Unit 6 Reflections: Conductivity Activity and Worksheet #1

My endearing students have started leaving chemistry jokes on my board before or after class:

At least not everyone has a bad attitude!

I'm still plugging away at as much of the modeling curriculum as possible, despite being ridiculously far behind.  As I mentioned in my last post, I made a decision to replace the "Sticky Tape Lab" with a simple static electricity activity.  It seemed effective and we were able to replace a lot of the time we would have spent on the sticky tape discussion with some coverage of the periodic table and the ionization trends on the periodic table.

I got it in my head that I really wanted to use the property of conductivity to help students identify the difference between ionic and covalent.  Every year I seem to want to demonstrate conductivity, but my school has never had any conductivity probes.

Some wonderful teachers at my modeling workshop introduced me to the idea of making conductivity probes out of 9v batteries and Christmas lights.  I found a couple ideas about the best way to do this online, then headed to Walmart to see what I could find.

I came up with these:

Homemade conductivity probe

Basically, I bought a strand of $5 Christmas lights and cut a lightbulb out of the strand for each probe.  The length of wire between each bulb is extremely short, so I also used the excess loopback wire and an alligator clip to give more length.  The alligator clips were in the automotive department surprisingly, and cost about $2 for a pack of 6.  The batteries were less than a dollar a piece, and the entire thing is taped to a popsicle stick for ease of use.  I spent around $20 and it took me about an hour to construct a set of 7 for my classes.  If I had more time, I would have had the students make them on their own.  I know a lot of my boys would have really dug that.

Of course, the day we hit this in the curriculum, all of the science labs were being used by other teachers.  So, I grabbed an assortment of substances that would be easy and safe to use in the classroom:

The procedure was to classify the elements as metals, nonmetals, and metalloids and test their conductivity.  Then students classified the compounds as M-NM or NMs only and tested their conductivity dry and then dissolved in H2O.

As usual, my first class of the day nailed it.  I was particularly proud of this group:

We just whiteboarded the data for comparison and verbal conclusions

Their terminology could have been better, but you have to understand that this was a group of 4 extremely low-performing and unmotivated boys who saw the pattern immediately.

After we discussed how M-NM compounds are conductive dissolved in solution, but NM-NM compounds are not, I gave my students Worksheet 1.  We did not have time to even touch the electrolysis of copper chloride lab.

Worksheet 1 is hard.  Ideally, the goal is for students to deduce both the ratios of ionic compounds and the oxidation numbers of each group all via the concept that solid M-NM do not conduct electricity.  That's a lot to discover on their own.  Now, before the conductivity lab, I taught my students about oxidation numbers when I taught them cations and anions.  But they still had to determine why compounds form in certain ratios.  It's a really good worksheet.  And oh my gosh, did my students complain and whine.  But... they figured out how to write binary ionic compounds all on their own and could articulate why.   I'm sure hoping the retain this understanding over the weekend!

I did not plan on going into great depth on the structures of ionic vs. molecular solids, but I did download the Mercury Software.  I like the idea of showing them the difference, but I could not get the software to work correctly for me and just didn't have time to play with it.  Instead, I'm falling back on an old powerpoint and some direct teaching for the differences between the chemical bonds and nomenclature.  I do intend to complete worksheets 3 and 4 next week and Quiz 1, but then I'm moving on.