Categories of ways that students think about the concept of chemical identity.
“What are the effects of using and producing different matter types?” is a question of consequence evaluation, because chemistry depends on context and affects the human experience. The life cycles of materials, including production, consumption, and disposal, have benefits, costs, and risks in many dimensions. These include social, economic, political, ethical, environmental, and ecological consequences. While the ultimate aim of chemistry is to improve the human condition, the design of chemical processes involves making decisions based on limiting consumption of energy, using renewable resources, and reducing or eliminating production of toxic byproducts. This chemical thinking question is often central to sustainable action work, such as evaluating which refrigerants are better than Freon, or designing a greener battery.
“How can the effects be controlled”? Is a question that involves making choices about which internal and external parameters to modify to maximize benefits and minimize costs and risks. While outcomes can be predicted based on models, in real processes there are often many variables which cannot be easily controlled, and many conditions that constrain the processes. Feedback loops of testing and refining are often used, resulting in design processes that converge on a desired outcome (maximizing, minimizing, or stabilizing output), usually making tradeoffs among different properties (such as price, quality, safety, and environmental impact). This chemical thinking question is often central to design activities, such as producing biomass or reducing the toxicity of combustion exhaust fumes.
“How can chemical changes be controlled?” is a question that involves understanding how changes in conditions affect the relative stability of the species involved in a chemical process. Control can be achieved by selecting reactants with structural features that change their energetic stability, varying the concentrations of reactants or capturing and removing products, adding substances which react with intermediates to facilitate or inhibit different mechanistic steps, changing temperature to activate chemical species,, or choosing solvents that facilitate or inhibit certain interactions. For example, controlling the replication of a virus may involve tuning conformations of a substance involved in the replication to block one pathway in the process. This chemical thinking question is often central to chemical process design and analysis activities, such as improving solar cell operation, analysis of battery efficiency, or characterizing the degradation of a dye.
“What affects chemical change?” is a question of identification of internal and external variables that affect the extent and rate of chemical processes. The extent to which reactants are transformed into products and the rate at which the transformation occurs depend on internal factors such as the composition and structure of the particles involved and their concentration in the system, as well as on external factors such as temperature, pressure, and the nature of the environment in which the reaction takes place (e.g., type of solvent, pH). Identifying these factors and their effects on reaction extent and rate allows us to design and control chemical processes for particular purposes.
“What interaction patterns are established?” is a question that involves selecting which classification systems and models are most relevant to understanding chemical processes for a particular purpose. Chemical reactions exhibit patterns that allow us to classify them in different groups to facilitate prediction, explanation, and control. Multiple classification systems and chemical models are often used simultaneously to analyze a process depending on the purpose and context. For example, a process may be thought of as a redox reaction when used in an electrochemical cell but as an addition reaction if used for synthetic purposes. Acid-base reactions can be understood by accounting for proton transfer or by paying attention to electron sharing. This chemical thinking question is often central to the analysis of chemical processes used to synthesize and analyze substances, and to harness chemical energy.
“What determines the outcomes of chemical changes?” is a question of dynamic behavior and probability. When the particles that make up different substances interact with each other, their interactions may lead to a variety of structural changes (e..g., some atoms may change positions or separate from a molecule). Which of these random changes are more likely to occur depends on the relative potential energy between the original particles and the new particles that are formed. More stable particles (with lower potential energies) are more probable to form and will determine the path and outcome of the reaction. Understanding the relationship between structure, stability, and reaction mechanism allows us to predict, explain, and control the products of chemical processes.
“What types of matter are there?” is a question of classification. Classification is a very important tool for predicting and explaining the properties of substances in our surroundings. For example, classifying a material as a metal versus a nonmetal allows us to predict that it may conduct heat and electricity quite well. Similarly, identifying a substance as an ionic compound allows us to explain why its aqueous solution conducts electricity. Classifications are often loose categories with gray areas, but they support chemical thinking when seeking to synthesize new substances, determine the identity of a material, or control a chemical process.
“What properties of matter types emerge?” is a question of the origin of properties. Predicting or explaining properties of substances often requires analysis of structural rather than compositional aspects of substances, and involves reasoning about emergence rather than arguing based on a central cause. Explaining behaviors of substances involves examining what influences energetic stability and how behaviors on one distance scale emerge from dynamic interactions between structural components on a smaller scale. There are many different scales at which these structure-property relationships are built (from multiple entities in mixtures down to electronic structure). This chemical thinking question is often central to predicting properties of substances, e.g., which oil is best for lubricating a transmission or frying plantains or making soap.
A formative assessment designed by an ACCT cohort member designed to investigate student understanding of chemical mechanism.