Units 3 & 4

Unit 3: Energy and Moving Particles (duration: ~2 weeks, or 8 class meetings)

Since the first bit of the traditional “Unit 2” was covered in the previous unit, this unit starts with pressure. Otherwise, there was no real modifications to the normal sequence.

Unit 4: Describing Substances (duration: ~2-3 weeks, or 10-12 class meetings)

We started this unit with a lab activity: Comparing Water and Ethanol. Students used some concepts and skills from previous units to compare two substances that look pretty similar. In the modeling materials, much of this is done in discussion/demo. But as my students needed to work on lab skills, it made sense to make this a mini-lab activity. Some major additions:

  • Introduces “vapor pressure” as a way to measure how easily a substance evaporates. Since we’d just used the pressure sensors, it was pretty easy for them to collect data.
  • Students investigate the pure substances as well as mixtures of the substances (50/50 and 70/30).

At the end of the activity, students compared data with their tablemates, then we compared results as a whole class. There was some variation from group to group on the values for vapor pressure (mainly due to the quality of the individual pressure sensors) but all groups observed that the vapor pressure for ethanol was nearly twice that of water.

Students then went on to complete the post-lab activity with their lab partners. In addition to particle diagrams, students make a connection between vapor pressure and rate of evaporation. There’s room also for them to predict what might make some substance evaporate faster than others. We had some great discussions about this, and it helped also to understand why heats of vaporization and fusion differ for different substances. Students do look up information thermodynamic data for the substances (Fun fact: Siri searches Wolfram Alpha for this info). Next, they sketch heating curves for ethanol and water, then make predictions about how they could separate the two.

The next day, we go over the post-lab activity. Then, I ask them to sketch what a heating curve would look like for a 50/50 mixture of ethanol and water.  Then, using a LabQuest 2 and a distillation set up, we collected the data.

After this, we go through the iron/sulfur/iron-sulfur/iron sulfide sequence from the modeling materials. Then the rest of the unit proceeds in the normal sequence.

I like the changes in this unit — it is great to have students make the connections to unit 2 and 3. It was also great to be able to revisit their predictions when we study intermolecular forces in more detail later in the course.

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