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Decomposers | Lesson 3 - Investigating Bread Molding

Lesson 3: Investigating Bread Molding

Students conduct an investigation to explore what happens when bread molds. They use two process tools in this lesson to record their ideas: the Predictions Tool and the Evidence-Based Arguments Tool.

Guiding Question

What happens when bread molds?

Activities in this Lesson

  • Activity 3.1: Predictions about Bread Molding (50 min)
  • Activity 3.2: Observing Bread Molding (60 min over 2 days)
  • Activity 3.3: Evidence-Based Arguments for Bread Molding (50 min)


  1. Measure mass changes in molding bread and other materials.
  2. Detect changes in CO2 concentration in chambers with decaying materials.
  3. Construct arguments that use evidence about changes in mass of decaying materials and carbon dioxide concentration to defend claims about movements of atoms and chemical changes during decay.
  4. Find patterns in data collected by multiple groups about changes in mass of decaying materials and carbon dioxide concentration.
  5. Draw and explain movements of materials during 1) growth of a decomposer and 2) function/movement of a decomposer, including air and decaying matter entering the decomposer, and waste, air enriched in carbon dioxide and water vapor leaving the decomposer.

NGSS Performance Expectations

Middle School

  • MS. Structure and Properties of Matter. MS-PS1-1. Develop models to describe the atomic composition of simple molecules and extended structures.
  • MS. Chemical Reactions. MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.

High School

  • HS. Chemical Reactions. HS-PS1-4. Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends on the changes in total bond energy.
  • HS. Chemical Reactions. HS-PS1-7. Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
  • HS. Matter and Energy in Organisms and Ecosystems. HS-LS1-7. Use a model to illustrate that cellular respiration is a chemical process whereby the bonds of food molecules and oxygen molecules are broken and the bonds in new compounds are formed resulting in a net transfer of energy.

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 things decay. 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 on the BSCS website 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 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 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 experience from the Animals unit). They use the Evidence-Based Arguments Tool to record their arguments at this phase.

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

Lesson Map:

See the Decomposers Unit Read Me document for more details.

Lesson 3 Map

Unit Map

Decomposers Lesson 3 Unit Map

Talk and Writing

At this stage in the unit, students will complete the inquiry and application sequences for bread molding—they go both up and down the triangle. This means that they will go through the Predictions Phase, the Observations Phase, and the Evidence-Based Arguments Phase in one lesson. The tables below show specific talk and writing goals for these phases of the unit.

Talk and Writing Goals for the Predictions Phase

Teacher Talk Strategies That Support This Goal

Curriculum Components That Support This Goal

Treat this as elicitation and brainstorming (like the Expressing Ideas Phase), but with more directed questioning.

Now that we have set up the investigation, we want to predict what we think will happen to matter and energy.

Three Questions Handout

Predictions Tool

Elicit a range of student ideas. Press for details. Encourage students to examine, compare, and contrast their ideas with the ideas of other students.

Who can add to that?

What do you mean by _____? Say more.

So I think you said _____. Is that right?

Who has a different idea?

How are those ideas similar/different?

Who can rephrase ________’s idea?

Investigation Video (first half)

Encourage students to provide evidence that supports their predictions. .

How do you know that?

What have you seen in the world that makes you think that?


Have students document their ideas to revisit later.

Let’s record our ideas so we can come back to them and see how our ideas change.

Predictions Tool


Talk and Writing Goals for the Observations Phase

Teacher Talk Strategies That Support This Goal

Curriculum Components That Support This Goal

Help students discuss data and identify patterns.

What patterns do we see in our data?

How do you know that is a pattern?

What about ______ data. What does this mean? 

Class Results Poster

Class Results Spreadsheet

Encourage students to compare their own conclusions about the data and evidence with other groups and other classes.

What about this number? What does this tell us?

How is group A’s evidence different from Group B’s data?

How do our class’s data differ from another classes’ data?

Class Results Spreadsheet

Class Results Poster

Investigation Video (second half).

Make connections between the observations and the data/evidence.

It says here that our BTB turned colors. What does that mean?

You recorded that your bread lost weight. What does that mean?

Observations Worksheet

Have students consider how their predictions and results compare.

Let’s revisit our predictions. Who can explain the difference between our class predictions and our results?

Who had predictions that were similar to our results? Has your explanation changed? How?



Talk and Writing Goals for the Evidence-Based Arguments Phase

Teacher Talk Strategies That Support This Goal

Curriculum Components That Support This Goal

Press for details. Encourage students to examine, compare, and contrast their ideas with the ideas of other students.

Who can add to that argument?

What do you mean by _____? Say more.

So I think you said _____. Is that right?

Who has a different argument?

How are those arguments similar/different?

Who can rephrase ________’s argument?

Investigation Video (second half)

Students provide evidence from the investigation (not just experiences in the world) to develop arguments.

Does your argument include evidence from the investigation?

What evidence is most important here?

What does this evidence tell us about what happened?
What evidence do we still need for a complete picture of what happened?

How do you know that?

Evidence-Based Arguments Tool

Class Results Poster

Class Results Spreadsheets

Investigation Video (second half)

Data from other classes

Focus on how matter and energy were transformed at different scales.

What does this evidence tell us about how matter is changing?

What does this evidence tell us about how energy is changing?

Three Questions Handout

Evidence-Based Arguments Tool


Revisit predictions and examine change in thinking.

Let’s revisit our Predictions and see how our thinking changed now that we know what happened.

Evidence-Based Arguments Tool

Predictions Tool

Encourage students to consider the questions they don’t have answers to.

This investigation told us many things about what happen to matter and energy during ____. But what questions do we still have?