1. Activity 25: Conservation of Mass

2. Read B-64
Challenge:
How does the total mass of the reactants compare to the total mass of the products in a chemical reaction?

3. Procedure
Follow procedure on pgs. B-65 to B-66

4. Data/Evidence:
Student Sheet 25.1

5. Which system is a closed system, and which is an open system?
Open System: the contents of the jar can mix with the air surrounding the outside of the jar.
Closed System: the chemical reaction is contained inside of the jar - all reactants and products stay in the jar.
Today, it is not an absolutely closed system - some heat and light will be able to escape.

6. Can of Soda When it is sealed, it is a closed system.
When it is opened and the gases are released it is an open system with the air around the can.

7. What type of system is a person - open or closed?
Open System - we take in food, oxygen, and water, from outside our bodies and excrete waste, carbon dioxide, and other substances.
In reality, there are very few absolutely closed systems in our universe.
Our planet, for example, is an open system in that it exchanges mass with the solar system and galaxy.

8. What is mass? How much matter is in something.
The greater the mass, the more of the substance you have.

9. Chemical Reactions Signs of a Chemical Reaction:
Change in Color of Products
Production of Gas
Emission of Light or Heat
Appearance of a New Solid (Precipitate) or Liquid.
These changes in physical appearance or characteristics of the reactants are a way to visually detect a chemical change.
Change at the atomic level is not visible directly.

10. Demonstration: Open System
1 mL of copper chloride solution and 1 aluminum washer in the bottle.
What do you think will happen to the mass of the system - the bottle and reactants - as the reaction occurs?
Will it increase, decrease, or stay the same?

11. Exothermic Reaction
Releases energy as the reactants interact chemically.
This is felt as heat.
Other exothermic reactions:
Combustion (Burning) and Rusting (Oxidation).
Do not grip your reaction chambers while conducting the reactions in closed systems.

12. What evidence have you observed that indicates a chemical reaction occurred?
Formation of a new solid
Formation of bubbles
Change in color of the solution
Heat generated
What happened to the mass?

13. You will also…
Conduct a second reaction with 1 mL 100,000 ppm copper chloride solution and 4 mLs of sodium hydrogen phosphate solution.
The reaction will be complete in approximately two minutes.

14. Lab: Closed System Use the balances carefully - be accurate!
Fix the lid onto the jar before beginning the reaction.
Place the reaction chamber on the table - do not keep it on the balance.
Do not let the substances mix before you have done the initial measurement of mass.
Press your thumbs against the lids while inverting the container to start the reactions.
Record detailed observations - how do you know a reaction occurred?

15. Discussion Record change-in-mass calculations
What trend do you see in the data?
What is this telling us about what is happening to the mass of the reactants as compared to the mass of the products?
What could explain why the mass may have changed?
How can we reduce error in this activity?

16. Law of Conservation of Mass
Mass cannot be created or destroyed in a chemical reaction, but it can change form.
Mass stays the same in an open system and a closed system, but in an open system, the gas can escape.
Toxic substances that will have negative effects on humans, animals, or the environment, will still be present after a chemical reaction.

17. Analysis Question #2
The Law of Conservation of Mass goes against our perception of what we experience every day.
When we observe food cooking or wood burning, it appears that mass is lost.
This is because one of the products formed in these reactions is a gas we cannot see.
If we could collect all of the gas and water produced when cooking or burning, we would find that mass is conserved.