LeaPS 2011
January 28
Conservation of mass and evidence for atoms
KY Standards addressed
(6th grade) Mass is conserved in all transformations
(7th grade) Equal volumes of different materials may have different masses
Matter is composed of atoms
Mass is conserved and this is linked to the idea of atoms
(8th grade) Atoms; Conservation of mass is linked to atoms
Learning targets
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I can cite everyday evidence consistent with the idea that matter is composed of very small
particles.
I recognize that all matter possesses mass, which can be measured.
I can use the idea that gas has mass to explain how some changes seem to show that materials
lose or gain mass.
I can associate the particulate nature of matter with the conservation of its mass through any
kinds of change.
I can connect the idea of the particulate nature of matter with the idea of atoms.
I can explain, using the idea of atoms, how a particular change is either a chemical or a physical
change. (well, this one will be addressed next time)
Probe – what’s in the bubbles?
Please complete on your own. Be sure to provide an explanation for your choice
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Activities
A. What is a gas? Does it have mass?
Introduction
Discuss in your group whether a gas has mass. Recall that a helium-filled balloon rises in the air.
What do you think is the mass of a gas such as helium? How might you make a measurement of the mass of a
gas?
Think about the following everyday occurrence: When you pour water into a saucepan, place it on the stove,
and bring it to the boiling point, in what way is the mass of the water changing? When you let the water boil for
some time, how is the mass changing? When all the water has boiled away, what is the mass? Where did the
mass of the water go after the water has fully boiled away?
Describe in your notebook how you could determine the mass of the lost water.
Gas cans
You should have a canister of pressurized gas in front of you. These are used for cleaning the dust out of
electronics such as a computer. Inspect the can. What does it say is the mass? Is this the mass of the whole
can, or just the mass of the gas inside the can? What evidence do you use to make this distinction? Record
your response in your notebooks.
What is the type of gas inside the canister? What evidence do you have for this? Is this the same thing as air?
Why do you say this? Record what you think the type of gas is into your notebook along with your reasoning.
Determine the mass of the can in grams, and record this value in your notebooks.
With the can facing away from you, and away from anyone else, depress the lever for exactly 2 seconds to
release some of the gas.
[Note, for another time, the changed temperature of the can after releasing the gas. We will do nothing
with this for now, but may return to this observation later. In science notebooks, it is always appropriate
to note any observed changes, even if they seem to be irrelevant.]
Weigh the can again, and record the final mass directly under the original mass.
Does the released gas have mass? What evidence do you have? Record your response in your notebook. If
you feel it does have mass, then how much is there? Again, cite your evidence in your notebook.
In what way does this activity answer the question of whether a gas has mass? What other properties of gasses
do you observe with this activity? Record your responses in your notebooks.
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Balloons
Use the canister to blow up a balloon and tie it. Blow up a second balloon the usual way and tie it too. Make
sure the two balloons are approximately the same diameter.
Spend a few moments tossing the two balloons back and forth, and write in your notebooks any observations
you make about differences in their behavior.
Weigh the two balloons, and record their masses in your notebooks. Which has the larger mass? In what way
does this activity respond to the question of whether a gas has mass?
Provide evidence for your answer in your notebooks.
Basketballs
Turn your attention to the basketball. It should be tightly inflated. If it is not, then pump it up until it has a normal
level of inflation, and it bounces like a basketball should.
Determine the mass of the inflated basketball. Also, determine its diameter using the measuring tape. Record
both these in your notebooks.
Using the pump, place 4 additional strokes of pumped air into your basketball. Weigh it and measure its
diameter again, and record these into your notebooks.
In what way does the mass of the basketball help to respond to the central question of this activity?
Check: Discuss with a facilitator your observations and statements you wrote. What evidence do you
have that a gas has mass? How do these experiments improve your understanding of whether a gas has
mass?
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B. Is mass conserved through chemical transformations?
Chemistry in an open beaker
In each of the following steps, collected data, such as mass, should be recorded in your notebook.
You will need two beakers: a 400 mL beaker, and a 100 mL beaker. They should be dry.
Weigh out about 5 g of sodium carbonate (Na2CO3) onto a piece of weighing paper and place the sodium
carbonate into the 400 mL beaker. Record in your notebook the actual mass you measured.
Weigh the empty 100 mL beaker and place into it 20.0 mL 3 M HCl. Weigh this beaker again and by subtraction
determine the mass of the HCl solution (it should be close to 20 g). Record your mass in your notebook.
Place the first beaker with the Na2CO3 onto the balance and record the mass of the total (Sodium carbonate plus
beaker).
Predict the resulting mass when all of the liquid is added to the Na2CO3. Is this the sum of the components (the
beaker plus the acid plus the carbonate)? Write it down.
Now slowly add all of the HCl to the sodium carbonate sitting on the balance, and make sure every drop is
added. It will bubble quite a bit, so adding slowly should keep the bubbles from overflowing the beaker.
Swirl the beaker and when the bubbles subside, measure and record the total mass of the system. Is
this what you predicted? What is the difference from what you predicted?
Discuss with your partners what you think happened to the missing mass.
Compare the measured total mass with the predicted mass. Calculate and record on the report sheet the mass
of the escaping gas. Write in your notebooks a procedure for determining the mass of a gas.
Note that the mass of the components in the beaker is dropping at the same time the reaction proceeds
and the gas escapes.
Discuss whether this proves the escaping gas has mass.
Three other students took up this topic and discussed it:
Larry: Well, this certainly proves that gas has mass as we all know the law in science that mass is always conserved
in reactions and so therefore the disappearing mass had to be contained in the gas.
Moe: Wait a minute. I don't think this experiment proves anything of the sort. The conservation law was established
by measuring the disappearing mass in the escaping gas. This sounds like circular reasoning to me.
Curly: I think it just happens that the gas escapes at the same time that the mass goes down. I think that since
gases rise, they obviously have no mass at all or more likely negative mass. Since we are actually measuring the loss
of mass in this experiment, we have therefore proven that the law of conservation of mass is false and that at least
sometimes mass can just vanish.
Discuss whether any of these students make sense, and ask between yourselves what we still need
to measure to determine whether gas has mass. Kids at the middle school level and older will generally state
that mass is conserved, but when pushed don’t really get this well. What are some of the drawbacks in their
understanding?
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Chemistry in a closed pop bottle
To the soda-pop bottle, add about 12 mL 3 M HCl, and replace the cap. Weigh this bottle to the nearest 0.1 g
and record its mass in your notebook.
Set your balance to zero, then place a 4"x4" square piece of tissue (Kleenix™ or Kimwipe™) on the balance
pan and add sodium carbonate (Na2CO3) until the total mass is about 3 g. Record the actual reading the
balance says in your notebook.
Fold up the tissue with the powder like a burrito, so that the powder is contained well inside the tissue and won't
easily spill.
Hold the bottle sideways and remove the cap so the acid inside won't spill out; then wedge the tissue into the
neck so it doesn't come into contact with the liquid. Keep the bottle sideways and screw the cap on very tightly.
With the bottle still sideways, weigh it (this is the weight before the reaction) and record the weight. Is this
weight the same as the sum of the tissue/powder and bottle/acid? Up to this point, has any mass been lost? If
so, how much, and can any of this be accounted for? Take a moment to record your thoughts in your science
notebook.
Turn the bottle upright and knock it so that the tissue with powder falls to the bottom and mixes with the HCl. If
the tissue doesn't fall down, then invert the bottle – the point is to allow the HCl and Na2CO3 to mix and react.
What evidence do you see to indicate a reaction is occurring?
You should discuss this with your
partners, and note at least three separate pieces of evidence for this.
Keep the cap on tightly, and swirl the liquid around well until the fizzing subsides.
Weigh the bottle again and record this mass. Is its mass still the same as it was just before the reaction took
place?
Release the built-up pressure slowly by partially unscrewing the cap. After the pressure is fully released, weigh
the bottle one final time and again record the mass.
Discuss whether the mass after the pressure is released is different from the mass before release. Is it
more or less? By how much more or less?
What is the mass of the released gas?
Does the mass in this reaction change as a consequence of the reaction itself, or does it change because the
leaving gas takes the mass with it? How do you know? Write out a statement in your science notebook about
conservation of mass, and evidence supporting this
Check: Discuss with a facilitator your observations and the statement you wrote. How does this second
experiment improve your understanding of whether a gas has mass? How can you respond to the students in
the previous dialog?
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C. Are both mass and volume conserved?
You will be asked to determine the mass and volume of several objects. To determine the
mass of something in this activity, place an empty graduated cylinder onto the scale, set it to
zero, gently place the material to measure into the cylinder, then again place it on the scale, and
record the value.
a) Use a 100 mL graduated cylinder to measure as close as possible 50 mL of marbles.
Determine their mass and place into a plastic cup.
Use the graduated cylinder again to measure 50 mL of the rice.
Determine the mass of the rice, add to the marbles and swirl to mix.
Pour this mixture back into the graduated cylinder; determine both volume and mass of the
mixture. Record
For each of the questions below,
notebooks along with your reasoning.
discuss with your partners, and write answers in your
Is the mass of the mixture equal to the sum of the marbles plus the rice?
Is the volume equal to the sum of the two?
b) Repeat this activity with sand and the rice. Record results
Is the sum of the masses of 50 mL of sand and rice equal to the mass of the mixture?
Is the sum of the volumes of sand and rice equal to the volume of the mixture?
c) Repeat this again using marbles and water. Record results
Is the sum of the masses of 50 mL of marbles and water equal to the mass of the mixture?
Is the sum of the volumes of marbles and water equal to the volume of the mixture?
In each of these activities, is mass conserved through mixing?
Is volume conserved through mixing?
Looking at the systems – marbles and rice, sand and rice, and marbles and water, why do the volumes
behave as they do upon mixing?
Check: Describe your observations to the facilitator, and provide an explanation for your observations.
d) Suppose we had done this activity with materials with very similar particle size, for example salt and
sugar. What would you expect the outcome to be, and why?
Repeat the activity using rice alone (50 mL rice plus 50 mL rice). Determine both mass and volume
of each separately and when mixed together. How are these results similar to and different from the
previous trials? Can you explain this?
In your notebooks, write a rule describing situations in which volumes of mixtures are conserved
following mixing and in which volumes of mixtures are not conserved.
e) Finally repeat this activity using distilled water and ethanol – measure out as accurately as you can
50 mL of each, and determine the mass of each. Mix them in a cup, and swirl to be sure they
are well-mixed. Then pour them back into the graduated cylinder and determine the final mixed
volume and mass.
Is mass conserved? Is volume conserved?
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Applying your observation of mixing the large objects, what conclusions can you reach about the
characteristics of water and ethanol?
Check your rule with the facilitator, and describe your observations about mixing water and ethanol.
What does this say about the properties of these materials?
Discuss in your group how each set of activities in these lessons is consistent with a particle
model of matter. In what way is this also consistent with the observations on temperature from the
previous session? How does such a model help you to understand the meaning of pressure?
Develop a list of everyday observations that may be more easily explained with a particulate model of
matter than with a continuous model of matter.
How is the conservation of mass linked with particles of matter?
Discussion:
To be considered by the whole group:
Place about 200 mL water into a saucepan and weigh them together. Place onto a hot stove and allow
the water to boil for 1 min. What will be the change in mass? Will it become greater, less, or stay the
same?
Place a weighed nail or other iron object onto a plastic weigh boat, and set aside for many years.
During this time, it completely rusts. Will its final mass be the same as, greater than, or less than its
initial mass? Explain, based on your observations in today’s activities.
Obtain some pure copper ore (consisting only of copper oxide) and weigh it. Heat it to a very high
temperature over a Bunsen burner for an hour or so, and pure copper will result. Will the final mass of
this copper be less than, greater than, or the same as the mass of the ore? Explain.
Do any of these activities violate the principal that mass is conserved in all transformations? Can you
think of a situation in which mass is not conserved? If someone presented YOU with a situation in
which it is claimed that mass is not conserved, what would be your response?
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Materials
Part 1
Gasduster™ cans of compressed gas (from OfficeMax; other brands may work as well)
Balloons
Scale reading to 400 or 500 g, to the nearest 0.1 g.
Balloons
Basketballs
Hand pump with needle
Tape measures (cloth is best)
Part 2
400 mL beakers
100 mL beakers
Paper towels
Sodium carbonate
3 M HCl (Dilute concentrated HCl 1 part with 3 parts water)
Plastic 100 mL graduated cylinders
20 oz pop bottles with screw-caps
KimWipes™
Scale (as above)
Part 3
Marbles
Rice
Sand
Distilled water
Plastic 100 mL graduated cylinders
6 oz or 8 oz plastic cups (preferably clear plastic)
90% ethanol
Scale (as above)
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