1. The reactants in the first reaction are .

2. What happens to the mass of an oxygen atom during a chemical reaction?

• The mass doesn’t change
• It depends on the reaction
• The mass decreases
• The mass increases

3. In the first reaction, sunlight causes oxygen molecules to break apart. This is an example of a reaction that energy.

4. Which statement accurately describes how a chemical change differs from a physical change?

• The chemical composition changes during a chemical change.
• Chemical changes involve changes in the state of matter.
• Chemical changes can be reversed.
• Chemical changes occur when energy is added.

5. The products of the first reaction are

6. One of your classmates is working on their model of ozone reactions and says, “You have to destroy ozone in order to make new ozone”. Do you agree with this statement, why or why not?

• Yes, Ozone gets destroyed from the sun’s rays and then new atoms are formed when the new molecule of ozone is made.
• Partially, the atoms from the beginning molecule get destroyed but it is considered oxygen, not ozone. The new atoms that are created then form together to make ozone.
• No, you have to break apart oxygen molecules to then combine them with more oxygen to make ozone. The oxygen that is broken apart isn’t considered ozone.
• No, when ozone gets broken apart it isn’t destroyed, the atoms just separate and then recombine.

7. Which of the following describes what happens during a chemical reaction?

• The type of atoms within molecules can change and form new molecules.
• Molecules can break apart and the atoms can change mass to form new molecules.
• The atoms in molecules can be destroyed and no longer exist.
• Molecules can break apart and the atoms can rearrange and form new molecules.

8. If 5g of O2 molecules broke apart into individual oxygen atoms. How many grams of oxygen atoms would be formed?

• 10g
• 2.5g
• 5g
• 4g

9. What would we expect to see in the products and reactant side of the reaction?

• a gain in mass on the product side
• a gain in atoms on the product side
• the same number of atoms on both sides
• more atoms on the reactant side

10. What is true of all the ozone molecules that are created in this reaction?

• they all share the same properties
• it depends on how they were formed
• all choices are correct
• they are all uniquely different ozone molecules

11.
You were doing some additional reading about the formation of ozone and found the reactions written in the form of the equations above. Does this representation accurately show that mass is conserved in each reaction, why or why not?

• Yes, because the number of oxygen atoms in reaction 1 is equal to the number of oxygen atoms in reaction 2.
• No, because the number of oxygen atoms in the reactants isn’t equal to the number of oxygen atoms in the products for each reaction.
• Yes, because the number of oxygen atoms in the reactants are equal to the products for each reaction.
• No, because the number of oxygen atoms in reaction 1 is less equal to the number of oxygen atoms in reaction 2.

12. Which model in figure A does a better job at communicating that oxygen atoms all have the same mass and why?

• Ivan’s model because all the circles represent one oxygen and they are drawn to be the same size.
• Ivan’s model because the number of oxygen atoms is consistent throughout each step.
• Sasha’s model, because the smallest circles represent individual atoms of oxygen that are the same size.
• Sasha’s model because atoms that contain more oxygen are larger in size than atoms containing fewer oxygen.

13. In the second part of the reaction, if 2 molecules of ozone are products, how many different atoms of oxygen do we need as reactants?

• 4
• 8
• 2
• 6

14. Your friend is analyzing models for the ozone reactions and thinks the mass of the atmosphere must be increasing since more ozone is being made. How could you use a particle model to help them clarify their thinking?

• A model could help them see that the atoms that form ozone have always been in the atmosphere. Increasing the ozone actually removes some of the atoms by creating larger molecules, which would decrease the mass not increase it.
• A model could help them see the atoms that form ozone have always been in the atmosphere. Increasing ozone doesn’t increase the mass, it just moves atoms of oxygen between molecules.
• A model could help them see that the atoms that form ozone are added in the form of sunlight. Since there are more atoms after the sun breaks apart O2, there is more mass in the atmosphere.
• A model could help them see that the atoms that form ozone get larger when they are bonded together. As the size of the atoms increases the molecule takes up more space, but the mass doesn’t change.

15. Two students are comparing their models for the ozone reactions. One A showed the first reaction with 12 oxygen molecules in the reactants, while student B showed 6 oxygen molecules in the reactants. They don’t think both of their models can accurately show conservation of mass. Do you agree or disagree and why?

• Disagree, it is possible for them both to show how the oxygen molecules in the reactants form ozone, student A will be able to form 4 ozone molecules while student B will form 2.
• Disagree, it is possible for them both to show how oxygen molecules break apart and then form ozone. Both students can form 2 ozone molecules, and student A can just take out any leftover oxygen.
• Partially agree, the masses of each student’s reactions will be different based on their different starting amounts. In order for student B to show conservation of mass, they will have to add in extra oxygen atoms in the products.
• Agree, if student A starts with more oxygen molecules in the reactant, their model will show more mass in the products than student B.

16. Refer to figure A. Which of the following revisions could be added to one of the models to improve how the model shows the Law of Conservation of Mass?

• Sasha’s model could have the circle labeled O2 be the same size as the circle labeled O3 so they clearly have the same mass.
• Sasha’s model could have a third circle labeled O after reaction 1 to show where the three atoms that form O3 came from.
• Ivan’s model could have 2 additional atoms of oxygen in step B so it is clear where the 6 atoms of oxygen came from in step C.
• Ivan’s model could have two additional molecules of O2 in step A and B, so there are 6 atoms of oxygen present throughout each step.

17. Refer to figure A. Your teacher tells you one of the models has errors that will make it difficult to accurately show changes in matter that occur during the reaction. Which statement identifies one of these errors?

• The product formed in Sasha’s model is different from the reactant.
• The oxygen atoms at step B of Ivan’s model are not bonded together.
• The product formed in Ivan’s model has 2 molecules of ozone that each contain 3 atoms of oxygen.
• Sasha’s model uses one particle to represent molecules that are made up of multiple particles.

18. The “hole” in the ozone layer is in part caused by reactions involving chlorine that is present in the atmosphere from harmful chemicals. Chlorine atoms (Cl) react with ozone molecules (O₃) to produce molecules of O₂ and molecules of ClO. Create a model to show this process.

19. Desean says, “Since the products of the reaction are both molecules instead of individual atoms, they are going to have more mass than the original reactants (chlorine and oxygen).” Use your model to explain why you agree or disagree with Desean’s claim.

20. Draw a model to show both steps of this reaction. Your product at the end of the second step should be 2 molecules of Ozone.

21. How does your model demonstrate the idea that mass is conserved in a chemical reaction?

22. Describe how your model shows the products and reactants from the first reaction as well as the products and reactants from the second reaction.