MU Chemistry Modeling Workshop: Day 2

Here’s more about my modeling chemistry workshop experience.

Day 2


After a brief discussion of Alberts’ article, we explored the modeling approach to measurement and (yes) significant figures. In student mode, we completed an activity where each group determines the area of a specific table. The catch is that we only have glugometers at our disposal (which happen to have no markings at all. So rather than reporting the area in traditional units, we report them in glugs2.  We had a discussion about what sorts of estimations we could make based on the device we used.  Next, we calibrated the glugometers using lined paper, and repeated the measurement of the table. Though we could report our areas to more digits, for whatever reason, our area values were even more wildly inconsistent from group to group. Rough morning?

The main goal of the discussion was that any calculated value couldn’t be any more precise than the least precise measurement. That is the extent of (our instructor’s) approach to significant figures. I can live with that! No rules to memorize, and more/less enough to get by without being bogged down.

Mass & Volume Lab

Now that we’d looked at mass, and at volume, it was time to see what the relationship might be between mass and volume. In student mode, we determined the mass and volume (by water displacement) of a two sets of cylinders of varying size. We plotted class data, and each group whiteboarded the equation of the lines of best fit.

Since everyone used the same data to create the graph, each equation should be the same. Our instructor asked us to look around and see if there were any differences. Based on the Volume Lab from yesterday, most substituted in the variables used in the y=mx+b equation. The main difference between the boards were significant figures and whether units were included with the variables. We observed that the slopes were identical (except for sigfigs), and that there shouldn’t be any uncertainty differences, since we were all using the same data.

The discussion started with the instructor asking what the y-intercept is, and what it means in the case of our graphs. Should it have a definitely value in the context of this lab? If so, what? If not, why not? Next we looked at the slope, and discussed what it means, in terms of variables. Rather than using the terminology “mass per volume,” she suggests using “for every,” since students rarely understand what “per” actually means. So density is how much stuff you have in a given space. When comparing the density of Substance A to Substance B, the higher slope of Substance A suggests that there is more substance A stuff (grams) than Substance B stuff in the same amount of space.

Next we “completed” worksheets 3 & 4, which are applied density problems. Rather than drill and kill, these problems are well designed to determine whether students conceptually understand the concept of density. These worksheets would typically be assigned for homework (separately), and each problem whiteboarded by a different group in class. Each group stands, summarizes their problem, and presents their solution. The resulting discussion reinforces the concept, clears up any lingering misconceptions, and reinforces use of a particulate model to describe density.

Density of a Gas Lab

Next we revisted the Alka-Seltzer experiment from the Mass & Change Lab and designed an experiment to determine the density of “Alka-Seltzer Gas.” The gas is collected via water displacement into an inverted Erlenmeyer flask in a half-filled water trough.

The results were all over the place; I think we dipped a little too far into “student mode,” with some of our lab techniques, but for the most part, we all observed that at the very least, the density of this gas was 2-3 orders of magnitude less dense than the solids in the Mass and Volume Lab, and any liquids referenced in WS 3 & 4. Using this, we can compare particulate diagrams of the three phases of matter based on how much “stuff” is in a given “space.”

I like the activity, but there are too many moving parts to make it work in the limited time available in my classes, so I may use butane or compressed air to do this instead of a chemical reaction.

Thickness of a Thin Layer

Next, we did a lab I’ve done many times before, where students determine the thickness of a sheet of aluminum foil. What was different here was that we looked at normal and “heavy duty” foil to determine if there was a difference. Once we had determined the values, we discussed what that meant, and what it might look like at the particulate level. Students tend to greatly underestimate or overestimate the size of atoms, so the instructor provides the accepted diameter of an aluminum atom, and the students determine how many atoms thick the foil actually is.

HW: Reflect on teacher’s guide to unit 1

At this point, my main concerns with the curriculum (at least in the context of Unit 1) were:

  • Pacing. The instructor of this course teaches primarily honors students, but she spends around 4 weeks on this introductory unit. I can’t imagine spending anywhere near as much time, given that we have substantially less time in class and in school. I think that my student’s prior experience with physics will  make conceptualizing graphs and equations a little easier.
  • Math level: All of my introductory students are enrolled in pre-calculus. It will be interesting to see whether they struggle with the graphical interpretations.
  • The exploding coffee can demo was excluded from the workshop, but doesn’t seem to serve any pedagogical purposes that contributes to the model. What interesting phenomena can we use that does get students thinking about the particulate nature of matter?

 But the main advantage to this modeling approach, particularly in the first unit, is that you’re covering the bulk of the “dry material” (observations, sig figs, measurement, density, volume, mass, physical change, chemical change, etc) in a real context, that will be built upon for the rest of the year. And you never have to open a textbook!

6 thoughts on “MU Chemistry Modeling Workshop: Day 2”

  1. This is great. I’d read a bit about modeling chem, but had no idea how it all fit together. I’ll second the voice of how helpful “for every” is over per for having students understand slope units. It’s so powerful that I took the idea to my middle school math department and tried to convince them to make the switch. The other hugely helpful think is insisting on horizontal fraction bars.

  2. The pacing was a big deal for me as well. I couldn’t see how we could possibly spend this much time going over Unit 1. If your kids are comfortable with the math part, you should be able to shorten that a little bit. My kids were NOT and truly struggled with transferring their knowledge of graphs to something that actually had meaning.

    I did the coffee can demo as a first day activity and it was a great introduction to the whiteboards. We talked about particles and what we thought they were doing. We also covered whiteboard conventions and what would be expected of each student during those whiteboarding sessions.

  3. Everything sounded great until you got to “And you never have to open a textbook!” So the students spend four weeks and still haven’t learned how to read technical material? What is the aversion to student reading that so many teachers seem to have?

    I’d like to recommend an excellent series of blog posts on Mylène’s blog shifting phases that talk about how to get engineering technology college students to learn to read technical material—something that they apparently had never been exposed to in K–12 education.

    1. I wouldn’t call it an aversion to student reading, its just that the curriculum isn’t compatible with the way most (all?) chemistry textbooks are written. There’s no rule that modelers can’t use textbooks (many, many do), and I will certainly use it when I can , but for this unit in particular, the corresponding textbook chapters are particularly bad. For instance, the textbook we’ve used for this course describes density in purely mathematical terms (“density is mass divided by volume”), which may be useful for concept review, but not while developing the concept in class.

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