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.
|Way of thinking||Defining characteristics|
Students may reference where a substance came from to establish its chemical identity or to differentiate it from other substances, they may think that an essential component or quality is imparted by the source.
Students may focus on the changes to a substance occuring through a process, event, or transformation, and reason about the chemical identity of the substance based on the type of change that is happening or has already happened or the external agents that may be invoking the change on the substance.
Students may consider the effect a substance has on a living organism to determine the chemical identity of a substance.
Students may rely on information provided to them by their senses in order to make judgments about the chemical identity of a substance.
|Composition & structure||
Students may use the components of the substance and their arrangement at either a macroscopic or microscopic level to reason about the chemical identity of the substance.
Students may place the substance into a more general class of substances in order to make inferences about the substance’s chemical identity.
Students may consider the function or purpose of a substance on its own or when in a mixture or object form to determine the chemical identity of the substance.
|Tests & experimental values||
Students may suggest performing tests or experiments on substances to aid in claims about chemical identity, and may wish to compare experimentally obtained values or observations to those defined in the literature.