Here is a quick recap of the model so far from the previous post in this model building series:
- 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.
Unit 1 was all about building a functional model of a particle. Unit 2 is the first time students will make an observation that doesn’t fit their model, and they will have to amend the model. Let’s jump right in!
I start Unit 2 with the diffusion demonstrations outlined the Modeling Instruction materials. First, I place something with a strong scent on a watchglass, and place that on a triple beam balance. I have used different extracts, fresh-squeezed orange juice and essential oils for this. They have all worked about equally well for me. I ask students to raise their hands when they smell the scent. Students observe that the smell travels around the room randomly. I then ask students to whiteboard what they think is happening. They come up with many different explanations but the conclusion we come to is that the particles must be moving! We have added something to our model!
We refine this addition to the model by observing what happens when you drop food coloring in warm and hot water.
From this observation, students infer that particles move faster in hot water and slower in cold water. Therefore, temperature is a measure of the average speed (kinetic energy) of the particles.
To end this demo day, I show students a thermal expansion demonstration.
Students observe two liquids (water and isopropyl alcohol) rise up capillary tubes as they are submerged in a hot water bath. The yellow liquid (alcohol), appears to rise more quickly than the red liquid (water). Students immediately respond that materials expand when they are heated but it takes a little more probing to get them to explain what is happening at the particle level. This allows us to talk about how a thermometer works. I also talk about Celsius and Fahrenheit here and where those scales come from.
Now that we know that particles move, we can ask some interesting questions, like, “how does a straw work?”
“How Does a Straw Work?” Discussion
For this discussion, I tell all my students to bring a drink to class with them. I then give every student 2 straws. The first questions is simple, take a drink with your straw and tell me how it works.
I get all sorts of answers to this question and hear lots of sciencey words like “pressure, vacuum” and my favorite “sucks.” It always becomes very clear that students actually have no idea how a straw works. The first step is to get them to see that nothing is being “pulled.” That means, the liquid is being pushed up the straw. But by what? Enter pressure. I also have students try to drink out of their straws with one on the inside of their drink and one on the outside. This helps cement the idea that you remove particles from your mouth to create a partial vacuum when you use a straw. Then the air on the outside of your drink is able to push your drink up the draw. I end class that day by asking the question, “is there a maximum straw length?” and showing the Veritasium video, “World’s Longest Vertical Straw.”
Now that we have defined pressure, we can figure out what affects it.
I have moved to completely separating each gas law and then combining them in the end as opposed to deriving all the gas laws during one lab. I start the discussion by blowing up a balloon and asking, “what could I do to change the pressure in this balloon?” Students are able to come up with, “change the volume, change the number of particles, and change the temperature.” From there we explore each of the gas laws, starting with Boyle’s Law.
To explore Boyle’s Law, students use Vernier Lab Pros connected to TI-83 plus calculators to graph the relationship between pressure and volume. Students push in and pull out the plunger on a syringe connected to a gas pressure sensor to collect 8 data points.
Students whiteboard their data and we are able to determine that pressure and volume are inversely proportional by the shape of the graph. I no longer have students linearize this graph because I do not think it adds enough to the discussion to be worth the time it takes to teach them how to linearize.
For each gas law, I have students draw 3 particle models to show what is changing and how that change affects the pressure. After discussing the relationship between pressure and volume, students work on just pressure-volume problems with initial-final-effect tables.
For Gay-Lussac’s Law, students again derive the relationship by collecting data with Vernier LabPros. This is an interesting whiteboard meeting because students collect their temperature data in Celsius and so their y-intercepts are far from zero. I ask students, “what would a y-intercept of zero mean?” Students are able to reply that it would mean the particles are no longer moving which would be absolute zero. Usually one student in the class remembers something about Kelvin at this point and what temperature absolute zero is. We then have a quick discussion about why scientists use Kelvin and the need for a temperature scale without negatives. With all of this in mind, students then work on just pressure-temperature problems.
I cut the Avogadro’s Law lab this year to avoid confusion with terms like “puffs” because students do not know how to count particles yet. Instead we talked about Avogadro’s Law and Charle’s Law and how they make sense at the particle level. Avogadro’s Law will come back at the end of the year when we get into PV=nRT.
This year, I added a day of gas laws review day before jumping into combined gas law problems. For the review, students whiteboarded a graph with particle models and a statement of relationship for each gas law. Students also put this information on a summary sheet for themselves. From there, we were able to cleanly transition into combined gas law problems with initial-final-effect tables.
That is the last major topic in Unit 2! All that’s left is a challenge problem and a practicum!
To wrap up the gas laws, I give students a challenging ranking task that a colleague of mine created. It challenges students to use their conceptual understanding of the gas laws instead of relying on their math skills. The ranking task gives students various scenarios like, “the volume is doubled and the temperature is halved”, and “the volume is tripled and the temperature is tripled.” The students must determine what effect the scenarios would have on the pressure and rank them from least to greatest final pressure.
For my Unit 2 practicum, I use the term “practicum” loosely. For the practicum, students are not challenged to solve a problem using their model but rather explain various demonstrations using their model.
I have students explain a lung model,
a marshmallow expanding in a syringe,
an water balloon being pushed into the mouth of a flask,
and the always fantastic “Crush the Can” demo using the gas laws. The students always find these demonstrations are not as simple to explain at the particle level as they first appear.
To sum up what we added to our model so far…
- Particles are always in motion
- Temperature is a measure of the average speed of the particles
- Pressure is a measure of the number of particles colliding with a surface
- Pressure and volume are inversely proportional (Boyle’s Law)
- Pressure and temperature in Kelvin are directly proportional (Gay-Lussac’s Law)
- Pressure and number of particles are directly proportional (Avogadro’s Law)
- Volume and Temperature in Kelvin are directly proportional (Charle’s Law)
Whew! That is a hefty unit! Look out for some more additions to the model concerning energy in the next post of this series!