Differentiate ionic and molecular compounds using multimeters

Have you ever done this?  You finally decide to change a lab, and then wonder, why didn’t I do this years ago?  That was me this week.  For years I used batteries and lights to measure the conductivity of solutions.  I used it in my lab exploring the physical properties of ionic and molecular compound.  But I found that the voltage was too low if I used D cell batteries.  And if I used 9 V batteries the kids would burn out all the lights.  It was a constant battle.  Here is how I fixed it.

Use the river metaphor for electricity

At the point of first-year chemistry, most students are pretty ignorant about electricity.  I take a couple of minutes to explain the difference between voltage and amperage.  I use a river analogy.  The voltage, or potential, is the slope of the river bed.  The higher the difference from one end to the other, the greater the potential to fall.  The Mississippi River has very little potential, since it is so flat.  But the North Fork of the Payette (an extreme kayaking destination near here) has LOTS of potential.  The current, or amperage, is how much water flows in the river.  You could have a high voltage but a low amperage.  It would look like a stream trickling down a cliff.  The kids then recognize that the Mississippi has a lot of amperage.  I don’t really talk about resistance in this lesson, but it can be a dam or the narrow cliffs impeding the river.

Set up the multimeter

This year, I finally switched to using a multimeter instead of lights.  What a great difference!  I just drew a simple diagram and printed copies to set on the lab stations.  The key points were to use nails and alligator clips to make probes to go in the solutions, so that the meters that I borrowed from physics wouldn’t get damaged.  We used a 9V battery to power the system.  Most students were able to set it up with little to no instruction.  You can see my drawing here.

Test the solutions

Students would turn the meter on and then read how many milliamperes were flowing through the solution.  It was great.  They got quantitative results, which they wouldn’t have if they were using batteries and lights.  They also didn’t have to worry about their circuit failing near as much. 

What I will do differently next year

Here are a couple of things that I will do differently next year.  Since we are measuring amperage, the depth of the water in the beaker matters a lot.  In the past, the lab was completely qualitative, so no precise measurements of volume were needed.  But now it would be beneficial.  Use the 10 A port.  I managed to fry a bunch of the internal fuses by using the red milliamp port on the multimeter.  The milliamp port was great for getting more precise numbers, until they clicked the leads together and popped the fuse.

The last thing I learned for next year is to stop being an old dog afraid of new tricks.  Edit labs.  If something always bothered you in a lab — change it.  The effort that you invest in creating new methods and ideas is paid off for the benefit of every kid that walks into your room.  They are worth it.

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