Air Exerting Pressure

Mable McMiller Reavis Elementary School
834 East 50th Street
Chicago IL 60615
(312) 535-1060

Objectives: Three ideas are presented for classroom demonstrations in upper elementary and junior high, intermediate level, including special education students, and bilingual students. To understand that we live on the bottom of an ocean of air. It is sometimes difficult for us to understand that air has weight and exerts pressure as a result of its weight. Scientist have calculated that each square inch of our body has 14.7 pounds of pressure. Scientist have calculated that each square centimeter of our body has 1 kilogram of pressure. On the entire surface of human body there is a total air pressure which varies from 10 to 20 tons! Just as oceanic animals are not crushed by the weight of water because their bodies exert an outward pressure, our bodies push outwards to equalize the air pressure pushing down upon us. To show how air occupies space and exerts pressure. To show how great the effects of air pressure can be. Materials needed: Experiment I Bionic Finger 1. One heavy rubber plunger (to unplug a sink). 2. A stool or chair with a smooth seat. Experiment II The Collapsing Can 1. One empty gallon (or 4-5 liter) can, or any other tin can that can be closed off air-tight. 2. A hot plate or burner and tripod. Experiment III The Balloon In A Flask
1. An Erlenmeyer flask (150-200 ml).
2. A round balloon with a large mouth (uninflated).
3. A hot plate or burner and stand.

Strategy: Experiment I Bionic Finger 1. Make a small hole in the plunger with a scissor's point. 2. Show the students the plunger and ask: "What is under the plunger when I place it on the table?" (anticipated answer: 'nothing'). 3. Ask one of the students to come up and put his/her cheek close to the hole in the plunger. 4. Push the plunger in: air rushes out and blows against the cheek! Air occupies space! 5. Show the students the plunger on top of a stool. 6. Tell them that you possess a bionic finger and that you can hold down the plunger against the stool with one finger. 7. Push down on the plunger and hold it down with one finger covering the hole (a wet finger will work better), and ask a student to come up and pull the plunger up. The whole stool will stick to the plunger and be lifted! Experiment II The Collapsing Can
1. Put about 20 ml of water in the can (just enough to cover the bottom)
and heat it over the hot plate or burner.
2. Let the water boil vigorously for about 2 minutes (vapors should
come out of the can).
3. Take the can with the boiling water off the heat (don't burn your
fingers!) and immediately close off with the cap very tightly.
4. Let it stand upright on the table and cool off to room temperature,
or, for faster results, cool off with wet towel.

Experiment III
The Balloon In A Flask
1. Put a little water (about 20 ml) in the flask and heat it to a boil.
2. Let the water boil vigorously for at least one full minute.
3. Take the flask off the hot plate and immediately place the balloon
with the mouth over the flask's mouth.
4. Let cool slowly at room temperature (the balloon will be pushed
inside out into the flask).

Performance Assessment: Experiment I Have students collect items that operate on this principle: suction cup darts, soap dishes, coat hooks, portable pencil sharpeners, dashboard cups, etc. Have students make comments or write poems about how they use "Air pressure in their lives." Experiment II Set up four work stations for students to be assigned to for an experiment using soda pop cans. Fill each can with 5 ml of water and heat it over the hot plate or burner. Let the water boil vigorously for about 2 minutes. Using a pair of tongs take the pop can with the boiling water off the heat. Use the tongs to grip the bottom of the pop can. Invert the can in a pan of cold water. Explain what happen? (The can popped, and crushed immediately.) Why? Experiment III Have students divide a chart into two parts. Label one side "Air pressure helps" and the other "Air pressure hinders." Brainstorm ways in which air pressure helps people. Record ideas on the chart. Have students be on the lookout for instances of air pressure helping/hindering people. Add these to the chart. Conclusions: Bionic Finger When holding the plunger down with one finger, the hole was covered and this prevented the air from coming back in under the plunger, causing a lower pressure under it. A moist finger works better to plug the hole because the water acts as a seal. The force holding down the plunger is equal to the surface area of the plunger multiplied by 1 kg (about 75 kg for a plunger with a 10 cm diameter). The Collapsing Can

When the water vapor which pushed out the air (which took up the interior
space of the can) after heating was allowed to cool, its volume was reduced by
approximately 1000. When the volume decreased inside the can, the outside
pressure crushed the can.
The total force working on the outside of the can is the total of the can's
surface area in cm2 multiplied by 1 kg.

The Balloon In A Flask

By boiling the water in the flask, it was changed from the liquid state
into the vapor state. The cooling of the flask will slowly condense the water
vapor and thus create a partial vacuum in the flask. This will cause the
balloon to be pushed in and the atmospheric air pressure will further blow up
the balloon inside the flask.


I would like to thank Jim Effinger of the Apple Program for teachers, Ed
Guzdziol my mentor, Elaine Granger my assistant, and of course Ken Schugg who
made my demonstrations successful.

References: Liem, Tik L. Invitations to Science Inquiry, St Francis Xavier University,
Department of Education, Ginn Press, 1985.

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