Just What is a Chemical Change?

If a teacher reviews chemical versus physical changes in high school or even in a freshman course, the chances that he’ll come across misconceptions regarding the topic are as good as those of seeing autumn colors in Quebec. This is normal because classifying change is a tricky concept given that students barely know the subject. In order to make sense of the physical versus chemical dichotomy, as they learn more chemistry, they actually have to review it a few times and relate new material to the definitions.
For instance, many students seem to think that physical changes are reversible and that chemical ones are not. But breaking glass is physical, but it’s not reversible. And while there are tons of irreversible chemical changes like combustion, many chemical equilibria are reversible reactions. Even the oxidation of mercury can be reversed with gentle heating.
 P1020430Bringing energy into the definition also leads to conceptual errors because energy changes are associated with both physical and chemical changes. Condensation, for instance, is exothermic enough to drive thundercloud formation.  Chemical changes on the other hand are neither necessarily explosive nor largely endothermic.
So it’s probably best to emphasize that chemical changes create different compounds and/or elements, and that we can check the behavior of the products to see if a chemical change has occurred.  For instance, the fact that evaporation is physical  can be verified by testing both water vapor and liquid water with cobalt chloride paper. Both make it pink by forming the hydrate. Meanwhile burning paper is chemical because the ashes don’t burn, which of course is a key behavior of paper towards oxygen.
A great indicator of chemical change is a change in color. Rusting, other forms of oxidation, digestion of food, the formation of red autumn colors or the decomposition of chlorophyll to reveal preexisting carotenoids or xanthophylls, are all chemical changes involving changes in color.
Since there are so many mechanisms of color, looking for such evidence to see if chemical change has occurred also leads to all sorts of other intriguing scientific questions.

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