From: PERShadow@aol.com
Date: Tue Jan 14 2003 - 16:29:58 PST
From: PERShadow@aol.com Message-ID: <144.7d6a2e3.2b560586@aol.com> Date: Tue, 14 Jan 2003 19:29:58 EST Subject: CO2 and balloons
I've done similar experiments w/students using the baking soda and vinegar
reaction, but I give it as a problem for the students to solve, i.e., prove
the law of conservation of mass. I had tried doing it myself and found that
when I weighed ziplock baggies, etc., I always lost .01-.1 gram. What I
figured out (I could be wrong) is that yes, the CO2 is small enough to
diffuse out through the plastic of the baggie (or rubber of a balloon, or
plastic of a 2L bottle of soda, which is why they go flat so much easier than
glass bottles). It will do this to reach equilibrium with the atmospheric
CONCENTRATION (not mass) of CO2, which is something like 0.5% or less, while
inside the CO2 has greatly increased due to the reacting chemicals. After
futzing around w/it to try to get it to work, I thought, why work so hard
when there's slaves, I mean students, to do the work for me.
The way I do it is towards the beginning of the year, when going over the
metric system and the scientific method, I explain the law of conservation,
then demo it very sloppily with a baggie into which I toss some crudely
weighed vinegar and baking soda. When I weigh it afterwards, it of course
has lost some mass. After brainstorming w/the kids why (poor measurement,
some lost as gas when I was slow to close the baggie), I have them come up
w/ways to capture all of the gas. The next several days are spent with them
assembling and trying out their systems. Invariably, students wind up having
to repeat the experiment several times, which ensures they are all experts
w/a triple beam balance, etc., as well as learning that science is not as
clear cut as it appears to be in the textbooks and requires a lot of
technical ingenuity, not just "book smarts".
Pat Roisen
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