A Picture's Worth A Thousand Words

A few years ago my PLC made it our goal to help students understand chemistry at the particle level.  To that end, we designed at least one assessment item per unit that asked students to draw pictures of particles or interpret pictures of particles.  At first glance, this looks easy, but we were surprised at how much we learned about what our students didn't understand about the language of chemistry.  Years later, I remain intrigued at how telling these questions can be.

This week we have been working on limiting reactant problems.  My students took a test over reactions and stoichiometry (the math of chemical reactions that can predict how much of a reactant is needed to make a certain amount of product) just before our semester ended in December.  I ran out of days to squeeze in limiting reactant problems, so that was first on the docket when we returned in January.  I'll write more on how this went in a subsequent post.

In the meantime, please consider one of the questions from the quiz my students took this week, a question developed by my PLC years ago.  I was absent when they took it and my substitute told me that my students told me that they had no idea what they were doing, so I was anxious to see if their quizzes said that too!  The front side of the quiz is a limiting reactant math problem with results pretty much what I would expect:  Several students aced the math, most had the right idea but got stuck in a few places, a couple were entirely lost.  The food for thought, though, was on the reverse side in the particle question.

 
I like this question because it asks them to draw on a lot of chemistry knowledge.  They have to understand the difference between what a subscript tells them and what a coefficient tells them.  They have to interpret a chemical equation like a recipe.  They have to have a conceptual understanding of limiting reactants.  They have to demonstrate the law of conservation of mass.  On the reactions and stoichiometry test in December, students did well.  I know they can balance an equation, explain conservation of mass, and solve stoichiometry problems.  But, I think you'll agree that these papers show a disconnect between the mathematical interpretation of chemistry and the conceptual interpretation of chemistry.

Several students are confusing subscripts and coefficients.  Most of these students agree on which substance is limiting and which is in excess, even though they have all drawn different pictures and have different numbers of molecules.  Two of these papers violate the conservation of mass law.  One student is confused about which substance is hydrogen and which is nitrogen.  I also have noted that some students crossed out particles as they "used them up," some created a key for themselves, others wrote themselves notes for later.  I am not going to say which one is the correct answer.  If you are a chemistry teacher, or have a science background, you know which one is correct.  Each one shines a little light on a different misconception.  And an opportunity to clear one up. 

There are some ways I would improve the question.  And some things that I might do differently before I use it again.  I was fairly generous as I graded it.  I don't like to take away points for the same mistake twice, so if the picture was wrong, but the numbers matched the picture, I only took off for the picture.  Many of the students whose work is displayed here scored very well on the quiz, despite the misconceptions uncovered on this question.  That brings up the question for me: does the grade really reflect what they know and are able to do?  Does the weight of this question vs the mathematical portion reflect what I think is important to emphasize? 

If you are a chemistry teacher, I would love it if you would use this item or a similar item and share your results or your thoughts.