Unit #1

Activity 14
Don’t Be so Pushy ... Oh, You’re a Gas. That’s OK.

Activity Overview

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

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 Workbench software to experiment with a contained gas. They experiment with changes in pressure, temperature, and volume, and discuss their observations.

Learning Objectives

Students will:

• 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.

Conceptual Prologue

Macro-Micro Connection

On the ground, when the balloon is initially being "filled", a volume of air that is smaller than the balloon's volume is heated. This heating causes the air to expand in volume. The atoms eventually push out far enough so that the balloon is full. It's not that more air has been added, but the same air is now taking up more space. When the balloon has been inflated to it full volume any increase in heating causes an increases in pressure, forcing air out of the opening at the bottom of the balloon, resulting in less air inside the balloon and a lowering of its mass. Less air inside the balloon means the air inside is “lighter” than the air outside.

Other macro connections:

• Air pressure is greater at the surface of the Earth, because the concentration of atoms is much higher than that found at high altitudes.
• Because of our understanding of gas pressure we can explain why climbers need to bring extra oxygen to the peaks of extremely high mountains. The lower pressure is due to the atoms being more spread out, so they can’t inhale as much oxygen into their lungs on top of the mountain as they can at sea level.
• SCUBA tanks and other gas tanks have to be built to withstand very high pressures, because a very large number of gas atoms are placed inside these containers.
• When air is compressed inside a sealed syringe, the pressure increases as the volume gets smaller. This explains why the syringe gets harder and harder to squeeze and why it springs back to its original position when you let go of the plunger.
• The end of a bike pump gets hot because the gas at the end of the pump is hot from the increase in pressure inside the pump chamber.

Science Concepts

Although we can’t see any physical evidence of how a gas can exert a pressure, we certainly can feel that pressure. To understand how a gas exerts pressure we need to recall the underlying atomic model: a gas is a bunch of atoms bouncing around like superballs. When an atom bounces off the walls of its container, the container feels the impact in the same way you would feel an impact from a ball bouncing off of a tennis racquet. However, the impact felt by the wall of the container is extremely small. The bouncing of one atom off of the wall of a container would be virtually insignificant. It takes millions upon millions of impacts between atoms and the walls of their containers concentrated on a vary small area to register a measurable pressure.

There are two primary factors which explain the magnitude of the pressure exerted by atoms: the frequency of impacts and the force of those impacts.

There are several ways that gas pressure can be increased due to increased frequency of impacts.

• Put more gas in the container. If you have more gaseous atoms in a container then there will be more frequent impacts against the walls of the container causing a greater pressure on its walls.
• Make the container smaller. If you have the same number of atoms crammed into a smaller space, then they will hit the walls more frequently with more atoms hitting the same area repeatedly, increasing the pressure on the walls.
• Raise the temperature. If you make the atoms move faster then they will hit the walls more frequently, increasing the pressure on the walls.
  

We can also increase gas pressure if each atom hits the wall of its container with greater force. There is one primary way to make this happen:

• Raise the temperature. By raising the temperature you add kinetic energy [energy of motion] to the atoms, therefore, increasing their velocity. If they are moving faster when they hit the wall of the container, then the impact against the wall will be greater, increasing the pressure on the walls.

Naive Conceptions

• Gasses consist of a continuous substance.

Students will often maintain that a gas can be made of atoms but that there must be something between the atoms. The idea of total vacuum or "nothingness" between the atoms is often hard for them to believe.

• Gasses only push back when their atoms are packed close enough to touch each other.

Gasses are always exerting a pressure. If all the atoms in a gas were close enough to touch each other before pushing back then only liquids or solids could exert a pushing force. Even now, you are experiencing an enormous amount of pressure from the air surrounding you. Every square inch of your body is feeling a force equivalent to 15 pounds pushing on it. You are used to this pressure and so don't feel it. However, it is this constant pressure that surrounds us that is responsible for our common experience of suction whenever there is a region of pressure that is lower than the "normal" pressure we always experience.

Activity Design and Execution

Major Science Concepts:	• Pressure • Gas laws
Assumed Previous Knowledge:	Assumed Previous Knowledge: • That atoms behave like superballs with 100% elasticity. • That kinetic energy is based on both mass and velocity. • That temperature is a measure of the average kinetic energy of the atoms or molecules of a substance. • That hotter temperatures mean higher kinetic energies.
Time:	• Part A: approximately 30 minutes • Part B: approximately 75 minutes
Materials:	For each group: • An empty 2 liter soda bottle. • A balloon which will fit over the mouth of the soda bottle. • A large bucket filled with hot water. • A large bucket filled with cold water. • A computer with Workbench software.
Advanced Preparation: (if any)	• It may take some time to heat a large bucket of water.

Investigative Question: How do gasses behave in a container if you change the volume, pressure and/or number of atoms?

Part A:

1. Prepare the soda bottles by placing a balloon over each one. Try to make sure the balloon has as little air in it as possible.
2. Ask students to alternately place the bottles in hot water and cold water, leaving them in the water baths until they notice a change in the balloon.
3. Bring the class together to discuss an explanation of why the balloon inflated in hot water and deflated in cold water. Ask them what must be happening at the atomic level to help explain this. Prompt them to use atomic motions in their explanations.
   

Part B: (you may want to have the class pause after steps 4, 6, and 8 for discussion at that time)

1. Have students use Workbench software to open the "Gas Laws" activity.
2. The software will present the students with a cylinder that has a top which can slide up and down, making the pressure constant. Also present will be a way for students to add or remove heat energy, as well as, graphs of pressure, temperature, and volume.
3. The software will ask them to experiment with changing the temperature of the container and noticing what happens to all the graphs when the temperature is increased or decreased. It will then ask them to describe what relationships they see between temperature, volume, and pressure.
4. The software will explain that the pressure is measured by the number and strength of impacts of atoms against the walls of the container. It will then ask them to notice and explain why the pressure first increases and then goes back down to the original level when the temperature is increased.
5. The software will then hide the temperature control and allow students to change the number of atoms. It will then ask them to describe what relationships they see between temperature, volume, pressure, and the number of atoms.
6. The software will explain that the pressure is measured by the number and strength of impacts of atoms against the walls of the container. It will then ask them to notice and explain why the pressure first increases and then goes back down to the original level when atoms are added to the container.
7. The software will then hide the number of atoms control and allow students to change the volume of the container. It will then ask them to describe what relationships they see between volume, temperature, and pressure.
8. The students should notice that both temperature and pressure increase when the volume is decreased. The software will then ask the students to explain why both of these things happen by referring to the number and strength of impacts.
9. The software will then allow them to control, atom number, temperature, volume, and pressure while asking the following questions:
   • If the volume of the container is fixed (so that the top doesn't slide up and down) predict what would happen if you added or removed atoms to the container. Explain your reasoning. Try this and see if you were right.
   • If the volume of the container is fixed (so that the top doesn't slide up and down) predict what would happen to the pressure if you added or removed heat energy to the container. Explain your reasoning. Try this and see if you were right.
   • When you pump up a bike tire the end of the pump near the tire gets warm (not from friction). Explain why this happens.
   • SCUBA tanks have to be built to withstand very high pressure because a lot of gas molecules are put inside them. Why would putting more molecules inside cause high pressure.
   • Try to make the pressure low any way you can without changing the temperature. How does a high pressure gas compare to a low pressure gas? Climbers who go to the top of Mt. Everest, the tallest mountain on Earth need to bring extra oxygen. The air at that altitude is very low pressure. Why do you think they need to bring oxygen with them because of this low pressure?

10. Have a discussion with students about all they have noticed in doing these simulations. Bring up many of the “Macro to Micro Connections” from above and have students try to explain some of these using the concepts they have just learned.

Assessment

Have students write several things in their notebooks:

1. Their answers to the above questions from step #9 above.
2. How does a gas exert a pressure if you can't see anything pushing back?
3. List two ways you can increase the pressure in a container with a fixed volume and explain why this causes a pressure increase.
4. 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 when compared to the air at high altitudes.

Internal Notes:
• See computer lab G for mockup of this simulation.