Quick recap from the first post of this series: I start the year with some underpinnings (scientific process skills that are necessary to survive in a Modeling classroom) activities. It is there that we establish how to build a scientific model.

Continuing my series on model building, let’s talk about Democritus.

Democritus’s Atomic Theory is the foundation for all of chemistry and is incredibly relevant today. This is where the chemistry Modeling Instruction curriculum starts. Democritus made the *observation* that if you break a rock into tiny pieces, those pieces are still made of rock. He then *inferred * that if you broke that rock into tiny particles so small we can’t see them, they would still have the same rock composition. Therefore, all matter must be made of teeny tiny indestructible particles that Democritus called atoms (I don’t use the word atom until we get to Dalton to avoid confusion about compounds). The first model of the atom was born! There are some other parts to Democritus’s model like the properties of atoms are determined by the shape of the atom but I don’t address that.

I start all of chemistry with the above story about Democritus and tell my students that this is our current model of the atom because we do not have any other evidence to tell us otherwise. Then I do the exploding can demonstration because chemistry is all about blowing things up, right?

**Exploding Can Demo**

The exploding can demonstration helps establish the practice of drawing particle diagrams. Students are asked to draw a particle diagram before the can is lit, while the can it lit, and when the can explodes. They come up with all sorts of explanations with their particle diagrams. Sometimes they are dead on, sometimes not. The right answer is not as important as the discussion of particles.

From the exploding can demonstration you can generate some particle diagram rules. The 4 I always have them come up with are:

- Particles are represented as circles, not dots
- Different particles should look different
- Include a key so we know which particles are which
- You don’t need more than 20 particles in your diagram

I always get at least one group that tries to represent particle motion with whooshies or arrows. When I see this I ask, “why are there lines coming off your particles?”, to which students usually reply, “because it’s a gas and gas particles are always moving.” I then ask, “do you have any evidence that particles move?”, to which students usually reply, “yeah, my 9th grade science teacher told me they do!” I followed that up with, “but how do *you* know?” It sometimes takes a few more questions to convince the students that particle motion is not currently in our model but we may add it later if we have evidence to support it. I do not tell students how the exploding can works here because they do not have the background knowledge to fully appreciate the chemistry. Instead I bring it back on the last day of class and have the students try to explain it again with their more robust model of the atom.

The discussion of particles and particle diagrams leads us straight into the “Mass and Change Lab.”

**Mass and Change Lab**

The “Mass and Change Lab” is a fairly standard conservation of mass lab. I have edited the lab so it is not exactly the same as what is in the Modeling Instruction materials but it includes a variety of chemical and physical changes that gain, lose and keep the same mass (depending on how you define the system). I have students use triple-beam balances during this lab to continue to reinforce the concept of significant figures.

After every group has collected their data, I have the class compile their data on the main board in the class. Each group writes whether the experiment gained or lost mass and if so, how much? The data will not be perfect. You can usually spot which groups forgot to account for the mass of a test tube or beaker and use it as an opportunity to talk about sources of error. Once we have established the mass change for each experiment, we whiteboard a before and after particle diagram for each mini experiment.

During this whiteboard session I ask students, “how are you going to show if the mass changed or stayed the same?” This is where students make the connection that the number of particles represents the mass. If the system gained mass, it must have gained particles. If the system lost mass, it must have lost particles. Students can then answer the questions, “where did the extra particles come from?” or “where did the particles go?” These questions can lead to a discussion on “what is a system?” and “what are open and closed systems?” After we have established particle diagrams for each mini-experiment, I ask students to come up with a definition for the Law of Conservation of Mass. The class usually comes up with something like “the total number of particles stays the same in a closed system.”

Now that we have established that the number of particles represents the mass, we can move on to density.

**Mass and Volume Lab**

I introduce the concept of density with a set of density balls I got from Education Innovations.

The two balls have the same mass but the smaller one *feels* heavier than the larger one. I ask students to account for this observation by drawing particle diagrams of both balls. I do not give this explanation the name “density” yet. We simply discuss it in terms of “the mass to volume ratio.”

Next we do the density lab. I have a few sets of aluminum cylinders and PVC cylinders of various sizes that I use for this lab. Any standard density lab kit would work. I ask students to find the relationship between mass and volume for the aluminum pieces and the PVC pieces. At this point in the year the students are well versed in finding relationships so I set the students loose to collect and graph their data. They come back with completed whiteboards and a lot to discuss.

Students quickly see that their data split into two lines so they have to calculate two slopes and write two statements of relationship. On the boards pictured above, you will notice that I have my students additionally draw in the line for water so we can determine if the pieces will sink or float (steeper slope than water will sink, a shallower slope than water will float). I also have the students represent both substances with particle diagrams so they have a quantitative and qualitative representation of density. I ask many questions throughout the board meeting like, “what would be more massive, 20 mL of aluminum or 20 mL of PVC?” Or the converse, “what would take up more space, 50 g of aluminum or 50 g of PVC?” At the end of the whiteboard discussion, we establish that the slope is the mass to volume ratio which we call “density.”

I follow up this lab with some worksheets on density adapted from the Modeling Instruction materials with qualitative (particle diagram) and quantitative (graphing and proportional reasoning) density questions.

I also give students a density practicum based off of Flinn’s “Don’t Sink the Boat” activity.

Once students are comfortable with the densities of liquids and solids, we can determine the density of a gas.

**Density of a Gas Lab**

The “Density of a Gas Lab” is a standard collection of gas by water displacement (see Flinn’s “Scientific Laboratory Techniques Guide” for a good diagram). The gas is CO2 generated by Alka-Seltzer and water. Outlining the procedure for this lab can be a little cumbersome but my students always get great data (though there are always a few groups that need a few tries to get there).

After students have collected their mass and volume measurements of the gas they collected and calculated the density, I have them record their data on the whiteboard in the front of the room. Immediately students notice that the density of a gas is a really small number. I have students put that number in scientific notation and compare it to the densities (in scientific notation) of liquids and solids we know of. This allows us to discuss the term “order of magnitude”. I ask students “how many orders of magnitude greater is the density of water compared to the density of carbon dioxide?” Students can easily determine water is three orders of magnitude denser. What students don’t realize is that means water is *1000 times **denser* than carbon dioxide! That usually catches them off guard so I ask them to represent the average densities of solids, liquids and gases in 3 particle diagrams.

Students either overthink it and want their particles diagrams to be exactly quantitatively correct or they underthink it and just draw each diagram with an arbitrarily smaller number of particles. Each group presents the reasoning behind their boards and we compare each board to the actual data. After a few comparisons, students realize that to truly represent the density of a gas, they would have to draw a fraction of a particle. Since fractions of particles do not fit our model, they settle for drawing one particle in the gas particle diagram. This representation is not congruent with many textbook particle diagrams and is a big misconception among students.

We have now learned all sorts of things about how the number and arrangement of particles affects properties of matter but we still have one burning question; how tiny are these tiny particles?

**Thickness of a Thin Layer Activity**

I wrap up the first chemistry unit with the “Thickness of a Thin Layer” activity from the Modeling Instruction materials. In this activity, students must determine the thickness of a piece of regular foil and the thickness of a piece of heavy duty foil using what they know about the density of aluminum (calculated in density lab).

From this activity, students can determine a minimum particle size if the aluminum foil is 1 particle thick (the heavy duty foil is about 1.5 times thicker than the regular foil, so the minimum particle size is 1/3 the thickness of the heavy duty foil). I then show students a clip from “The Ring of Truth” about particle size. The examples in the clip get the minimum particle size down even smaller. You could also drop a known volume of oleic acid in a large bowl of water, calculate the area of the circle it forms and then calculate the thickness of the layer to get a smaller minimum particle size.

I wrap up the discussion by showing students the “Scale of the Universe” applet. This site does a great job of putting the size of a particle into perspective for the students (as well as the size of the universe). Make sure to show it with the sound turned up, the music is awesome!

That is the end of the first chemistry unit! To sum up the model so far…

- All matter is made of tiny, indestructible, hard sphere particles
- The number and size of the particles determines the mass of the substance
- The number of particles in a closed system does not change
- The number of particles in a certain amount of space determines the density of the substance
- Particles are really small; on the the order of 10^-9 m or 1 nanometer.