Student Heuristics in Learning Chemistry

In my most recent post, I mentioned a monograph I’ve been reading: Concepts of Matter in Science Education. One of the articles that made me really stop and think was by Vicente Talanquer, titled: “How do students reason about chemical substances and reactions?” How do they indeed? It turns out that they often rely on heuristics – some of which mislead them. What are heuristics? Here’s how Talanquer describes it from p340 of the monograph minus the many references. (If you’re interested I encourage you to read the article in full.)

“Heuristic reasoning in judgment and decision-making has been analyzed from a variety of research perspectives. Despite differences in conceptualization and approach, existing frameworks highlight the capacity of the human mind to make decisions with very little time and information, using implicit and preconscious reasoning mechanisms. These types of reasoning strategies have been characterized as fast and frugal because they employ a minimum amount of time and information to generate a choice or decision and [are] adaptive or ecologically rational because they fit to the structure of the in which they are used. Heuristic processing can be expected to dominate over more analytical ways of thinking when a person has less knowledge, capacity or motivation to do well in a task. Although heuristics usually provide satisfactory answers, they do not always lead to the correct solution and seem to be responsible for many systematic biases and errors in human reasoning.”

Let’s look carefully at that second last sentence and apply it to students. When a student has less knowledge or capacity, there is a reliance on applying heuristics rather than more a careful analysis. That’s certainly true in my experience. The less knowledgeable and less capable students do provide heuristic type answers on an exam. Some of these have the “correct” buzzwords but are used incorrectly, and I can tell that the student doesn’t have a clear understanding of the concept being applied. Others are just plain wrong. On the other hand, the students who have a stronger grasp on the underlying concept are able to justify their arguments rationally and logically.

But sometimes students who seem to be able in a class discussion or in my office to make coherent, logical, rational arguments in answering a conceptual question fail to do this on an exam. (When a student asks a question, I usually don’t answer them directly, but rather provide them the tools to formulate their argument through prompts.) I think this is where time pressure can lead a student to fall back on a heuristic. I have timed exams in the majority of my classes (although I have experimented with other open-ended forms) because I think a time constraint does provide some measure of how well the student actually knows the material. It’s not a perfect measure by any means, and there are disadvantages to having timed exams. (Open-ended exams however have a different set of disadvantages.) For that matter, using exams in both teaching and assessment has its pros and cons. I think the pros outweigh the cons in a number of chemistry courses especially in the first year of college

Talanquer’s article makes reference to studies in first and second year college chemistry (usually General Chemistry and Organic Chemistry) whereby a large proportion of students “rely on heuristic strategies, rather than analytical thinking based on atomic-molecular models of matter... Heuristic reasoning allowed participants in our studies to reduce cognitive effort by minimizing the number of cues that needed to evaluate to make a decision.” This has echoes of cognitive load theory, and I think the cognitive load is particularly high in chemistry because of the back-and-forth between atomic-particle level explanation and macroscopic observations mediated by a symbolic language and a dizzying array of models. This problem in chemistry is often referred to as the Johnstone Triangle.

A few heuristics are highlighted in the article. Recognition is used when an object is recognized and exhibits the known property. The example provided is that students often select NaCl as being more water-soluble than NaBr simply because the first is more recognizable. One-reason decision-making is the tactic of searching for a single differentiating cue to answer a question. The example used is that the student might assume that BaO has a higher melting point than MgO because Ba is “heavier” than Mg, and simply stop there without any further analysis.

As an expert, I also use heuristics of a sort when thinking about chemistry, but my knowledge base is much wider and deeper than the student’s. Thus my recognition heuristic isn’t limited to just a few representative cases, nor would I quickly stop at a single differentiating cue or use a one-reason approach. The question is how to help my students move towards a deeper analysis. Clearly widening and deepening the knowledge base is important – and that is why content is important in the study of chemistry. But the process of reasoning through multiple factors can also be modeled in the classroom.

This semester I was starkly reminded of the differences between students who had knowledge and capability in chemistry and those who did not. I am teaching the Honors second semester General Chemistry course, i.e., the students are all strong students interested in being science majors who’ve had first semester chemistry the previous semester. Some of them also have very strong mathematical skills and are enrolled in physics and biology courses. On the other hand, the students in my non-science-majors course, while they may be knowledgeable and capable in many other areas, are typically not knowledgeable in thinking chemically. The wrong use of heuristics was apparent time and again, and I needed to do a much better job at helping the students build the necessary foundations to reason in chemistry. This summer one of my goals is to think about how to better build that foundation.