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Systems and Scale | Lesson 3 - Investigating Soda Water Fizzing

(Optional) Lesson 3: Investigating and Explaining Soda Water Fizzing

Students investigate changes in mass and CO2 concentration for soda water fizzing. Then they explain results using molecular models and chemical equations to answer the Movement Question and the Carbon Question. 

Guiding Question

What happens when soda water loses its fizz?

Activities in this Lesson

Note: Lesson 3 is optional depending on your knowledge of your students and learning goals. This lesson is recommended for middle schoolers to introduce them to using investigations, molecular models, and chemical equations to describe a simple chemical change. See the Systems and Scale Unit Read Me file for more information to consider when making this choice.

  • Activity 3.1: Predictions about Soda Water Fizzing (20 min)
  • Activity 3.2: Observing Soda Water Fizzing (30 min)
  • Activity 3.3: Evidence-based Arguments about Soda Water Fizzing (50 min)
  • Activity 3.4: Molecular Models for Soda Water Fizzing (40 min)
  • Activity 3.5: Explaining Soda Water Fizzing (40 min)


  1. Measure mass changes in materials undergoing chemical change (soda water fizzing).
  2. Detect changes in CO2 concentration in air samples (soda water fizzing).
  3. Use evidence about changes in mass of fuels and CO2 concentration to defend claims about movements of atoms during chemical changes (soda water fizzing).
  4. Find patterns in data collected by multiple groups about changes in mass and CO2 concentration.
  5. Draw and explain movements of materials during a chemical change (soda water fizzing).
  6. Explain the chemical changes as atoms being rearranged into new molecules (soda water fizzing).

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-PS1-7. Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

Background Information

This lesson (which is optional for high school students) uses a simple chemical change, the decomposition of carbonic acid, to introduce students to using investigations, molecular models, and chemical equations to describe chemical changes.

Carbonic acid is a weak acid, meaning that it always exists in equilibrium with dissolved CO2 in water, which in turn is in equilibrium with gaseous CO2 in the air. Increasing temperature and decreasing pressure drive the equilibrium in the direction of gaseous CO2, so when the soda water “loses its fizz,” carbonic acid is dissociating, and dissolved CO2 is escaping into the air.

In this lesson students are introduced to two of the Three Questions. All Carbon TIME Units share a focus on understanding the chemical changes involved in complex processes, such as combustion and growth of plants and animals. We will consistently focus on the idea that understanding these processes involves answering Three Questions: 

  • The Matter Movement Question: Where are atoms 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?)

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.

In this lesson, students will focus only on two: The Matter Movement Question and the Matter Change Question. They turn to the Energy Change Question in the following lesson when they investigate burning ethanol.

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 system of the soda water.
  • 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 soda water fizzes. 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 conduct the investigation for soda water fizzing, 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 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 soda water fizzing 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 3.4 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 3.5 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 soda water fizzes. 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: Decomposition of carbonic acid

Lesson Map:

See the Systems and Scale Unit Read Me document for more details.

Unit Map for Lesson 1

Unit Map

Unit Map for Lesson 3

Talk and Writing

At this stage in the unit, students will complete the inquiry and application sequences for soda water fizzing—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 shows specific talk and writing goals for these phases of the unit. 

Talk and Writing Goals