Monday, October 6, 2014

Isotopes Make Cents

When most educators talk about flipping lessons, they are referring to delivering content outside of class so that students can apply and practice concepts in class with the teacher there to support their learning.  There are many variations of "flipping" and, like all other teaching innovations, people who teach effectively and ineffectively with all the variations.  The flipping that I am interested in is taking a lab or activity that has been done in a traditional cookbook way and turning it into a discovery or inquiry-based activity instead.

Many chemistry teachers do an experiment to help students apply atomic mass calculations to a sample of something.  I have used beans to ask students to find the atomic mass of "beanium."  Students count three kinds of beans in a sample (they are all beans but have different masses) and find the average mass of each type.  Then they find the average mass of a bean in the sample, the atomic mass.  It does give kids a chance to practice a calculation, but it isn't really an experiment -- there isn't an essential question to answer or a problem to solve.

Years ago, when I taught ChemCom, we did an experiment that compared pennies to isotopes.  Pennies minted before 1982 were solid copper and have a heavier mass than the zinc/copper pennies minted after 1982.  They are all pennies and all worth $0.01, but they have different masses.  Just like isotopes.  Take a look at these pictures:

Can you figure out how many pre- and post-82 pennies are in the 
random pennies picture?

I have a FoodSaver.  Instead of using it for meat and cheese like the infomercial recommended, I created vacuum-sealed bags of 10 pennies, each with a different combination of pre- and post-1982 pennies.  I challenge the students to determine the number of each in the bag.  The vacuum-sealed bags hide the dates on the pennies well, so the students are intrigued to find the answer.  I use codes like 55 to indicate 5 of each or 73 for 7 pre-82 pennies and 3 post-82 pennies, so I can tell them immediately if they solved correctly.  Though this isotopic abundance calculation is harder than finding a weighted average like in "beanium," there is still a high degree of success.  

The flip isn't the activity, though.  It's how you use the activity.  If you pass out a worksheet that tells them all the steps - in the lab and the calculations - it won't require much original thinking.  Instead, maybe show these pictures.  Or a video of weighing the sets of pennies.  Give them a bag (or an opaque box) and have them guess at how many of each type of penny are in there.  Ask them to figure out what they need to figure it out.  Let them craft their own route.  I have seen students describe some very interesting ways of solving this problem.  Try it before you do isotopic abundance calculations and then you can refer to it when it becomes about isotopes.  Let them open the box and see if they were right.  

I'll keep writing about my experiences in flipping labs like this and post them on my newest page, Inquiry Labs.  Check back for more activities from time to time.  Try a few.  Got a great one?  Suggest it in the comments.

Read the followup to this post here.


  1. Oh man, I love this. Love the vacuum-sealed bags. Love the alternative definition for "flipped." Can you help me understand how the question is even solvable with the given information, though? I can calculate the average weight of a pre- and post-1982 penny but it seems like a random bag of pennies would be impossible to sort out.

    1. Just added a new post to respond to this. Thanks for commenting, Dan!