IIT/Field Museum – High School Transformation Project
Chemistry
Conservation of Mass in Chemical Reactions
Chemistry: Matter and Change (Glencoe)
Unit 5 States of Matter & Earth’s Atmosphere
Chapter 13 States of Matter
Section 13.1 Gases
Section 13.3 Liquids and Solids
Context of Lesson
One of the most commonly held misconceptions among chemistry students is the belief
that things “disappear” or “disintegrate.” While this misconception is addressed early in most
chemistry courses, this is usually only done through a definitional approach. Accordingly, many
students will cling to their misconceptions and/or be unable to relate the law of conservation of
mass to an experimental or real world scenario. Moreover, the application of the concept, critical
for mastery understanding, occurs much later in the chemical reaction lessons. Thus, this
investigation reinforces the student learning about the Law of Conservation of Mass from
Chapter 3 and provides an opportunity for the mastery of Chapter 10 and Chapter 13 content.
One of the best ways to dispel any lingering misconceptions about the conservation of
mass, as well as link text and experience, is for students to develop their own procedure for an
investigation that will demonstrate the law. Because the students will be measuring the mass of a
gas, the results will typically vary somewhat based on the success of their procedures. This
provides an opportunity to review or teach investigation reproducibility and percent error.
This lesson is located as part of the chapter on states of matter to aid the students’
development of understanding that all matter has mass. Many students mistakenly think that
phase changes result in mass changes, especially when gases form. This lesson provides a
concrete experience to dispel that misconception.
Main Goals/Objectives
While studying the chemical reaction of seltzer tablets in water, students will develop a
procedure that demonstrates the Law of Conservation of Mass. From doing so, the students will
be able to:
 State the Law of Conservation of Mass
 Explain supposed violations of the Law of Conservation of Mass using the differing
behaviors of the states of matter in the explanations
 Develop and carry out a procedure, for a solid-liquid-gas reaction, that solves the posed
problem of containing the gas from the chemical reaction to obtain its mass
 Apply the Law of Conservation of Mass to chemical reactions involving solids, liquids,
and gases by examining written expressions for the reactions and explaining how the law
is demonstrated
 Explain how scientific theories are different from scientific laws: Scientific laws are more
likely observable patterns and scientific theories are explanations for observable patterns
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Explain that scientific knowledge should be based on empirical data
Explain that scientific investigations all begin with a question, but do not necessarily test
a hypothesis
Explain that inquiry procedures are guided by the question asked
Explain that there is no single scientific method and provide at least two different
methods
Explain that all scientists performing the same procedures may not get the same results
General Alignment to Standards
STATE GOAL 11: Understand the processes of scientific inquiry and technological design to
investigate questions, conduct experiments, and solve problems
A. Know and apply the concepts, principles and processes of scientific inquiry.
 11.A.2a Formulate questions on a specific science topic and choose the steps
needed to answer the questions
 11.A.3d Explain the existence of unexpected results in a data set.
 11.A.4c Collect, organize and analyze data accurately and precisely
 11.A.4e Formulate alternative hypotheses to explain unexpected results
STATE GOAL 12: Understand the fundamental concepts, principles and interconnections of
the life, physical and earth/space sciences
C. Know and apply concepts that describe properties of matter and energy and the
interactions between them
 12.C.4a Use kinetic theory, wave theory, quantum theory and the laws of
thermodynamics to explain energy transformations
 12.C.5a Analyze reactions (e.g., nuclear reactions, burning of fuel,
decomposition of waste) in natural and man-made energy systems
 12.C.5b Analyze the properties of materials (e.g., mass, boiling point, melting
point, hardness) in relation to their physical and/or chemical structures
Materials
Part 1
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Per student or team
Seltzer tablets
100 mL graduated cylinders
100 mL or 150 mL Beakers (2 per student or team)
Class set
Electronic Balances
Safety goggles for each student
Part 2
 Will vary based on students’ procedures and investigation designs; however, all
the materials from Part 1 will be needed again
 Any other necessary materials need to be supplied by the students
 Students often design investigations using balloons and zipper-type plastic bags
(quart or gallon), so having these ready will enable all students to participate
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Safety
 Safety goggles should be worn at all times
 All bag/balloon openings should be pointed away from people as various procedures
for part 2 sometimes produce unintended projectiles (rare and not powerful, but
overemphasize safety just in case)
 Clean all spills and overflows as they happen
 Keep balance pans clean and dry to ensure precision and accuracy
The Lesson
Day 1
Bell Ringer
As a review question from chapter 3, write the following on the board or overhead and ask the
students to answer either in their notebooks or on pieces of paper.
“What does the Law of Conservation of Mass state?”
Discuss answers to the bell ringer.
Ask a student to provide a one sentence statement of the Law of Conservation of Mass (LCM).
Review the difference between a Law and a Theory.
Ask a student to explain why Conservation of Mass is a law and not a theory.
Activity
Give each student a copy of the lab sheet “Conservation of Mass Lab” (attached at end of
lesson). After the students read the sheet, review the introductory paragraph and general
procedures for Part 1 of the lab. You may have to clarify the use of “conserve” but try not to be
too elaborate since the experience will guide them to the meaning; moreover, part of the
experience is to notice the loss of mass and relevant explanations.
Direct the students to complete part 1. Potential useful/necessary questions include:
 How did you calculate the mass of the reactants?
 How did you calculate the mass of the products?
 Was the LCM violated? Explain using evidence from your investigation.
Once a team has completed part 1, tell the team members to answer the question at the bottom of
the lab sheet, if they have not already done so.
When the question for part 1 is completed, instruct the students begin to work on their
procedures for part 2. While the students begin part 2, circulate and ensure that each team has
done part 1 successfully. This is a great opportunity to remind students that all investigations
begin with a question, but not all of them a test hypothesis. Ask: What question is guiding the
investigation? Why is testing a hypothesis not part of the investigation?
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Note: There will most likely be some confusion among the students as they try to determine their
objective for part 2. It is important that while you guide their inquiry, you do not provide them
with too much direction. As you circulate among lab teams, using some of the following
questions/ideas might be helpful:
 Was the LCM demonstrated in part 1? Why not?
 What information/data do you have that shows you that the LCM was not
demonstrated in part 1? Make sure the students develop a clear connection between their
statements and the data they have to support those statements. Be explicit about the
connection. Remind them that scientific knowledge requires empirical data and
conclusions require evidence.
 Can you explain your data outcome from part 1? What was responsible for the outcome?
 What do you need to do in order to demonstrate the law of conservation of mass?
 What should your data look like in part 2? Use your data from part 1 to guide you.
Depending on your students’ abilities and class length, most students will not complete both
parts in one period. Accordingly, the homework for Day 1 should be for the students to finish
their investigation plans and bring in any necessary supplies.
Assignment
Students need to complete the procedures for part 2 and bring in any necessary supplies based on
their team’s procedure. The students’ ideas work best if the instructor requires explicit steps to
the procedure.
Day 2
Bell Ringer
Instruct the students to put their procedures for part 2 on their desks for you to review. Then
allow the students to begin gathering materials for conducting their investigations. As students
gather materials for part 2, circulate to review and approve procedures. For teams which have
major flaws in their procedure, tell them you will return shortly and do so after checking all other
teams’ procedures. This allows as many teams as possible to get the necessary teacher approval
and begin their investigation. For a team that does not have an appropriate procedure, try to make
suggestions to bring them back on track (the most common problem students have with the
procedure is creating a system that will actually contain the gas). Based on time remaining,
provide additional help as necessary in order to allow all groups to complete the activity by the
end of day 2.
The students will work through part 2 in two stages. First, they will have to figure out
conceptually what they are trying to do experimentally. Secondly, they will have to work the
mechanics of how to successfully conduct the experiment.
Teacher Note: Determine the maximum mass the balance can handle and make sure the
students’ designs will not be over the maximum value. Some example procedures that might
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appear to work from the student perspective, but should not be allowed because of the possibility
of relatively large error (discuss issues with students to guide their modifications) include:
 Covering a beaker with something (paper/hand/plastic wrap) after dropping tablet in the
water (or adding the water to the tablet’s container).
 Capping bottle/flask after dropping tablet.
 Placing a balloon over a bottle/flask after dropping a tablet into the container.
 Squeezing tablet under plastic wrap.
 Be wary of zipper lock bag systems because they are usually too large for the balance and
hang off the balance pan resulting in poor precision and reproducibility.
 Generally any procedure in which tablet and water are mixed in a system in which there
is an opening in the system at the start or after the reactants are mixed.
(If teaching about experimental error is an important learning objective at this point in the
course, the above examples could be acceptable IF the students are required to explain how
their design increases the likelihood of measurement errors.)
Procedures that are generally acceptable (depending on your expectations and modifications):
 Tablet stuck to lid of plastic bottle (Gatorade type) then tapped off or bottle inverted.
 Tablet or water in a small container within a sealed larger container, then mixed.
 Tablet stuck to plastic wrap covering the bottle opening, then “flicked” off, but plastic
wrap has to be secured with taped or rubber band prior to release of the tablet.
 Same as above, but done with stoppered Erlenmeyer flasks; however, be aware that
sometimes the stoppers “pop,” so they may require securing.
 Tablet and water isolated in zip lock bags, one inside the other.
 Placing a balloon containing a tablet in small pieces over a bottle/flask then lifting to mix
reactants.
 Generally, any system in which the tablet and water are sealed in a closed system prior
to mixing and the reaction’s initiation.
Activity
Once approved students should conduct their investigations.
Students understand the LCM better if they actually see that there is no mass change while the
reaction is in progress. If you are comfortable that their system will not leak (water/solution),
have the students place the whole system on the balance immediately after initializing the
reaction. Ideally, the mass will remain constant throughout the reaction process.
Monitor their work to help them identify leaks and to praise their work.
Some systems will eventually leak, so if their system holds for longer than 10 seconds, that is
generally acceptable for obtaining the necessary data. Use any leaks that occur, regardless of
how long after the reaction begins the leak occurs, to discuss design and equipment as sources of
error in a scientific investigation.
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On 0.1 g accurate balance, students should observe no change in mass. On more sensitive
balances, you will need to predetermine the degree to which you will hold your students
accountable and the desired measurement precision.
Ask the students to compare their results from part 1 and part 2 and to explain why the results
differ.
Teacher Note: Some students, even if they seem to conceptually understand what they are trying
to accomplish, will still not be able to reconcile their differing outcomes. A common student
comment is: “I must have done it wrong because the mass did not change.” Students anticipate
that something should happen with regard to mass. An outcome of no change, in any
investigation, can be unsettling for some students. If time permits, allow/tell the student to run
another trial to confirm the data.
Ask students to return to their assigned seats for a whole class discussion. The discussion should
focus on the students’ success with their systems, sources of error, differences in systems, and
similar results. Important goals to explicitly address in the discussion are:
 Explain that there is no single scientific method and provide at least two different
methods that teams in the class used.
 Explain that all scientists performing the same procedures may not get the same
results. This construct is readily discussed using the results from part 1. If two
teams had very similar procedures for part 2 but obtained different results, use
those results as an additional example of this construct.
Suggested discussion questions include:
 Why did everyone get different results for the day 1 investigation, yet everyone
used the same methods?
 Why did almost everyone get the same results (no mass change) for the day 2
investigation even though each group used a different method?
 Why did the systems from day 1 seem to violate the LCM, but the systems from
day 2 did not?
 Using the results of this investigation as your evidence, why can we conclude that
all matter, including gases, has mass?
 What is different about gases, compared to liquids and solids, that makes
understanding that gases have mass more difficult than understanding that liquids
and solids have mass?
Assignment
Complete Conservation of Mass Lab sheet by answering remaining questions
Modifications/Accommodations
 To avoid students needing to bring equipment in, the instructor can simply make a large
variety of pre-selected equipment available to them and have them focusing on
determining how to utilize the equipment in their procedure. Each team could get a
different set of materials. Include a few items that will probably not be useful to them in
this investigation.
 Provide blank table to help guide students development of a data table for Part 2
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For students that struggle with writing, they could verbalize their ideas to another student
or instructor to write down.
Providing a paper with numbered lines for steps might help some students better
understand what they are doing in the planning stage.
Limited English Learners that struggle with writing in English could write their
procedure in their native language and then orally translate it for the instructor to
approve. The translation could also be done with the help of another student, teacher, or
aide that if fluent in the language.
Assessment
 Students’ answers to questioning during their investigations and planning
 Success completing the investigation from day 1 by demonstrating a loss in mass
 Students’ explanation for loss in mass from day 1 investigation
 Observations of investigations and planning throughout the process
 Student responses during whole class discussion
 Evaluation of completed Conservation of Mass Lab Analysis sheet
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Name __Key: Sample Answers_____________________ Date____________
Hour _____
Conservation of Mass Lab
According to the law of conservation of mass, matter is neither created nor destroyed during a
chemical reaction. Another way of saying this is that the total mass of the reactants in any
chemical reaction equals the total mass of the products. Sometimes the products of a chemical
reaction look different from the reactants. For example, if a gas is produced, you may not see it;
however, the gas’ mass must be taken into account when determining the total mass of the
products.
Purpose: To design a procedure to demonstrate the law of conservation of mass.
Materials:
Part 1: seltzer tablets; 100 mL graduated cylinder; 2 beakers; electronic balance
Part 2: To be determined by you and your partner
Procedure: Part 1
1. Place a seltzer tablet into a dry beaker.
2. Measure out 50 mL of water. Pour it into the second beaker.
3. Record on your data chart the total mass of the initial system (beaker with tablet and
beaker with water).
4. Slowly pour the water from the second beaker into the first beaker (do not let the reaction
foam over the edge of the beaker or you will need to start over).
5. After you have poured all the water in the first beaker, wait for most of the tablet to finish
reacting. Then measure and record on your data chart the total mass of the final system
(beaker with solution made by tablet with water and empty beaker).
6. Calculate the change in mass for the reaction (the difference between the mass of the
final system and the mass of the initial system) and place the result in your data chart.
7. Answer question 1 below the chart.
Data: Part 1
Object
Total Mass BEFORE reaction
(beaker with tablet and beaker with water)
Mass (g)
226.3 g
Total Mass AFTER reaction
(beaker with solution made by tablet with water
and empty beaker)
225.8 g
Change in mass for the reaction
0.5 g
1. Based on your data, did this reaction follow the Law of Conservation of Mass? Defend your
answer using evidence from the investigation. No, because there was a loss in mass of 0.5 g from
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before the reaction to after the reaction. If the law had been followed, there would have been no
mass change at all.
Part 2
Devise a procedure to show that mass IS conserved during the reaction between a seltzer tablet
and 50 mL of water. Determine the materials you will need to conduct your investigation in class
tomorrow. You may request to use any lab equipment or you may bring approved materials from
home/elsewhere.
Make sure that your procedure is clearly written and easy for any reader to understand.
After completing your procedure, create a data table for your investigation.
Your procedure must be approved by your teacher before you begin your investigation.
Procedure: Part 2
1.
2.
3.
4.
5.
6.
Put the seltzer tablet in a snack size zipper bag and seal the bag well.
Put 50 mL of water in a gallon size zipper bag.
Put the bag with the seltzer tablet in it inside of the large bag with the water in it.
Seal the large bag. Make sure the seal is completely closed.
Measure the mass of everything and record the value as the mass before the reaction.
Keeping the large bag sealed, carefully pull open the small bag and drop the seltzer tablet
into the water.
7. When the reaction has slowed down, measure the mass of everything and record the
value as the mass after the reaction.
8. Calculate the difference between the mass of everything before the reaction and the mass
of everything after the reaction.
Teacher Approval: _Mrs. Teacher_________
Part 2 Data Table:
Mass of everything BEFORE reaction
78.9 g
Mass of everything AFTER reaction
78.9 g
Change in mass
0g
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Name __Key: Sample Answers____________________
Date____________
Hour _____
Conservation of Mass Lab Analysis
Part 1
1. What evidence was there that a chemical change took place when you mixed the tablet
with the water?
There were bubbles coming out so a gas was formed as part of the reaction. When a new
substance is formed, like the gas, a chemical change occurred.
(Common Student Errors: 1. Some students may state that the tablet (solid) dissolving in
the water (liquid) is indicative of a chemical change. This is incorrect because dissolving
is a physical change. 2. Another possible incorrect response is that the liquid was boiling.
Boiling is a physical change; moreover, there was no indication of heating to make the
water boil.)
2. Why was there a loss of mass for the reaction in Part 1?
The gas that was formed as part of the reaction between the water and the seltzer tablet,
escaped from the beaker. The loss of the gas from the system resulted in the loss of mass.
3. Does the reaction actually violate the Law of Conservation of Mass? Explain.
No, the reaction does not violate the Law of Conservation of Mass because the gas was
allowed to escape from the system. The Law requires that all substance stay in the
system. If the beaker had a cover or if there was some other means of containing the gas
so it did not escape, the difference in mass between the mass before and the mass after
the reaction would have been zero. That would have shown the Law of Conservation of
Mass was true.
4. How do the results from Part 1 of this investigation indicate that gases have mass?
Since the gas was the only substance that escaped from the beaker, then the loss of mass
had to be due to the loss of the gas from the system. If gases did not have mass, then the
mass would not have decreased as a result of the gas escaping from the system.
Part 2
1. What problems did you encounter in your new procedure and what could you have done
to improve the procedure?
The answers to this question will vary based on the procedure developed and materials
used by the students. For the procedure above, the students might have had difficulty
opening the small bag containing the seltzer tablet through the larger bag. An
improvement would be to leave the small bag partially unsealed, but making sure the
opening stayed above the water until the reaction was ready to be started. Then the small
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bag would only need to be turned over to drop the tablet into the water or pushed into the
water to let the water into the opening.
2. What did you have to plan for in your Part 2 procedure that was missing in the Part 1
procedure?
We needed to plan for the gas being produced and making sure that it did not escape the
system.
3. How do you know if the reaction demonstrated the law of conservation of mass?
Since our system did not allow any components to escape and we found no difference in
mass between the before the reaction mass and the after the reaction mass, we know that
the reaction demonstrated the law of conservation of mass.
4. In Part 2, students often design and build perfect systems that contain the reaction; yet,
many of them determine the change in mass for the reaction to be 0.1g. If no gas escaped
the system and the students made no arithmetic mistake, how could you account for the
small change in mass?
The small change in mass is probably due to the precision of the balance. Even though
the masses for before and after the reaction should have been the same, the balance may
not have the precision to report the two measurements as the same, especially since they
were not done at the same time and other students used the balance in between our
measurements.
While the above is the best answer, other possible correct answers that essentially address
the same issue are: 1. The calibration of the balance shifted over time and 2. Changes in
temperature in the room or from student use caused shifts in either the calibration or the
zero point of the balance. If students provide these answers, review precision and discuss
how these factors are components of precision of the balance.
5. Why was it scientifically acceptable for each student or group to design and carry out
their own unique investigation in Part 2?
There is no one scientific method. Therefore, different students could use different
procedures, designs, and materials to answer the same scientific question.
6. Using the characteristics of a solid, explain why a seltzer tablet is considered to be a
solid.
Solids have definite shape and definite volume. The shape and volume of each tablet is
not changed by the container it is in and it will not flow like a liquid or gas. These
characteristics occur because the intermolecular forces among the particles of a solid are
so strong that the molecules that make up the tablet can only vibrate in a fixed position.
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(A common error by students is to view the solid tablets as a group. The will argue that
the tablets will take the shape of the container and could flow out of the container if there
are many tablets in the container. This issue is best clarified by comparing this idea when
discussing liquids and gases. With liquids and gases, different samples will behave the
same with regard to containers and flowing, while this is not true for solids. To help
minimize the likelihood of this misconception from this question, the question asks about
an individual tablet.)
7. Using the characteristics of a liquid, explain why water is considered to be a liquid.
Liquids have definite volume and take the shape of their containers. The 50 mL of water
did not change in volume when it was transferred from the graduated cylinder to the
beaker. The shape of the liquid did change, however, as it took the shape of the graduated
cylinder when it was in there and took the shape of the beaker when it was in there. Like
a liquid, the water also flowed when it was poured from one container to the next. These
characteristics occur because the intermolecular forces among the particles that make up
the liquid are not strong enough to hold the molecules in fixed positions, but the forces
are strong enough to limit the molecules range of motion.
8. Using the characteristics of a gas, explain why you know that a gas was produced by the
water and seltzer tablet reaction.
Gases take the shape of their containers and will change in volume to occupy as much
space as possible. When the gas was produced in Part 1, it escaped from the system and
kept spreading out as far as it could because it was not contained in the system. The gas
probably filled our classroom and took the shape of our classroom, but we could not see
it to be sure. In Part 2, the gas was contained in the system by our large plastic zipper
bag. We knew it was taking the shape of the bag and filling the entire volume of the bag
because the bag inflated from the gas. These characteristics occur because the
intermolecular forces among the particles that make up the gas are too weak to hold the
molecules together or limit the molecules range of motion.
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Name _______________________________
Date____________
Hour _____
Conservation of Mass Lab
According to the law of conservation of mass, matter is neither created nor destroyed during a
chemical reaction. Another way of saying this is that the total mass of the reactants in any
chemical reaction equals the total mass of the products. Sometimes the products of a chemical
reaction look different from the reactants. For example, if a gas is produced, you may not see it;
however, the gas’ mass must be taken into account when determining the total mass of the
products.
Purpose: To design a procedure to demonstrate the law of conservation of mass.
Materials:
Part 1: seltzer tablets; 100 mL graduated cylinder; 2 beakers; electronic balance
Part 2: To be determined by you and your partner
Procedure: Part 1
1. Place a seltzer tablet into a dry beaker.
2. Measure out 50 mL of water. Pour it into the second beaker.
3. Record on your data chart the total mass of the initial system (beaker with tablet and
beaker with water).
4. Slowly pour the water from the second beaker into the first beaker (do not let the reaction
foam over the edge of the beaker or you will need to start over).
5. After you have poured all the water in the first beaker, wait for most of the tablet to finish
reacting. Then measure and record on your data chart the total mass of the final system
(beaker with solution made by tablet with water and empty beaker).
6. Calculate the change in mass for the reaction (the difference between the mass of the
final system and the mass of the initial system) and place the result in your data chart.
7. Answer question 1 below the chart.
Data: Part 1
Object
Total Mass BEFORE reaction
(beaker with tablet and beaker with water)
Mass (g)
Total Mass AFTER reaction
(beaker with solution made by tablet with water
and empty beaker)
Change in mass for the reaction
1. Based on your data, did this reaction follow the Law of Conservation of Mass? Defend your
answer using evidence from the investigation.
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Part 2
Devise a procedure to show that mass IS conserved during the reaction between a seltzer tablet
and 50 mL of water. Determine the materials you will need to conduct your investigation in class
tomorrow. You may request to use any lab equipment or you may bring approved materials from
home/elsewhere.
Make sure that your procedure is clearly written and easy for any reader to understand.
After completing your procedure, create a data table for your investigation.
Your procedure must be approved by your teacher before you begin your investigation.
Procedure: Part 2
Teacher Approval: _________________
Part 2 Data Table:
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Name _______________________________
Date____________
Hour _____
Conservation of Mass Lab Analysis
Part 1
1. What evidence was there that a chemical change took place when you mixed the tablet
with the water?
2. Why was there a loss of mass for the reaction in Part 1?
3. Does the reaction actually violate the Law of Conservation of Mass? Explain.
4. How do the results from Part 1 of this investigation indicate that gases have mass?
Part 2
1. What problems did you encounter in your new procedure and what could you have done
to improve the procedure?
2. What did you have to plan for in your Part 2 procedure that was missing in the Part 1
procedure?
3. How do you know if the reaction demonstrated the law of conservation of mass?
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4. In Part 2, students often design and build perfect systems that contain the reaction; yet,
many of them determine the change in mass for the reaction to be 0.1g. If no gas escaped
the system and the students made no arithmetic mistake, how could you account for the
small change in mass?
5. Why was it scientifically acceptable for each student or group to design and carry out
their own unique investigation in Part 2?
6. Using the characteristics of a solid, explain why a seltzer tablet is considered to be a
solid.
7. Using the characteristics of a liquid, explain why water is considered to be a liquid.
8. Using the characteristics of a gas, explain why you know that a gas was produced by the
water and seltzer tablet reaction.