The goal of this project is to measure the
relationship between the volume of a gas and its temperature, when the pressure
of the gas is held constant.
This is a modern version of a classic experiment by
Jacques Charles (who was also interested in flying balloons). Charles studied
the volume of a sample of air—sealed in a glass tube with a U-shaped
curve—as he systematically changed the temperature by immersing the tube in a
water bath. The air was trapped by a column of mercury, added to the open end of
the tube. By changing the amount of mercury in the tube, Charles could maintain
a constant pressure on the trapped air as the temperature was changed. Charles's
apparatus was an example of a manometer, a device used to measure
You can learn more about how manometers work, and
even run a simulated Charles's Law experiment by visiting the Chemistry Applet
website (see Bibliography). This would be excellent preparation for doing the
experiment on your own, so we highly recommend it. Note: to calculate the volume
of gas in the applets, you will need to know that the inside diameter of the
applet's manometer tube is 4.286 cm.
You can repeat Charles's experiments for yourself
with an inexpensive, modern apparatus based on a disposable plastic syringe and
a water bath. (Mercury is a dangerous neurotoxin, so we'll avoid working with
Terms, Concepts and
Questions to Start Background Research
To do this project, you should do research that
enables you to understand the following terms and concepts:
Here are the Chemistry Applet webpages
mentioned in the Introduction. These will really help your understanding of
Charles's Law if you take the time to do the virtual experiments! To
calculate the volume of gas in the applets, you will need to know that the
inside diameter of the applet's manometer tube is 4.286 cm.
Blauch, D., 2004. "Gas Laws:
Pressure", Department of Chemistry, Davidson College. [accessed
January 23, 2006] http://www.chm.davidson.edu/ChemistryApplets/GasLaws/Pressure.html,
Blauch, D., 2004. "Gas Laws: Charles's
Law", Department of Chemistry, Davidson College. [accessed January
23, 2006] http://www.chm.davidson.edu/ChemistryApplets/GasLaws/CharlesLaw.html.
The Sizes.com website has an exhaustive index
of units of measure, including both degrees Celsius and kelvins:
To do this experiment you will need the following
materials and equipment:
35 ml syringe (available, for example, from
Science First ("Gas Law Demonstrator Kit", Science First, Buffalo,
a homemade clamp to hold syringe underwater,
which can be made with:
two sturdy chopsticks (or two sturdy wood
dowels) longer than the diameter of your cooking pot,
two rubber bands,
a weight (e.g., a can of soup);
small piece of wire,
thermometer (calibrated in °C, range at least
cooking pot, deeper than syringe is tall,
showing how to set up syringe. The thin wire between the plunger tip and
the inner syringe wall allows air to escape from in front of the plunger
in order to equalize pressure. It is removed before starting the
experiment. Diagram adapted from Gabel, 1996.
Before starting the experiment, do your
background research so that you are knowledgeable about the terms, concepts
and questions, above.
With the plunger removed from the syringe,
seal the tip of the syringe with a tight-fitting cap. If a suitable cap is
not available, you can try epoxy or silicone sealant. Allow the epoxy or
silicone the recommended curing time before proceeding with the experiment.
(Note: if you seal the tip with the plunger in place, you will probably not
be able to remove the plunger unless you destroy the seal. Why?)
When your sealed syringe is ready for use,
insert the plunger to the 20 ml mark of the syringe along with a thin wire
as shown in the diagram above. The wire will allow air to escape from
beneath the plunger, equalizing the pressure in the syringe with the
atmosphere. Use the lower ring of the plunger as your indicator.
Hold the plunger in place and carefully
withdraw the wire.
Make sure that the plunger can move freely in
the syringe, and that the tip of the syringe is well-sealed. Give the
plunger a small downward push, and verify that it springs back. If it does
not, you may need to lubricate the side of the plunger with a small
amount of silicone lubricant or you may not have sealed the tip of
your syringe properly.
When you are satisfied with the results of the
previous step, record the initial volume of air in the syringe and the
You will be immersing the syringe into a water
bath, and observing the changes in volume of the gas as you change the
temperature of the water. Since the air in the syringe will make it buoyant,
you need a way to hold the syringe under the water. If you have a ringstand
and clamp, you're all set. Otherwise, you can put together a homemade clamp
with materials you'll probably have around the house. Here's how:
Wrap a rubber band around the top of the
syringe tube, just below the finger flanges.
Insert the chopsticks (as noted in
Materials & Equipment, wood dowels can be substituted for
chopsticks) through loops of this rubber band, one on either side of the
syringe. Slide the syringe so that it is about 7–8 cm (3 in) in from
the ends of the chopsticks.
Wrap the second rubber band around the
short ends of the chopsticks. This will make a "V" shape, with
the syringe held tightly down near the point.
This second rubber band can also be used to
hold the thermometer upright in the water. Keep the bulb immersed in the
water, but not touching the side or bottom of the pot.
Place this assembly on the top of your
cooking pot, so that the chopsticks are supported by the rim of the pot
and the syringe sticks down into the pot.
To hold the syringe in place when the pot
is filled with water, place your weight (e.g., a can of soup) on top of
the wide end of the "V" made by the chopsticks.
Make any necessary adjustments to make the
syringe and thermometer stable, and make sure that you can read the
scale on the syringe.
Measurements and Presenting Your Results
Remove the syringe and thermometer assembly
from the pot and set them aside.
Place the pot on the stove, but don't turn on
the burner yet. Fill the pot with ice cubes and enough water to immerse the
syringe to somewhere between the 30 and 35 ml marks.
Replace the syringe and thermometer assembly,
and weight it down securely.
Allow several minutes temperature in the water
bath to stabilize and for the temperature of the air in the syringe to
equilibrate with the water bath. Gentle stirring may help, but be careful
not to break the thermometer or knock your weight off your clamp.
Record the temperature of the water bath and
the volume of the air in the syringe. You may want to tap the plunger
lightly to make sure it is free to move. (If necessary, carefully (and
briefly) lift the syringe out of the water to read the volume. You may want
to have an adult help you with this part.)
Turn the burner on (no higher than medium heat)
to gradually heat the water. At regular intervals (e.g., every 10°C), turn
the heat off and allow the temperature to stabilize. Again, record the
temperature of the water bath and the volume of air in the syringe.
Repeat the previous step up to 80 or 90°C. The
pot will be quite full, so it is best to avoid boiling the water.
As with any experiment, it is a good idea to
repeat your measurements to be sure that your results are consistent. We
suggest at least three separate trials. (Note that the temperatures used do
not need to be exactly the same from trial to trial!)
Make a graph of gas volume vs. temperature for
all of your data points. It's a good idea to use a different symbol for each
of your trials (if something was wrong with one particular trial, it may
help you understand what went wrong).
What is the relationship between volume and
temperature in your data set?
Can you extrapolate from your data to find the
temperature that corresponds to a gas volume of zero? How confident are you
with this result, and why?
Would your data look different if you used
kelvins for the temperature axis instead of degrees Celsius?
Was the assumption of constant pressure valid?
What are the possible sources of error in your