Categories of ways that students think about the concept of chemical identity.
middle school science
“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 drives chemical change?” is a question of why chemical processes happen. Chemical reactions occur to different extents and at different rates. To what extent reactants will be converted into products depends on the relative potential energy of their submicroscopic components as well as on the relative configurations these components can adopt. The speed of a process depends on the mechanism of the reaction and on the concentration of those species that limit the reaction rate. Both reaction extent and rate are affected by the temperature of the system. Understanding the drivers of chemical change is critical for predicting, explaining, and controlling processes of interest, from making soap to reducing pollution.
“How does structure influence reactivity?” is a question of connection between chemical structure and behavior. The specific ways in which the submicroscopic particles of matter interact with each other and are transformed into different chemical species depend on their atomic composition and molecular structure. The types of atoms present in a molecule and their relative arrangement in space affect the distribution of the electrons that can participate in bonding processes with other particles. Understanding how molecular structure affects electron distribution, and how this in turn determines how different particles interact and react with each other is critical to design the synthesis of desired materials and to control 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.