Found an Argument

“I have found you an argument.” That’s the title of a short excellent article by George Bodner in the Journal of Chemical Education. I’d read this a long time back (the article was from 1991) but stumbled across it again as I’ve been reading and thinking about student misconceptions in chemistry. The article is based on a conceptual knowledge chemistry test given to entering first-year graduate students at Purdue – note these are students who were likely academically very strong students in their respective undergraduate programs.

 

Examples of concept questions to be tested are: “What are the bubbles in boiling water?”, “How does salt melt ice?”, and “What happens to the weight of an iron bar when it rusts?” While many of the students provide something reasonably close to a decent answer, a significant number do not. Sometimes only a small fraction of the students can come up with a cogent argument.

 

The paper’s title comes from a quote by Samuel Johnson: “I have found you an argument; I am not obliged to find you an understanding.” This fits with the argument made by Mercier and Sperbier that coming up with reasons or explanations is often lazy and post-hoc. It’s why I try to emphasize the explanative parts of chemistry, often the hardest for students. I’d like to think that’s the value-added of having an expert interlocutor for students to interact with in real-time.

 

An example that Bodner discusses is one that I have emphasized strongly to my first-semester G-Chem students this semester. The prompt is: “Everyone knows that sodium metal reacts with chlorine gas to form sodium chloride. Explain why. In other words, what is the driving force behind this reaction?” Bodner reports that at first students mostly said “because of a decrease in Gibb’s Free Energy of the system”. Further prompting of why led to “a significant fraction not being able to answer the question”. Scarily, and perhaps not surprisingly: “By far, the most common explanation was based on the assumption that the octet rule drives chemical reactions.”

 

Every year I try to emphasize to my students that the octet rule is simply a rule of thumb for identifying good Lewis structures, and should not be invoked for explanative power. I even have a Happy Atoms Story to go along with this part of class. We explicitly go through the energetic calculations showing that the first ionization energy of sodium is not sufficiently compensated by the first electron affinity of chlorine. Then we invoke Coulomb’s Law and define lattice energy. Before all this is done, we’ve already discussed bond forming and breaking across a range of examples illustrating the general bond energy curve.

 

Here’s a snapshot showing some of the erroneous answers from the incoming chemistry graduate students.

 

But it gets worse…

 

The purpose of Bodner’s paper is not to ridicule students. It is aimed at us, the instructors. We should be thinking carefully about our teaching – it needs to be more robust, and we need to pay explicit attention to common misconceptions and the “Swiss-cheese knowledge” of our students. Here is my paraphrase of Bodner’s observations and conclusions: (1) Students are often only good at applying domain knowledge narrowly, and they need more “cross-training” examples; (2) Misconceptions can be quite sticky and difficult to dislodge; (3) Students can “know stuff” yet lack in understanding; (4) Misconceptions can often be caused inadvertently by instructors.

 

I will close with a different example. Here’s the prompt: “Ice is less dense than water, but steel is almost eight times as dense as water. Explain why both the Titantic and the iceberg it hit were able to float on water.”

 

One of the answers given by a student: “The Titantic was made from titanium, not steel.”