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
Shopping Bag or
Read more about key criteria of design tasks by visiting DITC's
Chooser Chart page.