Gases are a form of matter with much lower density than
solids of liquids. In this lab you will observe how the
volume of a gas in a balloon changes as the pressure of the
air around it is reduced at constant temperature. You will
also observe the change in volume of a gas at constant
pressure as it is warmed and cooled.
The procedure of this lab is step-by-step. Your analysis of
your observations and your calculations and your
conclusions should be step-by-step, too. I have written the
procedure. It is up to you to put the rest into an orderly
whole. I ask questions in the procedure to guide your
analysis. I also provide some generalized notes about how
to put together your lab report. The key to writing a good
lab report is taking good notes while you are in the
500 mL filter flask (vacuum
no-hole stopper to fit
lab notebook & pen
ice & water
3 small balloons
400 mL beaker
centrally located: 2 500 mL beakers
electrically heated to 35°C - 40°C
You will look at two of the basic proportional
relationships introduced at the beginning of your lessons
on the ideal gas laws. The first, Boyle’s Law,
relates pressure and volume at constant temperature:
P · V = k or P1 · V1 = P2 · V2
As you know, this relationship represents an inverse
proportionality. When pressure is increased, volume must
decrease. When pressure is decreased, volume must increase.
This is all fine and good to imagine but it is much easier
to understand if you can see it for yourself. Science is
more clear to the mind when it is clear to the senses.
The second relationship you will explore is the one
commonly called Charles’s Law:
V V1 V2
— = k or — = —
T T1 T2
Clearly, this is a direct proportionality and says that as
temperature rises, so must volume. Similarly, you can say
that as temperature falls, the volume must decrease. Keep
in mind that this proportionality is only accurate when the
pressure of the gas remains constant.
Tie back loose hair and clothing. If you are not wearing
shoes with closed toes and heels then you should be! The
aspirator you will use in this lab provides a powerful
source of suction and can reduce pressure inside sealed
vessels to a remarkable degree. The sealed vessel you will
use is designed for the purpose of supporting the immense
pressure of the Earth’s atmosphere when the air
inside is drawn out but that does not mean there is no
danger of implosion. For this reason you
must wear safety
goggles at all times! No chemicals will be used in this lab
but be careful anyway.
Part One: Boyle’s Law
Inflate a balloon to a diameter of about 2 cm and
tie off. This balloon must be very small in order to
fit through the opening in the vacuum flask.
I will make one observation for you: balloons
contain gas at the same pressure as the pressure
pushing on them from the outside. The balloons you
inflate will be at atmospheric pressure as long as they
are in the atmosphere. If there is a barometer in the
room find out what the atmospheric pressure is today
and record this data. Convert the reading you record to
Measure as accurately as you can the diameter of
the balloon. Put it into the filter flask and stopper
the flask tightly.
I have already connected the flask to an aspirator.
When the flask is stoppered tightly enough and you turn
the water on full blast in the faucet it draws air
through the tube, evacuating the flask of air. Turn the
faucet on full blast.
It may be a few minutes before you notice anything
happening. If nothing does happen for about 5 or 6 min,
check the connections: ask for help with this.
In all of this I am assuming that you are using
your lab notebook as described in the candle
observation lab: write down the name of the lab and the
materials; take notes on each step of the procedure as
you perform it. Write down observations of the
unexpected and the obvious. In short, at this point
please consider taking the necessary steps to address
the problem that you have not done any of this yet. Or
did you read ahead? Good, then you are already doing
this. By the way, are you using a pen?
Write down in your lab notebook what you see once
the pressure inside the flask goes down. What happens
to the balloon? What was the pressure inside the
balloon before you started the aspirator? Is the
pressure inside the balloon greater or less now that
the aspirator has been working for a few minutes? How
do you know?
Try to measure the new size of the balloon
inside the flask using the ruler. Treat all
measured diameters as the diameters of perfect
spheres (a decent enough approximation). Find the
radius and use V = (4/3)πr3 to find
the respective volumes. Use the same units for each
calculation and show the correct units for the
result. If the balloon is more cylidrical than sperical use V = πr2h.
In your lab notebook use Boyle’s Law to
estimate the pressure inside the balloon according
to your measurement of its volume before and after
subjecting it to reduced pressure. Assume that
the pressure in the balloon was 1 atm before you
put it in the flask. Alternately, if there is a
barometer in the room, find out what the
atmospheric pressure actually is and convert that
number to atmospheres. Use that number in the
Part Two: Charles’s Law
Fill the 400 mL beaker just over half full with
ice. Add water until the beaker is about ¾ full.
Swirl or stir to mix thoroughly. If at any time the ice
seems to be melting away, replenish it.
Use a thermometer to measure the temperature of the
ice bath. Note this temperature in °C and convert
that value to K (K = °C + 273).
How does the water feel now that you have added
ice to it? Does condensation form on the outside of the
beaker? Notice anything else?
balloons and measure their diameters. Make them both the same size (about 2.5"). Calculate their volumes.
Put a balloon into the ice bath and
hold it down.
Keep the balloon submerged for a few minutes then
pull it out. Compare it to the other balloon that
you have not subjected to a tortuously cold ice bath.
Is it sensibly smaller or larger than the reference
balloon? What did you expect to happen? Measure the
diameter and calculate the corresponding volume.
Put the cold balloon back in the ice bath so that
it stays at the temperature you measured above.
One lab partner should now take the other balloon
over to one of the warm water baths maintained by your
teacher. This bath will be somewhere between 35°C
and 40°C. For reference, 37°C is human body
temperature (check if that’s true by converting
98.6°F to °C). This will not be hot enough to
hurt you. It will, however, be nearly 40° warmer
than the ice bath. Let the balloon warm up fully to the
temperature of the warm water (hold it submerged for
about 3 or 4 minutes). Note the exact temperature of the
warm water bath in your notebook.
Compare the balloon to the one being held in the
ice bath. Measure the diameter of the balloon you just
heated and compare its volume to the volume of the
balloon you chilled. Now you have direct measurments of
all four quantities in the second Charles’s Law
formula given in the background section. The volume and
temperature of the cold balloon and the volume and
temperature of the warm balloon. Are the balloons
different sizes? How well do your data fit the model?
Did you remember to do your calculations using K
Compare the V & T data collected for the heated and cooled balloons to their original V & T from before heating/cooling. Note: the T from before heating/cooling was the same as the room temperature. How do all of these data fit the Charles’s Law mathematical model?
Write a neat, grammatically correct, stylistically
sophisticated lab report. Use the first person (I, me, my),
the present tense, and the active voice:
“I held the balloon below the surface of
the ice water,” not “The
balloon was held below the surface of the ice
water.” Please write in 12 pt Times,
1.5-spaced, typed; written: write neatly! Tell me what you
did, why you did it and what you found out.
Your observations in this lab were meant to lead you to a
better understanding of what the mathematical models,
represented by the ideal gas laws, look like in
‘real life’. Relate the
mathematical models (the formulas) to what you found in the
course of the observations made during this lab activity.
Science proceeds by observing nature, recording
observations, building a model (in this case mathematical),
and checking that model against nature. It is important to
connect your imaginations about nature (the model) against
the real thing. When the model doesn’t match up with
what you observe, then the model must be modified. That is,
unless you can think of some reason why your observations
were not exact enough to verify or disprove the model.
Organize your report using the following outline:
Introduction: This is, ironically, the last section
you will write. In this section you should briefly
summarize (50 words or less) the results of your
observations. Just the results, nothing else. What did you
find out? What do you know now that you did not know
before? How does it relate to the conceptual/factual
learning you are doing in class?
Materials: Copy (yes, copy) the list of materials
from the procedure I wrote. Well, not exactly copy. Please
do not write down anything you did not use. Also, add any
items you used but which were not listed on the procedure
that I wrote. This will help me to make the lab better.
Procedure: In your own words (this is very
important) in your own, your very own
words, write down what you did. It does not have to be a
lot of writing but it does have to give a reasonably
complete picture of your activity in the lab. Use the
procedure I wrote and your own labs notes to create this
picture. Think of it as a story you are writing about what
you did in the lab. Keep it short and to the point and for
this lab it should not require more than about 450 - 500
words. No comments on results are expected or desired in
this section. Observations are welcome and should be
Data: This section is where you should present all
numerical data you collected. For this lab you measured the
diameters of several balloons under different conditions. Create a table showing the
conditions under which each diameter was measured.
Specifically note the temperature of each measurement, and
the pressure, if possible. For each balloon I asked you to
calculate the volume before and after changes. Include this
in the data table as well. If there was a barometer in the
room and you figured out how to read it, include the
reading in this section.
Results: This is where it all comes together. You
gave your observations in the procedure section, you
provided your data in the data section. This is where you
write about what it means and what you learned.
The following is a list of the observations, measurements, and
calculations that I expect you to write about in this report:
The sizes of the balloons you inflated.
What happens to the balloon inside the filter flask
after you turn on the aspirator? To the cooled and heated balloons?
The visual comparison of all before/after pairs.
All physical changes and how things feel to you.
The atmospheric pressure as found on the barometer
in the room (if there is one)
The diameter of each balloon at room temp/pressure
and the relevant temperature and pressure before you do
anything to it
The diameter of the balloon inside the evacuated
filter flask and its temperature
The diameter of the balloon you cooled in the ice
bath and its temperature and pressure (room pressure)
The diameter of the balloon you heated and its
temperature and pressure (room pressure)
The volume of each balloon, based on the diameter
you measured and the assumption that it is roughly
spherical (V = (4/3)πr3) or cylindrical (V = πr2h). You should have
this volume calculation for each balloon before and
after each change.
The pressure inside the filter flask based on your
measurement of its volume, the volume of the unchanged
balloon, and the atmospheric pressure in the room. This
is just a Boyle’s Law calculation.
The predicted volume of each balloon when cooled or
when heated. Compare the predicted volume (found using
the initial volume, the room temperature, and the
cold/hot water bath temperatures) to the volume you
measured. Are they close enough that you could say that
Charles’s Law is confirmed? Or was there too much
uncertainty in your measurements and calculation of
If you have already written your introduction
(remember, I said to write it last) then go back and
look at it after you finish writing the rest of the
report. Re-write it if it seems necessary.