Activity 3.4 - Molecular Models for Soda Water Fizzing (45 min)

Tell students that atoms are too small to see, so scientists keep track of them in other ways. Write on the board “carbonic acid molecule” and the formula “H₂CO₃” and explain that this is the molecule that is responsible for the fizz in soda water. Tell students that the formula means that there are two hydrogen, one carbon and three oxygen atoms in the molecule.

Explain to students that they will use the molecular models to figure out how carbonic acid can break up into molecules of water and carbon dioxide when soda water loses its fizz.

Divide the class into pairs and give each pair a molecular model kit and Molecular Models 11 x 17 Placemat . Pass out one copy of 3.4 Molecular Models for Soda Water Fizzing Worksheet to each student.

• Use slide 3 of the 3.4 Molecular Models for Soda Water Fizzing PPT to explain the bonding of atoms in molecules. Tell students that the rules on this slide are important because they apply to all molecules that they will make in all Carbon TIME units
• Use Slide 4 to show instructions to construct a molecule of carbonic acid. Students can also follow instructions in Part A of their worksheet.
• Use Slide 5 to instruct students to compare their own molecule with the picture on the slide.
• Slide 6 shows an important message: after students create their reactant molecules, make sure they put away all unused pieces of their molecule kits. This helps reinforce that the matter and energy in the reactants are conserved through the chemical change, and that only the materials from the reactants are used to build the products.

Tell students to follow the instructions the worksheet to construct their Products.

• When soda water sits in the open, a chemical process occurs. Releasing the pressure on the soda water and allowing it to warm up allows the carbonic acid to decompose into carbon dioxide (CO₂) and water (H₂O).
• Show slide 7 of the PPT and have students re-arrange the atoms to make molecules of CO₂ and H₂O. To do this, they will need to move their molecules from the Reactants side to the Products side of the Molecular Models 11 x 17 Placemat. Explain to students that atoms last forever, so they should not add or subtract atoms when they change the reactant molecule into product molecules.
• Show students Slide 8 to compare the products and a comparison between reactants and products.
• Show students Slide 9 to compare the molecules they made to the molecules on the slide.

Show slides 10-14 in the PPT to encourage students to make connections between what is happening in the animation and the molecular models they made.

• For each slide, focus on different atoms in the molecule. The animation draws attention to where they atoms begin and end in the reaction.

Show slide 15. Tell students to complete Part C of their worksheet to trace the atoms during the chemical change.

Have students verify that the number of atoms before and after remained constant: Atoms last forever! Tell students that this means that the number of atoms before and after the reaction does not change.

Tell students that now that they have represented a chemical change using molecular models and in animations, they will represent chemical change by writing the chemical equation.

• Show Slide 16 of the presentation to guide students through the process of writing a balanced chemical equation for the decomposition of carbonic acid. Indicate that the decomposition of carbonic acid is a chemical reaction and that the chemical equation is a representation of this reaction. Tell students that these rules apply to all chemical reactions.
• Tell students to write their equations in Part D of their
• Have students write their own chemical equations before comparing them with the one on Slide 17.