Summary

In this demonstration, students will learn to appreciate the power of air pressure. This lesson is intended to be done after students have had an introduction to the properties of gases, probably in a unit on states of matter.

Students should already know that the particles of a gas are very far apart compared to the particles in liquids and solids. They should also know that the particles in a gas are in constant motion and barely interact with other gas particles other than to collide with them and bounce off.

For students of almost any age, the concept that the air around them which they can barely feel exerts a force is very counterintuitive. This lesson mainly serves to give students a chance to make some very surprising observations and to begin to explore some effects of air pressure.

Grade Level

Elementary or middle school

Objectives

By the end of this lesson, students should be able to

• Recognize that the gases in air exert pressure on their surroundings. Chemistry Topics

This lesson supports students’ understanding of

• Air pressure

Chemistry Topics

This lesson supports students’ understanding of

• Pressure

Time

Teacher Preparation: 60 minutes

Lesson: 45 minutes

Materials

• Water
• Cup (enough for each student plus one more)
• Playing card or laminated index card large enough to cover the top of the cup
• Straws (enough for each student)
• 12-16oz Plastic bottle with lid (Beverage bottle with plastic thicker and stronger than a disposable water bottle)Hammer and nail
• Empty bucket

Safety

No specific safety guidelines are necessary.

Teacher Notes

• Air pressure is not an easy topic for students to fully understand.
• Do these demonstrations over a bucket to avoid spills, just in case.
• Pressure is measured in Force per unit Area or P = F/A
• The force in the case of air pressure is the weight of a column of earth’s atmosphere above the point where it is being measured.
• At sea level, the weight of a 1 square inch column of air is about 14.7 pounds.
• This means that the pressure at the earth’s surface (at sea level) exerted by the air (air pressure) is 14.7 pounds per square inch.
• The very unintuitive aspect of pressure in a fluid (gas or liquid) is that the pressure measured at a point in the fluid is exerted equally in all directions – up, down, sideways, and at every angle. This means that the air exerts a pressure in all directions of about 14.7 pounds per square inch.
• We don’t normally feel this pressure because it is all around us. It’s like a fish not feeling the water pressure all around them.

Demonstration

PART I

Fill a small plastic cup to the top with water. 

Place a playing card or laminated index card on top of the cup. 

Note: Conduct the rest of the demonstration over a bucket in case of spills.

While holding the card in place, turn the cup upside down. 

Now, carefully remove your hand and the card should stay in place even though the cup is upside down. The water should stay in the cup.

Explanation: The air pressure exerted upward on the card is greater than the pressure caused by the weight of the water. Therefore, the air pressure keeps the card in place and the water stays in the cup.

You can further explain that the pressure is exerted in every direction so no matter how you hold the cup, the card should stay pressed against it and the water should not come out. At this point you should hold the cup in different directions so that the card is sideways, on top, and at various other angles to show this is true.

PART II

Before the demonstration, carefully use a hammer and nail to poke a hole in the side of the bottle about 2 inches from the bottom. 

While students are watching, put your finger over the hole and fill the bottle to the top. Don’t put the cap on yet.

Note: Conduct the rest of the demonstration over a bucket in case of spills.

Take your finger off the hole and show students that water comes out of the hole. 

While the water is coming out, screw the cap on the bottle. 

The water should stop coming out of the hole. Ask students what is keeping the water from coming out of the hole.

Explanation: When the bottle is closed, the air pressure cannot push on the water inside the bottle. Instead, air pressure is exerted on the outside of the bottle (and the hole). This pressure is greater than the pressure caused by the weight of the water in the bottle. Therefore, the air pressure prevents the water from leaving the closed bottle.

When the bottle is opened, even a small amount, the air pressure pushes on the water. The combined pressure from the weight of the water plus the outside air pressure pushes the water out of the hole.