One of the first topics in Chemistry I is states of matter and chemical vs. physical properties. Pretty much every sample of matter is either a solid, liquid, gas, plasma, or unknown. Similarly, the properties of matter fit neatly into the categories of chemical, physical, both, and unknown.
There’s a lot to be gained by asking questions that don’t have neat answers. For example, I was talking today about states of matter, and I had my students debate how to categorize ketchup and glass. For ketchup, they knew it was a liquid, but couldn’t come up with words to explain why it seemed to have a definite shape. (I actually got a bottle of ketchup from the cafeteria and poured some onto a watch glass so they could look at it.) Some of the students had also been told by previous teachers that glass is actually a viscous liquid. (I believe it’s actually an amorphous solid, but the point is the ambiguity.) The point of the discussion was that every time we try to put things neatly into categories, there will always be something that doesn’t fit. Science is about explaining what we actually observe. If we observe something that doesn’t fit the rules, we have to explain why it doesn’t fit the rules, and how we classify it (or whether we classify it at all) when that happens.
The subject of chemical vs. physical properties and changes is much the same problem. Dissolving sodium chloride in water breaks ion-ion bonds, which we will later teach our students are “ionic bonds”. However, we get the NaCl back when we evaporate the water, and evaporation of water is a physical process. The distinction between a purely physical change and a shift in the chemical equilibrium caused by removal of water—one of the substances that participates in the ionization of NaCl—is lost on high school students who have never taken a chemistry course.
I think the purpose of the chemical vs. physical topic at the beginning of the school year is no more than to get kids to think about the questions “What is a chemical reaction?” and “What isn’t a chemical reaction?” If they can identify a new chemical formula as a result of the change, they can say with confidence that a chemical reaction must have occurred. If it appears that no substance with a new chemical formula was produced, they can say with some confidence that the reaction was most likely physical. Even if the latter turns out to be wrong or incomplete, it has served its purpose of getting them to think in very simple terms about what we mean by the term “chemical reaction”.
If you believe Jean Piaget’s conclusions about the development of thought processes in children, he suggests that the typical child moves from concrete thinking (“concrete operational”) to the capacity for abstract thinking (“formal operational”) around age 16-17, though a few develop it much younger, some much later, and some never get there at all. (Unfortunately, the emphasis on low-level thinking brought about by NCLB-inspired testing appears to be delaying this transition, but that’s a separate rant.)
In the US, most high school students take chemistry in grade 10 or 11. Most tenth graders are 15-16 years old, and most 11th graders are 16-17 years old. This means our high school chemistry students are only beginning to develop the understanding that things don’t fall into neat little categories, and pretty much every rule of classification they’ve ever learned has boundary conditions that don’t fit anywhere neatly. (This is probably one of the reasons that some high school kids need to test every limit that a teacher describes. These kids are hands-on learners, and they are doing an experiment to enhance their understanding of the fuzziness of boundaries.)
So in the larger sense, we’re not just teaching them about what is and what isn’t a chemical reaction. We’re also teaching them how to deal with science (and the world in general) on a new level, in which the limits of certainty is a key concept.
Looking at some of the state standards for different subjects, English appears to be the only high school subject with a curriculum that is deliberately meta-cognitively arranged. In talking with high school English teachers, the literature students read intentionally moves from stories with concrete plots and easily accessible themes (such as Romeo & Juliet and Of Mice and Men in ninth grade) to more plots that have more complicated themes and less neat outcomes (To Kill a Mockingbird and Wuthering Heights in tenth grade.)
In terms of development of the adolescent mind, chemistry provides students with hands-on experiences that develop their thinking skills through understanding of the natural world in a way that complements their development of thinking skills through exposure to and discussion of literature in their English classes. Or, to put it another way, the chemistry teacher’s equivalent of To Kill a Mockingbird is a pile of ketchup on a watch glass.
Originally posted to the ChemEd-L discussion list.
