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scenarios students should start to understand the importance
of diversity and how small changes in or impacts on one part
of an ecosystem can ultimately result in major changes in the
rest of the ecosystem.
Extensions
• Have the class make one large food web on a classroom
wall. Don’t worry about the total budget. Just make sure
everyone is fed.
• Make a food web of a local community. Find pictures of local
animals, determine what these organisms eat, and create a
food web with them. Teachers can create a budget activity by
starting with a group of primary producers from any habitat
and determining who eats them. The energy amount values
will generally increase by 10 points as you move up each step of
the food chain. Very large organisms (e.g., a whale) will require
more energy than very small organisms (e.g., a coral), even
though they both feed on zooplankton. There are a number
of good websites on food webs (see Resources).
• For an advanced class, have students calculate the diversity
of their ecosystems based on the richness (total number
of different species) and evenness (equitability) of their
ecosystem. The webpage “Diversity of the Deep” provides
a good review on diversity indices (see Resources). Have
students calculate how this diversity would change based
on the removal of one organism.
Acknowledgments
This activity was developed as part of the Oceanography Camp for Girls
and the NSF GK–12 OCEANS program (0231843). I would also like to
thank S. Sawney who helped with the development of this project.
References
National Research Council (NRC). 1996. National science education
standards. Washington, DC: National Academy Press.
Tilman, D. 2000. Causes, consequences, and ethics of biodiversity.
Nature 405 (6783): 208–11.
Wilson, E.O. 1999. The diversity of life. New York: W.W. Norton &
Company.
Resources
Barry’s Clip Art—www.barrysclipart.com
Clip art, etc.—http://etc.usf.edu/clipart/index.htm
Diversity of the Deep—www.vims.edu/bridge/archive0505.html
Estuary food web—www.estuaries.gov/pdf/FoodWeb.pdf
Food chain—www.picadome.fcps.net/lab/currl/food_chain/default.htm
Microsoft Greetings Workshop—www.microsoft.com
Natural Resources Education Center—www.in.gov/dnr/nrec/programs
1
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January 2007
Bubbles on a soda
can: A demonstration
of Charles’s law
The bubbles-on-a-soda
can activity is an illustration of Charles’s law,
which states that for a
fixed amount of gas, there
is a direct relationship
between the temperature
of the gas and its volume.
In other words, if the
temperature of a fixed
amount of gas increases,
so does its volume. The opposite is true, as well: If the temperature of the gas decreases, the volume decreases.
I was first introduced to this activity by a student who
demonstrated it for an assignment as part of an elementary
science methods course I was teaching. For the past 10 years,
I have been using this activity at the start of the school year
to introduce my students to hands-on investigations.
Materials
The materials needed for this activity are soda cans,
bubble solution, rubbing alcohol, cotton balls, and petri
dishes (see Activity Sheet). Seventy-percent isopropyl
rubbling alcohol can be purchased at a grocery store; one
946 mL (32 oz.) bottle will be enough for five classes.
You will also need cotton balls, to absorb the rubbing
alcohol—approximately one per student. The soda cans
with tabs removed and the petri dish tops and bottoms
rinsed with water can be reused for each class. Rubbing
alcohol is a flammable material, but small amounts can
be flushed down a sink with a large quantity of water,
unless local laws prohibit such disposal. The cotton balls
can be rinsed along with the petri dishes and thrown
away in the trash. The main concern when using rubbing alcohol is the buildup of vapors in the sink, pipes,
sewer, or trash. Rinsing with large amounts of water will
reduce that risk with the small amounts of alcohol left
over from this activity.
Because they will be handling rubbing alcohol, all
students must wear safety glasses, and the room must be
John Burns ([email protected]) is a science educator
at Ramona Junior High School in Chino, California.
S C I E N C E
sampler
properly ventilated. Students should be warned that alcohol is highly flammable. Before the activity takes place,
consult with the school nurse to determine whether any
students have allergies.
hands around their cans. Ask students to describe what
happens to their bubbles now that their hands are warm.
(The bubbles increase in size more rapidly.) What do they
think is causing their bubbles to increase in size?
Next, distribute the petri dish lids containing cotton
Activity
balls that were soaked in rubbing alcohol. Ask students
I begin the activity by asking students, How many of you
to predict what effect the rubbing alcohol might have on
enjoy making bubbles? I explain that in today’s activity
the size of the bubble if placed on the sides of the soda
they will make bubbles on a soda can and try to make
can. Have students make a big bubble over the mouth
their bubbles larger and smaller. After passing out the petri
of the inverted can and then swipe the upper sides and
dishes with soap solution and soda cans, I demonstrate for
bottom of the can with the cotton ball. (The bubble will
students how inverting the can in the soap solution causes
decrease in size and may even disappear into the can.)
a bubble to form over the opening of the can. Tell students
At this point I ask students to graph the results of their
to do this gently, as agitating the soap solution will cause
observations (see Figure 1). Many of my seventh-grade
many small bubbles to form, making it more difficult to
students are confused by this request and immediately ask
get new, larger bubbles to form on the can. Give students
for help. I explain to students that a graph is a picture of
a few minutes to make and observe bubbles on their own
information, and encourage them to do the best they can.
soda cans. While they are making bubbles, ask students if
To try and coax something out of them, I tell students that
any air can get into the can once the bubble has formed.
the graph is worth five points and that they will receive
Can any air escape from the inside of the can as long as
at least four points for creating what they believe to be
the bubble does not burst? These questions set the stage
a graph of their observations of a bubble on a soda can.
for students using the bubble as a qualitative measure of
Some students literally draw pictures of the experiment,
the volume of a gas, i.e., a big bubble means more volume,
such as a hand holding a can. Others make a bar graph
and a small bubble means less volume.
with two bars: one large bar, labeled hands, and a small
When students are able to successfully form bubbles on
bar labeled alcohol. Most students do not label the x- or
y-axes. This activity lets me know the depth of students’
the soda cans, ask them if they can figure out a way to make
knowledge about graphing, and helps me determine what
a bubble bigger without squeezing the can or poking a hole
else needs to be taught.
and blowing air into the can. At some point, students will
begin to discover that simply holding their hands around the
At the conclusion of the activity or the next day, we
can increases the size of the bubble.
review students’ observations of a bubble on a soda can.
Ask students to predict what will happen to the size of
Students start by drawing pictures and using words to
the bubble if they rub their hands together before placdescribe how to make bubbles on a soda can and how
ing them around the can. As you listen to their answers,
to make the bubble bigger and smaller (see Figure 2). I
see if any students are making a connection between
circulate around the room as students complete the first
temperature and the size of the bubble. This allows
part of the worksheet. After students have completed
you to gauge students’ depth of
their drawings and explanations,
understanding and to discover
I place on the board an unlabeled
Typical student
FIGURE 1
bar graph
any existing misconceptions
graph with three data points
that they may have. The questhat, if connected, would form
tions you ask will help students
a straight line at about a 45°
to make a connection between
angle to the origin of the graph.
temperature and the volume of
Through discussion of variables
a fixed amount of gas. Once all
for the activity and the things
students are holding their cans
that may have caused the bubble
and watching their bubbles grow,
to change size, I try to elicit from
rub your hands together vigorstudents the idea that temperaRubbing
Hands
ously and ask students to do the
ture affects the volume of air in
alcohol
same, then have them place their
the can, which is demonstrated
January 2007
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S C I E N C E
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they discuss how to measure the effects of placing warm
hands or rubbing alcohol on the sides of the can.
By convention, the manipulated variable of an experiment is placed on the x-axis, so at this point I label the
x-axis of the graph temperature, reminding students that by
using their warm hands they controlled the temperature
of the can. I write volume on the y-axis, representing the
responding variable of the activity. Telling students that
the data point in the middle of the graph represents the
size of a bubble at room temperature, I
ask them to explain either of the other
two data points on the graph. (The data
Reviewing the bubbles-on-a-soda-can activity
by the change in the size of the bubble. Using their predictions from previous days and the drawings on their
worksheets, students are able to contribute their ideas
to the class discussion. It is the teacher’s responsibility
to help students focus on the variables of volume and
temperature. By having students describe the pictures
they have drawn, it is not difficult to elicit the term
volume as they talk about measuring the size of a bubble.
Similarly, students will likely use the term temperature as
FIGURE 2
Draw a picture of a bubble on a can.
Explain your picture.
FIGURE 3
Graphs before and
after discussion
Charles’s law states that there is a direct
relationship between the temperature of
a gas and its volume.
Explain how to make a bubble on a
can smaller. Use drawing and words.
Explain how to make a bubble on a
can bigger. Using drawing and words.
Questions
1. To measure the amount of space a bubble occupies, you would need to
know the ______________________ of the bubble.
Graph on the board prior to class discussion
3. Of the variables you have listed, as the experimenter, which of them were
you able to manipulate so as to change the size of the bubble?
4. Graph the results below. Your variables are the size of the bubble and your
manipulated variable.
Information
For a gas, there is a direct relationship between temperature and volume.
As temperature increases, the volume of a gas increases. As temperature
decreases, the volume of a gas decreases. This direct relationship has a
name: Charles’s law.
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January 2007
Volume
2. What are the variables that affect the size of the bubble? List as many as
you can.
Temperature
Graph as completed after class discussion
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sampler
point to the right represents an increase in temperature
and volume, so it is the point that corresponds to their
hands around the can. The data point to the left represents
a decrease in temperature and volume and corresponds to
the rubbing alcohol on the sides of the can.)
At this point, I connect the three data points and state
that the line represents a direct relationship between the
temperature and volume of a fixed amount of gas (see Figure
3). This particular direct relationship has a name: Charles’s
law. Students are then asked to complete the questions
on the worksheet (see Figure 2). As an assessment, I have
students state the variables that are directly related in
FIGURE 4
Thinking about the bubbleson-a-soda-can quiz
1. Charles’s law is a statement referring to two variables that
explains how gasses behave. Name the two variables.
2. In your own words, define Charles’s law.
3. Draw a graph that represents Charles’s law. Label both
the x and y axis.
the bubbles-on-a-soda-can activity, define Charles’s law
in their own words, and draw and label a graph showing
the relationship between the temperature and volume of
a fixed amount of gas (see Figure 4).
Explanation
A direct relationship is a positive relationship between
two variables; in other words, if one variable increases,
so does the other variable, if one variable decreases, so
does the other variable. To illustrate a direct relationship
with a bit of humor, I tell students that there is a positive relationship between the amount of time they spend
studying for tests and the grades they receive.
Charles’s law states that there is a direct relationship between the temperature and volume of a fixed amount of gas.
As the temperature of a fixed amount of gas increases, so does
its volume, and vice versa. When students place their hands
around their cans, they heat the cans, which in turn heat the
temperature of the air inside the cans, resulting in the bubbles
increasing in size as the volume of air in the cans expands. The
bubble acts as a lid for the can, not allowing air to enter or escape, so that the amount of air inside the can remains fixed.
Activity Sheet
Bubbles on a soda can: Discovering Charles’s law
Materials
• 946 mL rubbing alcohol (typically sold as a 32 oz. bottle) (Safety note: Use of alcohol requires proper ventilation,
and students must wear safety goggles.)
• 100 mL dish soap
• 500 mL water
• soda cans with tabs removed (one per student)
• 10 cm diameter, round petri dishes (one per pair of students)
• cotton balls (one per student)
• safety goggles
Preparation
Prepare soap bubble mixture by adding dish soap to water. Place enough soap solution (5–10 mL) to fill half of a petri-dish
bottom. The tops of the petri dishes can be used for rubbing alcohol–soaked cotton balls; pour 5–7 mL of rubbing alcohol
in each top, one top per student group, and add two cotton balls.
Instructions to students
Place the top of an empty soda can in your dish of bubble mixture, then turn the can upright. Observe the bubble that is
formed. Without poking a hole in the can and blowing air in or squeezing the can, can you make the bubble bigger? Can
you affect the size of the bubble by using the cotton balls and rubbing alcohol?
Explanation
For a fixed amount of a gas, there is a direct relationship between temperature and volume: As temperature increases,
volume increases, and as temperature decreases, volume decreases.
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Conclusion
The bubbles-on-a-soda-can activity is an inexpensive
way to illustrate Charles’s law (see Activity Sheet),
have students practice observation and graphing skills,
and introduce experimental variables. As teachers, we
have an opportunity to practice questioning skills to help
students discover science concepts and decide when to
use direct instruction. Most of my students need direct
instruction on graphing, especially the labeling of axes
with variables from an experiment. Asking students to
explain their drawings for this activity is a way to let them
make their own connections between the temperature of
a gas and its volume.
One direct application of Charles’s law is to explain
why a soda can, when exposed to heat, may spontaneously
explode. When a cold soda is opened, very little pressure
is released; as the temperature increases, sometimes the
pressure that builds up causes the soda to spill out of its
container. At times, soda cans exposed to direct sunlight
will explode because of the build up of pressure inside the
can (due to the increased temperature). This idea can be
revisited while studying topics such as heat-conducting
materials, phase changes due to an increase or decrease
of temperature, and evaporation, or a during discussion
of why sweating cools our bodies.
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January 2007
Word wall connections
A word wall has always been an important bulletin
board in my middle school science classroom, but I have
found a more interactive way to reinforce vocabulary
and connections between concepts. At the beginning of
each unit I make a list of the words and terms that we’ll
be discussing. I write the words on lengths of oaktag,
about 8 cm high, and attach a thin strip of magnet to
the back of each tag. I display the tags on one side of the
magnetic white board at the front of my room to create
my interactive word wall. Just above the word wall are
posted my classroom rules: “Be Ready, Be Responsible
and Be Respectful.” To the right of the rules is posted
the essential question for the unit. This arrangement
displays the vocabulary for all to see and allows me to
reference it as I introduce new terms, make connections,
and review concepts.
One of my favorite ways to use the word wall is to
create food webs. Students can arrange the tags by
producers, consumers, and decomposers and add labels
to show the flow of energy through the web. Arrows and
other types of symbols, as well as terms you don’t have
tags for, can be added using dry-erase markers. On the
sample word wall shown in Figure 2, the student uses a
marker to identify the consumers as either primary or
FIGURE 1
Sample word wall setup
PHOTOS COURTESY OF THE AUTHOR
Rubbing alcohol has a boiling point of approximately
82˚C. Heat of vaporization is the term used to describe the
amount of energy needed to change a liquid to a gas. Since
the boiling point of rubbing alcohol is relatively low as
compared to water, it evaporates at room temperature
rather rapidly. As the rubbing alcohol evaporates from the
side of the can, some of the energy needed for the heat
of vaporization for the phase change comes from the can
itself, thus lowering the temperature of the can. The air
inside a soda can with a lower temperature will decrease
in volume, as evidenced by the size of the bubble over
the can’s opening.
The concept of heat of vaporization can be used to
explain why sweating during vigorous activity helps
people to cool down. It is the evaporation of the water
in the sweat on the surface of our skin that helps cool
our bodies. One way to illustrate this without exercising is to have students place a small amount of rubbing
alcohol on the back of their hand and feel how the area
where the rubbing alcohol was placed feels cooler than
the surrounding skin.
Jan Staires ([email protected]) is a teacher at East Haven
Academy in East Haven, Connecticut.