Return to Chemistry IndexGases, Pressure, and Volume

Patricia A. Riley Lincoln Park High School

2001 N. Orchard St. Mall

Chicago IL 60614

(312) 534-8130 ext. 148Objectives:

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 stopperStrategy:

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 andquicklyinsert 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

happen?

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

syringe?

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.