Decomposers Lesson 3 Background Information

This lesson will be particularly helpful for students struggling to identify that mass of decaying materials is lost to the air. Students conduct an investigation with bread molding and observe an increase in CO₂ in the air using BTB. Students must explain where the carbon atoms in the CO₂ came from.

In this lesson the students return to the guiding question for the unit about how bread molds. We will consistently focus on the idea that understanding carbon-transforming processes involves answering the Three Questions:

• The Matter Movement Question: Where are molecules moving? (How do molecules move to the location of the chemical change? How do molecules move away from the location of the chemical change?)
• The Matter Change Question: How are atoms in molecules being rearranged into different molecules? (What molecules are carbon atoms in before and after the chemical change? What other molecules are involved?)
• The Energy Change Question: What is happening to energy? (What forms of energy are involved? What energy transformations take place during the chemical change?)

Matter (the Matter Movement and Matter Change Questions). We find that even students who have learned how to balance chemical equations do not appreciate the meaning of the procedure:

• Conservation of atoms (the Matter Change Question): The numbers of atoms on the left and right side of a chemical equation have to be the same because they are THE SAME ATOMS! A chemical equation just shows how they are being rearranged into new molecules.
• Conservation of mass (the Matter Movement Question): ALL the mass of any material is in its atoms (and none of the mass is in the bonds, which are just attractive forces between atoms). So, the mass of the products is always the same as the mass of the reactants.

Energy (the Energy Change Question). Chemists, physicists, and biologists have many different conventions for describing and measuring chemical energy. We have a deeper explanation of the conventions used in Carbon TIME units and how they relate to conventions used in different scientific fields in a document called Carbon TIME Content Simplifications. Here are some key points:

• All bond energies are negative relative to individual atoms. So, during a chemical reaction, it always takes energy (the activation energy) to break bonds. Then, energy is released when new bonds are formed.
• Whether a chemical reaction releases energy or not depends on the total energy of the reactants, compared with the total energy of the products. So, energy is released when the total bond energy of the products is lower (i.e., more negative relative to individual atoms) than the energy of the reactants.
• In systems like our atmosphere, where excess oxygen is always present, the most abundant sources of chemical energy are substances that release energy when they are oxidized (e.g., substances with C-C and C-H bonds).

Our research has consistently showed that these ideas are extremely difficult for students who have not formally studied chemistry. We therefore use the convention of twist ties to identify bonds that release energy when they are oxidized.

The investigations in all units will make use of two essential tools:

• Digital balances. Students can detect movement of atoms (the Matter Movement Question) by measuring differences in mass. In this activity students will be able to observe changes in the combined system that includes both bread and mold.
• Bromothymol blue (BTB) is an indicator that changes from blue to yellow in response to high levels of CO₂. Thus, changes in BTB can partially answer the Matter Change Question by detecting whether there is a chemical change that has CO₂ as a reactant or product.

Activity 3.1 is the Predictions and Planning Phase of the instructional model (beginning the climb up the triangle). During this phase, students record their predictions and express ideas about what happens to matter when bread molds. They use the Predictions and Planning Tool to do this.

Activity 3.2 is the Observations Phase of the instructional model (going up the triangle). During this phase, the students record the results of their bread mold investigation and try to identify patterns in their data and observations. The important practices students focus on in this activity are 1) making measurements and observations, 2) recording their data and evidence, and 3) reaching consensus about patterns in results. They use the Observations Worksheet and Class Results Poster to do this.

Activity 3.3 the Evidence-Based Arguments Phase of the instructional model (going up the triangle). During this phase, the students review the data and observations from their investigation of bread molding and develop arguments for what happened during the investigation. In this phase, they also identify unanswered questions: at this point they have collected data and observations about macroscopic scale changes (BTB color change and mass change), but they do not have an argument for what is happening at the atomic-molecular scale (unless they are able to make predictions based on their experiences from the Animals and/or Plants units). They use the Evidence-Based Arguments Tool to record their arguments at this phase.

Key Carbon-Transforming Processes: Digestion, Biosynthesis, and Cellular Respiration