The Driving Question The Systems and Scale Unit starts by asking students to express their ideas about the driving question about an anchoring phenomenon: What happens when ethanol burns? The unit helps students answer this question by using core disciplinary ideas, science practices, and cross-cutting concepts to develop scientific explanations of how matter and energy are transformed during combustion of different organic materials Carbon is the Key! In the unit, students learn to tell the story of how matter and energy are transformed as they move through systems where combustion of organic materials is occurring, including open flames and internal combustion engines. A particularly powerful strategy for explaining how flames transform matter and energy involves tracing carbon atoms. We have found that learning to use this strategy proficiently with flames—relatively simple systems—prepares students to study more complex living systems. Students’ Roles and Science Practices As students learn to answer the driving question by explaining how flames transform matter and energy, they play three different roles that encompass all of the Next Generation Science Standards science and engineering practices. Questioners: Students explore the driving question, clarify, and generate more detailed questions. Investigators: Students conduct two matter-tracing investigations of (a) soda water fizzing and (b) ethanol burning. They develop evidence-based arguments about key observations and patterns. Explainers: Students construct model-based explanations of how matter and energy are transformed when organic materials burn. The roles that students play are also embedded in the Carbon TIME Discourse Routine. The Discourse Routine guides how classroom discourse aimed first at divergent thinking and then at convergent thinking should be sequenced through the unit. Good Explanations Answer the Three Questions Students figure out how to answer the driving question by tracing the carbon-containing molecules in fuels through a series of movements and chemical changes inside flames. At each stage in these processes they answer Three Questions about what is happening: the Matter Movement Question, the Matter Change Question, and the Energy Change Question. Note that, in Carbon TIME, crosscutting concepts serve as the “rules of grammar” for producing a scientific performance. With respect to organic materials burning, high quality explanations should attend to the following rules that are implied by crosscutting concepts. Explanations should attend to… Scale by explaining events and phenomena at the appropriate scale (see more in the structure and function bullets below). Systems and system models and energy and matter by following rules for tracing matter and energy through systems and system models. For example, neither energy nor matter should be created or destroyed as it moves into, through, or out of a system. Structure and function by linking structures and functions in explanations at each scale. Macroscopic scale (tracing matter and energy through flames Atomic-molecular scale (tracing matter and energy through a chemical process—combustion—involving molecules with different structures and properties) Below, we use the anchoring phenomenon of ethanol burning as an example of how students learn to answer the Three Questions for combustion of different organic materials. The Matter Movement Question: Tracing Molecules Through Flames Students learn to tell the following story of how carbon-containing molecules move through the flame in burning ethanol. Ethanol (C5H2OH) evaporates and enters the flame, along with O2 molecules from the air. Carbon dioxide and water vapor leave the flame. The Matter Change and Energy Change Questions: Explaining How Combustion Changes Organic Materials Matter movement is an essential part of the story, but not the whole story. To answer the driving question, students learn to explain chemical changes that occur inside flames: Matter Change: Students explain how the reactant molecules (ethanol and oxygen) break apart in the flame, and how their atoms then bond together to create new products (carbon dioxide molecules and water molecules). Energy Change: Students explain how the chemical energy in the reactant molecules, indicated by high-energy C-C and C-H bonds, is released when the products form lower-energy C=O and H-O bonds, releasing energy in the form of heat, light, and movement of air. Students practice their explanations using multiple representations: (a) hand-on molecular models, (b) PowerPoint animations, (c) a chemical equation (C2H5OH + 3O2 -> 2CO2 + 3H2O), and (d) written paragraphs. How Much Detail? There are more complicated and more scientifically accurate ways of talking about chemical bonds and about changes in energy; we discuss some of those in detail in our educator resource: Carbon TIME Content Simplifications. But our learning progression research has shown that there is an important tradeoff here—many students get lost in the details and never learn a basic coherent story that answers the driving question. The Next Generation Science Standards take a clear position on this tradeoff; a coherent story based on principles such as matter and energy conservation is more important than the details. Consult the Unit Sequence tab to decide how much detail is appropriate for your students.
