S&S Lesson 3 Tab 3

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 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?)

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 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 soda water fizzes. 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 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