Gas Laws: Pressure

Interactive, scaffolded model

Overview and Learning Objectives

Class 1:

Part A: Students place a balloon on an empty two liter soda bottle, and then observe what happens when heated and cooled by placing the bottle in a hot and cold water bath. They then discuss what they think is happening.

Part B: Students use Molecular Workbench software to experiment with a contained gas. They experiment with changes in pressure, temperature, and volume, and discuss their observations.

Class 2:

Part A: A classic demonstration is done in which the "suction force" causes two plates or hemispheres to stick together. Students then discuss why they stick together. They then kinesthetically model why air gets "sucked" back in when the valve sealing the two plates is

opened.

Part B: Students use Molecular Workbench software to experiment with several different ways of creating an apparent pulling force of suction while being able to visualize the actual pushing forces of molecules. A tube will be created that has a movable divider. This divider can be "sucked" to the left by changing several aspects of the environment in the left side of the tube: volume, temperature, and numbers of atoms.

Part C: More demonstrations are done, each one discussed to explain how the visual result can occur through a pushing force, not the previous false notion of a pulling "suction force."

Students will be able to:

• define gas pressure in terms of atomic impacts;
• name two ways that the impacts of molecules could be altered to change gas pressure;
• predict what will happen to the pressure if atoms are added or removed from a container of flexible volume;
• predict what will happen to the pressure if the temperature of a gas changes in a container of flexible volume;
• predict what will happen to the temperature if the volume is changed in a container filled with a gas.

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Assessment

There are many questions embedded in this model-based activity. In addition, students may be asked:

• How does a gas exert a pressure if you can't see anything pushing back?
• List two ways you can increase the pressure in a container with a fixed volume and explain why this causes a pressure increase.
• At high altitudes, where many hot air balloons go, the air pressure is lower than on the ground. Draw a picture showing what the air on the ground looks like at the atomic level and compare it to the air at high altitude.

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Central Concepts

Key Concept:

Class 1: Gas pressure is a function of the frequency and force of impacts of molecules distributed over a certain area.

Class 2: Suction, which is usually thought of as a pulling force, does not exist.

Additional Related Concepts

Biology

• Waste Disposal

Physics/Chemistry

• Collision
• Concentration
• Gas
• Gradient
• Pressure

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Textbook References

• Web of Life - Chapter 2: Chemical Basis of Life

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Activity Credits

Created by CC Project: Molecular Workbench using Molecular Workbench + Pedagogica

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