Gases, Pressure, and Volume

Patricia A. Riley Lincoln Park High School
2001 N. Orchard St. Mall
Chicago IL 60614
(312) 534-8130 ext. 148


For students in sixth, seventh, and eighth grades
1. To demonstrate the meaning of pressure.
2. To demonstrate that gases exert pressure.
3. To demonstrate that gases take the volume of their container.
4. To demonstrate the relationship between the pressure of a fixed quantity
of gas and its volume.

Materials Needed:

For class demonstration:
8"-16" balloons 625-nail board
2-liter pop bottle 1-nail board
small plastic pop bottle 2-nail board
3 1-hole rubber stoppers with 3-nail board
glass tubing inserted overhead projector
2 pieces of rubber tubing that plastic petri dish with cover
fit the glass tubing small plastic beads
3 tubing clamps 4 small potatoes of same size

At each team work station:
ringstand syringe, 10 to 20 mL
test tube clamp flat, stackable weights or textbooks
solid rubber stopper


1. Sit on 625-nail board on top of a table where all can see; be sure to
swing feet. Introduce the lesson and then jump down from table. Hold up
board for all to see; invite everyone to come up and sit or stand on it.
a. Put out the 1-nail board, 2-nail board, and 3-nail board; again invite
everyone to come up and sit or stand on these boards.
b. Ask questions such as: Why are there no takers to sit or stand on the
1, 2, or 3-nail boards, but plenty willing to sit on the 625-nail
board? What would happen if someone actually did sit on these small
boards? Why can we sit on the 625-nail board but not the others?
c. Lead the discussion until students mention that body weight spread
over many different nails is what makes the difference. Now write the
formula for pressure on the board: Pressure = force/surface area.
Stress that surface area refers to the surface directly acted on by
the force. So what is this surface: your bottom or the nail tips?
Stress that a force is a push or a pull. So what acts as the force
when we sit or stand on the board? What about the space between the
nails? Drop one potato onto each nail board from the same height and
compare the results.

d. Write the formula for pressure on the board again, this time
substituting in the force and surface area used:

Pressure = body weight/area of nail tips

Ask students to determine for which board the pressure would be
greatest, least, and why.
2. Blow up a balloon and knot it. Ask the following questions to stimulate
discussion: Do gases, for example air, exert pressure? How do we know?
Is the air in the balloon exerting pressure? How do gases do this?
a. Using an overhead projector and small plastic beads inside a covered
petri dish, demonstrate the movement of the gas particles.
b. Ask students to relate this model to the nail board model and to the
balloon of air.
3. Insert a 1-holed stopper into the mouth of a second balloon. Blow up the
balloon to about the same size as the first balloon and clamp it closed.
Uncap an empty 2-liter plastic pop bottle and pour hot water into the
bottle and pour hot water into the bottle. Place the bottle in a pan of
hot water for several minutes. Attach a piece of rubber tubing to a
piece of glass tubing inserted into a 1-holed rubber stopper. Now dump
out the water in the bottle and quickly insert the stopper into the
mouth of the bottle and clamp the rubber tubing closed. Allow the bottle
to cool. Do the same with the smaller bottle. This will create a partial
vacuum in each bottle.
a. Ask the students to observe what happens to the bottles. Why did it
b. Attach the glass tubing of the second balloon to the rubber tubing of
the 2-liter bottle. Now remove the clamps. Ask the students to
describe what happens to the balloon, to the bottle. Why did it
happen? Did the volume, pressure, shape of the air change?
c. Blow up the second balloon again and attach it to the smaller bottle.
d. What conclusions can be drawn? Hopefully the students will say that
the pressure of a gas changes when its volume changes.
4. Let's try to test this conclusion. Divide the students into small groups
of three or four. They are to set the syringe volume at the maximum, seal
the tip with a solid rubber stopper and support the syringe with a clamp
attached to a ringstand. Tell the students to read the volume of air in
the syringe before adding any weights (books work best) and then to read
the volume after adding each weight. They should record the total weight
and the volume associated with it after each addition of weight.
a. What do they observe?
b. Why didn't the syringe plunger collapse? Why didn't it fly out of the
c. What happened to the pressure of the air inside the syringe?
d. What's the relationship between the volume of a gas and the pressure?
e. The students can graph the pressure on the x-axis and the volume on
the y-axis.
f. Students should be able to state that as the pressure on a gas
increases, its volume decreases.

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