Rediscovery

As a teacher, my gold standard for a good lesson, was one where students replicated the process of discovery. I did this accidentally once and realized it was the best part of the lesson. I was trying to teach conservation of mass. Your Chemistry textbook probably said verbatim, “mass is neither created nor destroyed, it can only change forms”. Set aside the fact that this is false for a minute. In normal laboratory conditions, it’s close enough. So, here I am, undergraduate science education student teacher, thinking I’m very clever. I’m not going to use someone else’s experiment to demonstrate this, I’ll just make up my own, and I don’t need to test it before the class because I know exactly what will happen.

1. Put baking soda in balloons and vinegar in water bottles, stick ‘em together with the balloon flopping over the side.
2. Measure the weight (or mass if you’re a pedant, or actually weight if you’re a true pedant because that’s what digital scales actually measure on Earth)
3. Hold balloon up and jiggle the baking soda into the bottle where it can mix with the vinegar.
4. Watch as the mixture effervesces, generates a gas, and the balloon inflates.
5. Measure the mass again.
6. Notice how the mass is the same even though the liquid and solid components converted into gas form.

Great experiment, right? Thanks. I’m feeling very proud of myself coming up with an experiment I can easily get materials for that should clearly and unambiguously provide clear evidence for this scientific law. I’m sure these eleventh graders will love it. I’m sure they’ll all understand conservation of mass much better now. So memorable and tactile! The students approach the scale at the front of the room taking their comparison measurements. The first group comes over to me with their lab notebooks.

“Mr. Flavin, we measured less weight the second time than the first time.”

“Interesting, let’s think about causes of error. Maybe you didn’t tare the scale the same way each time. Maybe there was some air pressure difference. Maybe the scale has some systematic error.”

Next group

“Mr. Flavin, we measured less weight the second time than the first time.”

This kept happening. The weights were similar, but I know systematic error when I see it. Every group measured less weight the second time. I was bewildered (in science education classes, we called this Piagetian disequilibrium), I was thrown off-balance. What happened? Why had this experiment, which I thought I had remembered doing before, turned out differently than I expected. Mass was conserved right? Maybe some exothermic energy resulted from the reaction and escaped from our not entirely closed system. But the classroom hadn’t exploded, so surely entire grams of mass had not been converted into energy. Maybe balloons leak much faster than I imagined and noticeable amounts of air had leaked through the balloons or the Scotch tape attaching them to the bottles. Then it hit me. I knew what happened. I went to my room, did some simple back of the napkin math, and showed up to class the next day ready to go.

“Yesterday, most of you measured less weight after the reaction than before, right?”

“Sure did Mr. Flavin.”

“Why do you think that happened?”

“Air escaped the balloon.”

“You said yesterday that we measured it wrong.”

“Maybe gases weigh less than liquids and solids.”

“Who can remind us about Lavoisier’s experiments in conservation?”

“He made closed systems and caused reactions in them to show that the mass was the same before and after.”

“Good. How were our experiments similar to Lavoisier’s?”

“We had closed systems.”

“And how were they different?”

“He used glass and we used water bottles and balloons.”

“Great, do you think that made a difference?”

“No, they’re both closed systems so it doesn’t matter?”

“Okay, but how are balloons and glass containers different?”

“Balloons can get bigger and smaller. Their volume can change.”

“Okay, great, when you keep mass the same and change volume, what else changes?”

“Density.”

You get the idea. The balloons inflated, so the water bottles took up more space. They displaced more air and the buoyant force acting on the system increased, lifting the balloon up. Imagine an uninflated balloon underwater. It might sink, but if you fill it with air, it will rush to the surface of the water. The same thing happens in air. The mass is the same, but the density decreases, the volume increases, more air is displaced, the buoyant force increases. The difference in weight measured by the scale is the weight of the air that the balloon had to push out of the way to inflate.

—true mass (constant) —scale reading —buoyant force (displaced air) —

The reason I’m telling this story is that at least for me, and hopefully for my students, it demonstrated the process of “rediscovery” which is the closest thing I’ve experienced to true discovery. It’s always bothered me that science textbooks tell the stories of scientists making discoveries. And then they tell you facts.

Lavoisier knew something was up. The theories of his day seemed off. Everyone says fire completely destroys stuff. Once it burns up, it doesn’t exist anymore. But that can’t be right. Something must be left over. If I can pour my exorbitant wealth into bizarre devices, I bet I can get just the right circumstances to prove that fire just turns stuff into other stuff.

And then your textbook says, “Trust us. We write textbooks. Stuff turns into other stuff. Matter is neither created nor destroyed. This is true at all times in all places. Don’t believe us? Doesn’t matter. This will be on the test.”

It’s the exact opposite of the process of discovery. What makes you a good scientist: generating hypotheses, designing experiments, examining evidence, updating hypotheses; is completely different than what makes you a good student in most science classes: believe your teacher, memorize facts, recite facts, explain away anomalous data in your lab notes as “measurement error”. My poorly designed lab experiment was much closer to the act of discovery. We didn’t demonstrate conservation of mass, but we inadvertently rediscovered the buoyant force.

I always loved the TV show Mythbusters. Their whole shtick was to take some idea that people accepted as fact and run a real, honest to goodness experiment. Were the experiments perfect or peer-reviewed or unimpeachable? No. Did they show you what happened if you tried stuff in the real world? Absolutely. Did you get to see lots of real-life explosions and car crashes? Also absolutely. Whether the myth was busted or not, you were rediscovering the evidence for or against it in the real world.

I don’t have anything against textbook knowledge. I trust most of the people who write textbooks most of the time and you’ll understand the world better if you read them than if you don’t read them. But I’ve always preferred a slightly more hands-on approach to understanding the world.

I wish I was planning on blowing up some cars for this blog, but the Discovery Channel isn’t paying the bills. Also, my career has led me to drift away from physical science and towards the more ethereal data science. But I do hope this blog will be an excuse for me to engage in some “rediscovery” and some “exploration of divergent phenomena”. And I hope that if you join me, you’ll get to experience some of the joy of rediscovery as well.