Lesson 4 - Investigating and Explaining Ethanol Burning

Students investigate changes in mass and CO₂ concentration for burning ethanol. Then they explain results using molecular models and chemical equations to answer the Three Questions.

Guiding Question

What happens to ethanol when it burns?

Activities in this Lesson

Activity 4.1: Predictions about Ethanol Burning (30 min)
Activity 4.2: Observing Ethanol Burning (30 min)
Activity 4.3: Evidence-Based Arguments about Ethanol Burning (50 min)
Activity 4.4: Molecular Models for Ethanol Burning (50 min)
Activity 4.5: Explaining Ethanol Burning (40 min)

Unit Map

Unit Map for Lesson 4

Target Performances

Lesson 4 – Investigating and Explaining Ethanol Burning (students as explainers)
Activity 4.1: Predictions about Ethanol Burning (30 min)	Students develop hypotheses about how matter moves and changes and how energy changes when ethanol burns and make predictions about how they can use their investigation tools—digital balances and BTB—to detect movements and changes in matter.
Activity 4.2: Observing Ethanol Burning (30 min)	Students record data about changes in mass and BTB when ethanol burns and reach consensus about patterns in their data.
Activity 4.3: Evidence-Based Arguments about Ethanol Burning (50 min)	Students (a) use data from their investigations to develop evidence-based arguments about matter movements and matter changes when ethanol burns, and (b) identify unanswered questions about matter movement and matter change that the data are insufficient to address.
Activity 4.4: Molecular Models for Ethanol Burning (50 min)	Students use molecular models to explain how carbon, oxygen, and hydrogen atoms are rearranged into new molecules during the oxidation of ethanol (the chemical change that happens when ethanol burns).
Activity 4.5: Explaining Ethanol Burning (40 min)	Students explain how matter moves and changes and how energy changes when ethanol burns (connecting macroscopic observations with atomic-molecular models and using the principles of conservation of matter and energy).

NGSS Performance Expectations

Middle School

Structures and Properties of Matter. MS-PS1-1. Develop models to describe the atomic composition of simple molecules and extended structures.
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.
Chemical Reactions. MS-PS1-5. Develop and use a model to describe how the total number of atoms does not change in a chemical reaction and thus mass is conserved.

High School

Chemical Reactions. HS-PS1-4. Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.
Chemical Reactions. HS-PS1-7. Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

Three-dimensional Learning Progression

This lesson will be particularly helpful for students struggling to identify that mass of a burning fuel is lost to the air. Students observe a fuel source losing mass inside a chamber. They also 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 what happens when ethanol burns. 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 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.
Weak bonds (like C-C and C-H) generally have MORE chemical energy than strong bonds (like C=O). The energy of the stronger bonds is more negative relative to individual atoms.
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 when ethanol is burned.
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.

Key Ideas and Practices for Each Activity

Activity 4.1 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 ethanol burns. They use the Predictions and Planning Tool to do this.

Activity 4.2 is the Observations Phase of the instructional model (going up the triangle). During this phase, the students conduct the investigation for ethanol burning, record data, 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 4.3 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 ethanol burning 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. They use the Evidence-Based Arguments Tool to record their arguments at this phase.

Activity 4.4 is the first part of the Explanations Phase of the instructional model (going down the triangle). Students construct molecular models of the chemical change they observed in the investigation to help them develop an atomic-molecular explanation for what happened.

Activity 4.5 the second part of the Explanations Phase of the instructional model (going down the triangle). Students use the Explanations Tool to construct final explanations of what happens when ethanol burns. Ideally, at this phase their explanations will combine evidence from macroscopic-scale observations during the investigation with their new knowledge of chemical change at the atomic-molecular scale.

Key carbon-transforming processes: combustion

Content Boundaries and Extensions

At this stage in the unit, students will complete the inquiry and application sequences for ethanol burning—they go both up and down the triangle. This means that they will go through the Predictions Phase, the Observations Phase, the Evidence-Based Arguments Phase, and the Explanations 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 and Questions 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 and Planning 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 and Planning 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 other 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 ethanol lost weight. What does that mean?
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 and Planning 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?

Talk and Writing Goals for the Explanations Phase	Teacher Talk Strategies That Support This Goal	Curriculum Components That Support This Goal
Examine student ideas and correct them when there are problems. It’s ok to give the answers away during this phase! Help students practice using precise language to describe matter and energy.	Let’s think about what you just said: Air molecules. What are air molecules? Are you talking about matter or energy? Remember: atoms can’t be created. So that matter must have come from somewhere. Where did it come from? Let’s look at the molecule poster again… is carbon an atom or a molecule?	Molecule Poster Three Questions Poster
Focus on making sure that explanations include multiple scales.	The investigation gave us evidence for what was happening to matter and energy at a macroscopic sale. But what is happening at an atomic-molecular scale? What is happening to molecules and atoms? How does energy interact with atoms and molecules during chemical change? Why doesn’t the macroscopic investigation tell us the whole story? Let’s revisit our scale poster… what is happening to matter at the molecular scale?	Molecular Models Molecular Modeling Worksheets Explanations Tool PPT Animation of chemical change Powers of Ten Poster
Encourage students to recall the investigation.	When did this chemical change happen during our investigation? How do we know that? What is our evidence? What were the macroscopic indicators that this chemical change took place?	Evidence-Based Arguments Tool Investigation Video
Elicit a range of student explanations. Press for details. Encourage students to examine, compare, and contrast their explanations with others’.	Who can add to that explanation? What do you mean by _____? Say more. So I think you said _____. Is that right? Who has a different explanation? How are those explanations similar/different? Who can rephrase ________’s explanation?	Explanations Tool