Unfortunate Son

Just thought I’d share this since it made me realize how easy it is to do inquiry and how many teachers aren’t doing it.

My apologies to anyone directly involved in this that I may offend if they happen to read this.

I’ve mentioned before that I get emails from the NSTA physics and chemistry listserv. Here is an email snippet from an in-service teacher we got today:

I wanted to do a conservation of mass lab with my students.  Thought that vinegar and baking soda would do fine.  Mixed them in a flask on the balance and saw a difference of .4g with about 5 g of baking soda and 15 g of vinegar.  I thought that was fine since carbon dioxide was forming.  I did the exact same thing again except cover the flask with the balloon with the baking soda inside so it never let the gas out.  The balloon expanded but I saw that same change in mass again.  I checked the scale to make sure it didn’t waver and it didn’t.

Any ideas on what happened?  Do you have a conservation of mass lab that works where you can show it needs to be a closed system?

I know I replied to him privately before any list members replied to the group. I dislike all the inane comments that follow most of these questions and their responses, so I wanted to privately address his question.

I asked him a few questions rather than give him an answer (slightly edited here to make me look smarter than I am):

  1. How do you know it didn’t work the way it was “supposed” to? (I actually told him it did work, then asked my questions and now I realize I should have asked him in the form of a question)
  2. Does the gas in the balloon have mass?
  3. What is the gas in the balloon?
  4. Why does the balloon “float” as the balloon fills with gas?
  5. You’ve obviously collected a gas in the balloon, why do you think that its mass isn’t counting towards the mass you are reading?
  6. I also had to ask a clarifying question since he used both balance and scale in his question – they measure 2 different things and if I could elicit anything towards what I knew was the reason for the discrepancy, maybe the use of scale might seem a little more intuitive to him once he made a realization.

I was disappointed to see “answers” (those are double finger quotes since almost none of them were answers) to his question quickly start rolling in.

The first responder actually steamrolled the answer past him: here is a modification that you might use that minimizes gas leakage and allows you to ignore air buoyancy because the container is a constant volume…gave instructions for modification.

Next: I have done this experiment in the zipper baggie for years, and we rarely get the same mass. Remember, if you put an onion into a baggie you can often smell it without opening the bag. Plastic is permeable.

Third responder gave him the answer: This is a very common problem and it is due to buoyancy. And then went on to do a very good job explaining it.

Fourth told him to do almost exactly what he had already done: The two most popular methods often are placing a balloon on the flask and trying to collect the gas in the balloon. The other method is placing the whole experimental apparatus into a plastic zip top bag. Both methods have at times produced excellent results, and other times these methods have yielded very little if any improvement. It all seems to depend on how the bag or balloon is sealed to the glassware…It all has to do with procedure…and the care that the kids take to implement their procedure.

And on they went – 13 responses in 6 hours with only one correct answer (I don’t count the first since he glossed over it like that part wasn’t important), and they were all things to improve the experiment or to “fix” the problem.

Many of them even made it sound like the CO2 was escaping rather than air being displaced, as did the original poster (density of air is less and therefore gets pushed out).

How unfortunate that they all passed up on a teachable moment for the teacher becoming a learnable moment he could pass on to his students; instead, they gave him “easy outs.”

My questions to him probably weren’t the best ones to elicit buoyant force and changing volume, but they were at least questions that might make him think. So, if any of you have any better questions to add, I’ll probably be using this in either chemistry or physics.

It’s better to give than receive,” but I’m taking literary license and am going to go with, “It’s better to question than receive.”

Edit: I should also point out that simply asking questions isn’t inquiry (at least not in the way I meant it to be applied). The questions are meant to prompt the student to want to try and figure things out by doing different investigations. What could he have done to figure out why the mass seemed to have “left” the system?

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