1. Living By Chemistry SECOND EDITION
Unit 3: WEATHER
Phase Changes and Behavior of Gases

2. Lesson 57: Air Force
Air Pressure

3. ChemCatalyst
If you blow up a balloon and let it go, it flies around the room.
Why does the gas inside the balloon come out?
How can you change how fast the balloon moves?
How does this demonstration provide evidence of air pressure?

4. Key Question
What evidence do we have that gases exert pressure?

5. You will be able to: describe and define gas pressure
explain what causes air pressure
complete simple air pressure calculations

6. Prepare for the Demonstration
Watch the demonstration as a class.

7. Discussion Notes
In each demonstration, air is trapped somewhere.

8. Discussion Notes (cont.)
Air pressure is the force per unit area exerted on objects as a result of gas molecules colliding with those objects.
Pressure: Force applied over a specific area. Force per unit area. Gas pressure is caused by gas molecules striking objects or the walls of a container.

9. Discussion Notes (cont.)
In examples like the ones seen today, there are generally two types of air pressure to consider: air trapped inside a container, and air from the atmosphere outside a container.
The mixture of gases that surrounds you at all times is called the atmosphere.

10. Discussion Notes (cont.)
Atmospheric pressure: Air pressure that is always present on Earth as a result of air molecules colliding with the surfaces of objects on the planet. At sea level and 25°C, there is 14.7 lb/in2 of air pressure from the air around us. This is referred to as one atmosphere of pressure, or 1 atm.

11. Wrap Up What evidence do we have that gases exert pressure?
Gas pressure is defined as the force per unit area caused by the molecules of a gas pushing on objects or on the walls of its container.
Atmospheric pressure is the pressure of the air around us.
Many generalizations you make about air can also be made about any gas, whether helium, argon, or any other.

12. Check-In
Give evidence that gas molecules exert pressure on the walls of whatever container they are in.