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Activity 2 Keeping Track of Stuff in a Closed System
MASS CONSERVATION
Activity 2: Keeping Track of Stuff in a Closed System
Shut the door behind you…or else!
Some years ago, a fascinating and unique experiment took place in the Arizona
desert. The building in which this experiment took place is an entirely enclosed multidomed structure. There are no plumbing or electrical facilities coming into or going
out of the building. There are many windows that allow sunshine to flood into the
building, but the windows do not open. The name of the structure
is “Biosphere.”
In 1991, a colony of 8 people set about to live inside for two years. The central idea
behind the experiment was to learn how to create and inhabit a self-sufficient
environment that could be the home
for humans on the Moon or Mars.
Answer these questions based on the reading.
2. What would happen to the balance of this system if some of the mass in this
system slowly leaked out? For example, what would happen if carbon dioxide
were slowly escaping to the outside environment?
3. Compare and contrast the planet Earth with the Biosphere.
Activity 3: Mass and Open Systems
3.2 g
Team 2
2.6 g
Team 3
4.8 g
Team 4
1.3 g
Team 5
0.0 g
Team 6
3.2 g
Team 7
2.8 g
Average
2.6
2.6 g
1. The amount of mass in the
Biosphere will remain the same,
because a closed system has
nothing added to, nor removed
from, the system. Even after
interactions, the mass of a closed
system remains the same.
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Unit 4 • Chapter 1
Activity 2 Practice
3. The Earth and Biosphere are both
essentially closed systems for mass
(except for the Earth’s relatively tiny
mass output of space equipment
like space probes, and mass input
from meteors.) Both systems will
support life: plants, animals, etc.
However, Earth would support a
far greater diversity of life because
of its many varied climates and
biomes (i.e., grasslands,
rainforests, desert).
Imagine that another class did the same experiments your teacher did with the baking
soda and vinegar. Assume that their experiments were fair tests. The table at left
shows the mass change recorded by each of the
teams after they mixed the baking soda and
Table: Baking Soda
vinegar, and let out the gas that was produced.
and Vinegar Mixture
The uncertainty of the mass scale used to make
Team
Mass Change
the measurements was 0.1 g.
Team 1
Activity 1 No
Practice
2. The balance of the system would
likely be disrupted, at least
temporarily. For example, a loss of
carbon dioxide would likely
adversely affect the plants’ growth,
which may in turn affect the
animals (and the humans) that feed
off of them.
1. If the Biosphere is a closed system for mass, will the amount of mass in the
Biosphere increase, decrease, or remain the same over a long period of time?
Write your reasoning.
The example on the next page shows how How
To Evaluate an Experiment Conclusion was used
to evaluate the conclusion of one of the
students, Nguyen.
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PRACTICES—ANSWERS
INTERACTIONS AND CONSERVATION
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Activity 3 Practice
1. Yes, the experiment is a fair test.
Yes, each supporting reason is
based on evidence, not opinion.
Example: Nguyen wrote the following conclusion and reason:
Conclusion: We concluded that, when air is released from the bag with the baking
soda/vinegar mixture, the mass of the bag decreases.
No, the supporting reasons use all
of the available evidence (data),
not just part of the evidence.
Reason: I think air has mass. Therefore, letting gas out of the bag must decrease
its mass.
Use the Criteria
Evaluate Nguyen’s Conclusion
Valid/Not valid: Jason's conclusion
is not valid
Check all the criteria for a valid
conclusion that you think apply.
Yes No
The experiment is a fair test.
Valid/Not Valid:
Reason: Jason just uses part of the
available evidence. He only
mentions his teams’ data. He
should include all of the class data.
Yes No
Each supporting reason is
based on evidence, not
opinion.
Yes No
The supporting reasons use
all the available evidence
(data), not just part of the
evidence.
Reason:
Nguyen only gives his opinion as
a reason for his conclusion. This
experiment was a fair test and
produced lots of data that supports
his conclusion, but he doesn’t talk
about the data at all.
2. Yes, the experiment is a fair test.
Nguyen's conclusion is not valid
(Questions 1-3) Use How To Evaluate an Experiment Conclusion
to determine if the following conclusions are valid or not valid.
Create and complete an answer box similar to the example above for
each question below.
Yes, each supporting reason is
based on evidence, not opinion.
No, the supporting reasons use all
of the available evidence (data),
not just part of the evidence.
Conclusion: When air is released from the bag with the baking
soda/vinegar mixture, the mass of the bag does not change.
Valid/Not Valid: Isabel's
conclusion is not valid.
2. Isabel wrote the following conclusion and reason:
1. Jason wrote the following conclusion and reason:
Reason: Our result (Team 5) was that the mass change
(Start Mass – End Mass) was zero after we let gas out of the bag.
Conclusion: We concluded that, when air is released from the bag with the
baking soda/vinegar mixture, the mass of the bag decreases.
Reason: Isabel uses reasons from a
different observation. She should
use the data from the class
experiments to back up her claim.
Reason: Air has mass. I know this because the mass of the soccer ball increased
after the teacher pumped air into it.
3. Chantel wrote the following conclusion and reason:
Conclusion: We concluded that, when air is released from the bag with the
baking soda/vinegar mixture, the mass of the bag decreases.
Reason: In our experiments with the bag, the class average for the mass change
(Start Mass – End Mass) was –2.6 grams. Since a decrease in mass of 2.6 grams is
greater than the 0.1 gram uncertainty, the decrease in mass is significant.
3. Yes, the experiment is a fair test.
Yes, each supporting reason is
based on evidence, not opinion.
Yes, the supporting reasons use all
of the available evidence (data),
not just part of the evidence.
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Valid/Not Valid: Chantel's
conclusion is valid.
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Reason: Chantel uses all of the
data available, and bases her
conclusion on evidence observed
(while taking into account the
uncertainty). Also, the experiment
is a fair test.
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Activity 4 Keeping Track of Volume in a Closed System
MASS CONSERVATION
Activity 4: Keeping Track of Volume in a Closed System
Read the conversation between
students from another class.
1. What happens to the amount of
mass in a closed system even after
interactions have occurred? (You
explored this in Activity 2.)
I was surprised in
Activity 2. I thought the
solutions would have
more mass after the
solid formed.
2. What can happen to volume in a
closed system after interactions
have occurred?
I was really surprised that
the volume of the alcohol
and water mixture was
less than the volume of the two
separate liquids. I thought
the two liquids would combine
and take up the same
amount of space.
3. In Activity 2, you were asked the
question, “How is it that scientists
ever come to accept an idea?”
You may wish to review the
conversation in Activity 2 among
the four team members—Rebecca,
Carlos, Otis, and Xuan. Write a
paragraph with the best answer
you can give to the question at
this time.
Activity 5: Interactions and Mass
Hot and Cold Packs
(Questions 1-3) Perhaps you have used an instant cold pack or hot
pack for an injury or to soothe your sore muscles. There are two liquids
inside the pack that are separated from each other. For the interaction
between the two liquids to take place inside the pack, you must squeeze
the pack to break the barrier between the liquids. Once the barrier is
broken, the two liquids can interact. If it is a cold pack, the pack will get
really cold; if it is a hot pack, it will become quite warm.
1. Is the pack an open system or a closed system? Write your reasoning.
2. After the interaction between the two liquids in the pack, does the
amount of mass in the pack increase, decrease, or stay the same?
Write your reasoning.
3. How would you set up an experiment to find out if the mass of the
pack increases, decreases, or stays the same?
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INTERACTIONS AND CONSERVATION
Activity 4 Practice
1. After interactions have occurred,
the mass in a closed system stays
the same. Evidence is the mass
stayed the same even though a
solid formed from the mixture of
two liquid solutions in Activity 2.
2. After interactions have occurred,
the volume in a closed system
sometimes changes. Evidence is a
change in volume was seen after
mixing water and alcohol, but not
after mixing water and vinegar.
3. I think that scientists base their
ideas (theories) on a collection of
evidence. If many experiments all
point to the same idea, then they
consider this idea as true.
However, scientists will quickly
change their idea if there is any
evidence that doesn’t agree. For
example, we conducted some
experiments where volume was
conserved in a closed system, but
we had one experiment that
showed it was not conserved. This
meant that we could not make a
general statement that volume is
conserved in closed systems. For
the conservation of mass, we
observed that mass was conserved
in all the experiments we did, and
that scientists have not observed a
contrary experiment. If they ever
do, we will no longer be able to
say that mass is always conserved.
Activity 5 Practice
1. The pack is a closed system for
mass, because there are no mass
inputs or outputs.
© It’s About Time
2. The amount of mass in the pack
stays the same, because in a
closed system the mass stays the
same during interactions.
3. Place the pack on a mass scale,
and read the mass. Then, squeeze
the pack to break the barrier
between the two liquids. After the
pack becomes cold or hot, use the
scale again to measure the mass.
Note whether the change in mass
is a greater amount than the
uncertainty of the mass scale.
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4. Start Mass + Input Mass
– Output Mass = End Mass
4200g + ??? – 0g = 4700g
Determining Masses in Open Systems
4200g + 500g = 4700g
(Questions 4-7) For systems with both mass inputs and mass outputs, you can use
this mathematical statement to find the mass of objects:
5. Start Mass + Input Mass
– Output Mass = End Mass
Start Mass
Input Mass
Output Mass
End Mass
For example, imagine a system consisting of a 100 g plastic pitcher. The pitcher is filled
with 1000 g of orange juice, and 400 g is poured out. What is the end mass of the
pitcher and orange juice?
15 MT + 30 MT – 20 MT = ???
15 MT + 10 MT = 25 MT
To find the end mass, set up a mathematical statement like this:
6. Start Mass + Input Mass
– Output Mass = End Mass
250g + 115g – ??? = 335g
Start Mass
100 g
Input Mass
1000 g
Output Mass
End Mass
400 g
?
The mass of the pitcher and the 600 g of juice remaining in it would total 700 g.
365g – ??? = 335g
100 g
Output Mass = 30g = Mass of balloon
(10g) and gas
(600 g)
700 g
Use this example to answer the next three questions.
4. Rain Collector. Meteorologists (weather scientists) wanted to determine the mass
of rain falling during a one-week period. They used a large pan as a rain collector,
and placed it on top of a mass scale. At the beginning of the week, the mass of the
pan plus the water already in it was 4200 g. One week later, the mass was 4700 g.
None of the water was allowed to evaporate or otherwise leave the system. What
was the mass of the rain that landed in the pan during the week? Show your work.
Mass of gas = 30g – 10g = 20g
7. The Mass Conservation idea is used
to solve problems where some of
the masses are known and you are
trying to find an unknown mass.
Since the problems all involved
an open system for mass, the
mathematical statement below that
expresses this idea can be used.
5. At the Gas Station. During a two-week
period at Bill’s Gas Station, a gas tanker came
and put 30 metric tons of gasoline into one of
the underground storage tanks. During the
same two weeks, Bill sold a total of 20 metric
tons of gasoline to customers. Bill started with
15 metric tons of gasoline in his storage tank.
How much gasoline does Bill have left in his
storage tank at the end of these two weeks?
Show your work.
Start Mass + Input Mass
– Output Mass = End Mass
6. Gas Mass. A flask containing a chemical solution had a mass of 250 g.
A mass of 115 g of another chemical was poured into the flask, and a
balloon was quickly attached to seal the top of the flask.
The combined solution bubbled for a few minutes, and the balloon
filled up with gas. After removing the gas-filled balloon, the mass of
the flask with its contents was 335 g. If the mass of the balloon is 10 g,
what was the mass of the gas that escaped into the balloon? Show
your work.
7. To solve Questions 4 to 6, what scientists’ idea did you use? Write a couple of
sentences to explain how the idea applies to these problems.
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Activity 6 Mass Conservation Problems
Multiple Choice
1. Which experiment done in class provided evidence about whether volume is
conserved or not?
a) Mixing water and alcohol showed that volume was not conserved.
b) Mixing water and vinegar showed that volume was not conserved.
c) Mixing water and alcohol showed that volume was conserved.
d) Every experiment we did showed that volume was conserved.
2. Which statement is true for an open system?
a) There is no mass input into the system, and there is no mass output from
the system.
b) Either mass is input into the system, or mass is output from the system, or both.
c) Some mass is always input into the system.
d) Some mass is always output from the system.
3. What happens to the mass of objects in open and closed systems after they
interact?
Activity 6 Practice
1. (a) Mixing water and alcohol
showed that volume was not
conserved.
2. (b) Either mass is input into the
system, or mass is output from the
system, or both.
3. (c) The mass always stays the
same in a closed system only.
4. (d) A car tire blows out on the
road, and the driver carefully
brings the car to a stop. The system
is the car, including the tires and
the driver.
5. (c) The mass decreases because
the gas that bubbled out of the
cake has mass.
a) The mass always stays the same in both an open system and a closed system.
b) The mass always stays the same in an open system only.
c) The mass always stays the same in a closed system only.
d) The mass can change in both open systems and closed systems.
4. In which situation would the mass of the system change?
a) Oil and vinegar are shaken up and mixed in a sealed glass jar for a salad
dressing. The system is the jar with the oil and vinegar.
b) A long stick of taffy candy is pulled apart into bite-sized pieces. The system
is the taffy.
c) A basketball bounces along the ground until it stops bouncing and rolls to a
stop. The system is the basketball.
d) A car tire blows out on the road, and the driver carefully brings the car to a
stop. The system is the car, including the tires and the driver.
5. While a cake is cooking, gas bubbles up through the cake batter to escape. How
does the mass of the cake change as the cake bakes, and why?
a) The mass increases because, as the gas bubbles escape, the cake increases
in its density.
b) The mass increases because when the batter cooks, it gets bigger.
c) The mass decreases because the gas that bubbled out of the cake has mass.
d) The mass stays the same because the escaped gas has no mass.
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Activity 6: Mass Conservation Problems
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6. (a) You cannot conclude that the
masses are different because of the
uncertainty of the scale.
6. On Friday, Hai starts an experiment by measuring the mass of a
flashlight and batteries system as 358.0 grams. She turns the
flashlight on and leaves it on for the weekend. When she returns
on Sunday, the flashlight is no longer shining because the batteries
have run down, and the mass of the system is 358.2 g. If the
uncertainty of the scale is 0.5 g, what can you conclude about
Hai’s experiment?
7. (c) The mass of the system
decreases.
8. (d) None of the others are correct.
(17.0 kg is correct.)
a) You cannot conclude that the masses are different because of the uncertainty
of the scale.
9. (c) The mass of Swiss cheese is
conserved.
b) The mass increases because the mass change is greater than the uncertainty.
c) The mass decreases because the mass change is greater than the uncertainty.
d) The mass increases because the mass change is within the uncertainty of
the scale.
7. A weak solution of an acid and a solution of sodium
carbonate in separate beakers are placed on a mass scale.
The mass is 603.9 grams. The sodium carbonate solution
is added to the other acid solution, and a chemical
interaction occurs.
XLT
RESET
START MASS
After some time, the mass of the new solution and the
beakers is measured as 601.7 g. If the uncertainty of the
scale is 0.5 g, what can you conclude about this experiment?
a) You cannot conclude that the masses are different
because of the uncertainty of the scale.
b) The mass of the system increases.
c) The mass of the system decreases.
XLT
RESET
END MASS
d) The mass of the system stays the same.
8. At Ed’s deli, sliced Swiss cheese is a popular seller. At the beginning of the week,
Ed started out with 11.5 kg of this cheese. He ends the week with only 4.5 kg of
Swiss cheese, even though he received 10.0 kg from Fred’s House of Cheese. How
much Swiss cheese did Ed sell during the week?
a) 3.0 kg
b) 6.0 kg
c) 14.5 kg
d) None of the others are correct.
9. What idea did you apply to solve the previous problem?
a) Swiss cheese is a popular cheese at Ed’s deli.
b) Fred’s House of Cheese sent more cheese than it was supposed to.
c) The mass of Swiss cheese is conserved.
d) The volume of Swiss cheese is conserved.
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Scientists’ Consensus Ideas Answer Keys
Unit 4
12. Mass Conservation
(Unit 4 Chapter 1, Activities 1–4)
5. In Activity 4, we did an experiment in which we mixed alcohol and water in a closed system. The teacher
measured the mass of the system before and after the mixing. This measurement showed that there was no
change in mass (within the uncertainty of the measurement.) We found that when we mixed the alcohol and
water, the volume of the mixture was less than the volume of the unmixed alcohol and water. This shows
that volume can change during interactions in a closed mass system.
6. In Activity 2 (dissolving sugar and water, mixing chemicals) and Activity 4 (mixing alcohol and water) we did
experiments in which various interactions took place in closed systems. In each of the three experiments
that took place, the mass of the system was measured before and after the interaction, and no change in
mass was measured within the uncertainty of the measurement.
7. In Activity 3, we measured the change in mass of an open system. We mixed vinegar and baking soda in a
© It’s About Time
bag. We measured the mass of the system before mixing. After mixing, we opened the bag and let some of
the gas produced escape. Then we measured the mass of the system again – after the gas was removed.
The system’s mass changed. This shows that in an open system, the mass of the system can change.
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