soda. The chemistry of the reaction is fixed, with a "correct" ratio that utilizes all of the chemicals with zero residue and results in maximum gas production. With this task, students from one year to the next could "bequeath" the solution to their younger counterparts. This is much less true with challenges like Cardboard Chair, Shopping Bag or Pop-Up Book. Read more about key criteria of design tasks by visiting DITC's Chooser Chart page.

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