Eight ways students think about chemical identity

Answering “What is this substance?” is part of nearly every problem that chemistry is used to solve. We identify substances by classifying them and by differentiating them from each other. As students learn chemistry, their chemistry knowledge becomes more sophisticated in being able to answer two questions: (1) What types of substances are there? and (2) How is one substance different from another substance?

When students or chemists work on a chemistry problem, some aspect of the problem is likely to require classifying or differentiating substances. This is basically about comparing and contrasting. Comparing involves looking for what is the same, and then making generalizations about other entities that fall into a class that share the same properties. This can be done in intuitive ways, such as natural vs. artificial, or more normative chemistry ways, such as polar vs. nonpolar molecules. Contrasting involves focusing on differences that permit telling entities apart. This can also be done in intuitive ways, such as whether a substance is breathable or not, or in more normative chemistry ways, such as determining whether a compound contains a carboxyl functional group. Expert chemists sometimes rely on the same intuitive aspects that beginner chemists do, but they use them with deeper sophistication. As students reason, it can be helpful to listen for the ways that they use chemical identity thinking, and to recognize which ways of thinking they are using. You can respond to the substance of students’ chemical identity thinking by helping students see what is productive in how they are thinking.