Lab: Conservation of Mass

Leif Eggen Cloquet Middle School

Summary

In this lab students will conduct an investigation to find out if substance either lose or gain mass after a chemical reaction takes place. Students will combine (and measure the mass of) 200 ml of water with a measured mass of Alka-Seltzer. They will add these two masses together BEFORE combining them. Students will then combine the two substances together in a capped bottle and record observations. Students will wait for the reaction to finish, then measure mass of the system. Students will then answer lab questions, and balance the chemical equation for this reaction.

Learning Goals

Students will observe that mass is not created or destroyed during a chemical reaction.

Students will balance a chemical equation accounting for all atoms in the reaction.

Students will measure mass and volume of a liquid and a solid.

Key concepts: A closed system is one in which the reaction is isolated from its environment.

An open system is a system that can interact with its environment.

A physical change when a substance changes state.

A chemical change is when a substance interacts with another in the presence of energy to change chemically.


Key vocabulary terms: closed system, open system, physical change, chemical change

Context for Use

This lab should be conducted in groups of two.
Students should have 45 minutes to conduct the activity (each group should do it twice).
Equipment and supplies needed: Balance, graduated cylinder, Alka-Seltzer tablets, empty bottle with a cap.
Students should be familiar with balancing simple chemical equations and understand the concepts of reactants and products.
I integrate this lab into the FOSS sequence on chemical reactions.
This activity could be done in a classroom setting, just stock up on paper towels!

Subject: Chemistry:General Chemistry:Chemical Reactions
Resource Type: Activities:Lab Activity
Grade Level: Middle (6-8)

Description and Teaching Materials

LAB: CONSERVATION OF MASS

Minnesota State Academic Science Standards Addressed:
6.2.1.2: Substances can undergo physical changes which do not change the composition or the total mass of the substance in a closed system.
Benchmarks:
- Describe how mass is conserved during a physical change in a closed system.
- Identify evidence of physical changes, including changing phase or shape, and dissolving in other materials.
6.1.3.1: Designed and natural systems exist in the world. These systems consist of components that act within the system and interact with other systems.
Benchmark:
- Distinguish between open and closed systems.

Problem of Study: Do substances lose or gain mass during chemical reactions?

Materials:
balance
graduated cylinder
Alka-seltzer tablets
empty bottle with cap

Procedure and Results:
1. Measure 200ml of water and carefully pour it into the empty plastic bottle. Recap the bottle.
2. Measure the mass of half an Alka-Seltzer tablet and the plastic bottle containing 200ml of water with the cap on.

Mass of the "system" BEFORE combining. System is ( bottle with cap, alka-seltzer and water ) = __________ grams (1 pt.)

3. With the bottle on the lab bench, place a half tablet in the bottle and IMMEDIATELY screw the cap on VERY tightly.
What do you observe? (3 pts.) ____________________________________________________________________________________________________________________________________

4. Place the closed bottle back on the balance and measure its mass it AGAIN.
Mass of the "system" with closed cover (CLOSED system): __________ grams (1 pt.)

- Wait until the Alka-seltzer tablet has completely dissolved. Place the bottle in the white tub and SLOWLY loosen the cap and leave the bottle uncapped. Wait 30 seconds. DO NOT SPILL!
- Place the cap and filled bottle back on the balance and find the mass of the "system".
5. Mass of the "system" (bottle, cap, alka-seltzer and water) with the cap off – (OPEN system): ______________ grams (1 pt.)
Questions:
1. Did a physical change occur at any time during this experiment?______ What evidence do you have of that? (3 pts.)


2. Did a chemical change occur at any time during this experiment? _____ What evidence do you have of that? (3 pts.)


3. Was the mass of the closed bottle system AFTER the chemical reaction (step 4) the same as the mass of the uncombined system, BEFORE the reaction (step 2)? ______
If not, attempt to explain why not. (1 pts.)



4. After the chemical reaction, was the mass of the closed bottle system (step 4) the same as the open bottle system? (step 5) ______ If not, attempt to explain why not. (1 pt.)


5. During a chemical reaction does the total amount of mass change? (1 pt.) _____
During a chemical reaction, can the form of matter change? (1 pt.) ______

6. What would happen to the mass of a package of firecrackers set off inside a closed steel box? (3 pts)



Follow-up Review:
The chemical formula for this reaction is:

H2O + Na(HCO3) à CO2 + NaOH + H2O

In the chart, indicate the number of atoms of each element which are shown on the Left and Right sides of the chemical equation. (8 pts.)

ELEMENT Atoms on the left: Atoms on the right:

Carbon (C): ______________ ___________
Hydrogen (H): ______________ ___________
Oxygen (O): ______________ ___________
Sodium (Na): ______________ ___________

Teaching Notes and Tips

Review chemical equations (balancing) before doing this lab. That way students can balance the equation at the end on their own.

Monitor students closely to ensure each group CLOSES the bottle cap tightly!

Stock up on plenty of Alka-Seltzer so students can perform the experiment a second time (they SHOULD do this)

This activity is different because I incorporated the terms "open" and "closed" system into the investigation.

Assessment

A lab sheet to be completed during the lab AND as homework worth 27 points.

Formative teacher observation of students performing the activity. (i.e. measuring correctly, following scientific writing on lab sheet.)

Questions:
1. Did a physical change occur at any time during this experiment?______ What evidence do you have of that? (3 pts.)


2. Did a chemical change occur at any time during this experiment? _____ What evidence do you have of that? (3 pts.)


3. Was the mass of the closed bottle system AFTER the chemical reaction (step 4) the same as the mass of the uncombined system, BEFORE the reaction (step 2)? ______
If not, attempt to explain why not. (1 pts.)



4. After the chemical reaction, was the mass of the closed bottle system (step 4) the same as the open bottle system? (step 5) ______ If not, attempt to explain why not. (1 pt.)


5. During a chemical reaction does the total amount of mass change? (1 pt.) _____
During a chemical reaction, can the form of matter change? (1 pt.) ______

6. What would happen to the mass of a package of firecrackers set off inside a closed steel box? (3 pts)



Follow-up Review:
The chemical formula for this reaction is:

H2O + Na(HCO3) à CO2 + NaOH + H2O

In the chart, indicate the number of atoms of each element which are shown on the Left and Right sides of the chemical equation. (8 pts.)

ELEMENT Atoms on the left: Atoms on the right:

Carbon (C): ______________ ___________
Hydrogen (H): ______________ ___________
Oxygen (O): ______________ ___________
Sodium (Na): ______________ ___________

Standards

6th Grade Science Standard: 6.2.1.2: Physical changes in a closed system

6th Grade Science Syandard: 6.1.3.1: Designed and natural systems

References and Resources