Objectives | Materials | Invitation to Learn | Lab Procedure | Closure

After investigating the effect of volume changes on the pressure of a confined gas held at constant temperature, students will be able to :

1. Relate the changes in volume of gases to changes in pressure (i.e. inverse relationship).
2. Explain why pressure of a gas increases as volume decreases.
3. Predict the pressure of a gas when its volume is specified.
4. Construct a mathematical expression for the relationship between pressure and volume (i.e. P x V is always equal to a constant value dependent on temperature).

Preliminary Knowledge:

1. Basic math skills and some simple algebra
2. Graphing skills
3. Familiarity with the principle of the particulate nature of matter (a concept presented in the 7th grade Science Core Standards)
4. The natural motion of particles in matter
5. Common units of measurement for volume such as liters (L) and units for pressure such as mm Hg or atmospheres (atm)

• Computer Lab with Internet connection or a single machine and suitable projection equipment. We recommend a Pentium class computer (166 MHz, with at least 32 MB ram).
• Java-capable and enabled browser with the Java plug-in installed. For questions/assistance write <support@cosmic.utah.edu>.
• Calculator
• Lab notebook or Student Packet - This is a printable version of the lab materials (instructions, tables, questions, graph formats, and questions/problems) where students can record their lab.

1. Teachers may introduce this lab by having students blow up a balloon, then attempt to compress the sides of the balloon. Have them describe what they feel. Hopefully they will realize that the gases inside the balloon push back. They may need prompting to find the word "pressure" to describe the pushing back that they feel. Alternatively, an empty two liter bottle with a tightly fitting cap will work for this preliminary activity.
2. As the balloon is pushed inward, students should be challenged to describe what is happening to the volume of the balloon. (It's getting smaller.) Have them describe whether there is any change in the amount of gas in the balloon. (There is no change in the number of particles, nor in the mass of the particles). Make sure students realize that they can change the volume of a gas without changing the number of gas particles.
3. Ask students if they could imagine any other variable which could cause the balloon and its contents to change. Conceivably, some students may realize that temperature will have some effect. Challenge them to design an experiment to test the validity of their ideas.

Pre-Assessment:

Here are several questions that will gauge your students' understanding of the effect of volume changes on the pressure of a gas.

1. If you want to reduce the pressure of gas in a tire, what could you do?
2. As you let the gases in a hair spray can escape, what happens to the volume and pressure of the gas?
3. If you want air to enter your lungs, what must you do to the space inside your chest cavity?
4. What is the relationship between gas volume and gas pressure when temperature is kept constant?

Directions for teaching the lab:

Invite students to proceed to the beginning of the Student Lab. They should first read the introductory statements and other information that pertains to the lab. As students scroll down the page, they will see the lab setup. The gas particles will be shown as colored dots inside of a transparent container. They will be moving randomly and at constant velocity. The values for the volume (in liters) and the pressure (in mm Hg) will be displayed. The teacher should emphasize that the gas container, shown as a square on the screen, is actually a sliced-section of a larger cubic container. Teachers may want to have a large cubic box to show what a sliced-section actually represents. Students will be able to vary the volume of the container using the Increase Volume and Decrease Volume buttons. Students will have the option of running the experiment at temperatures between -50 and 150 degrees C. Different temperatures can be selected by moving the top of the mercury column up or down. During the analysis of the data, students will discover that P x V is always a constant number when a single temperature value is used. If the temperature is changed, the product of P x V is different. To avoid varying temperature and volume simultaneously, data will only be entered if the selected temperature remains the same. If a new temperature is selected, the data table will be cleared and volume and pressure for the new temperature will be entered. Remind students that they need to record the data from the experiments into their own lab notebooks. After making several changes in volume, students should continue to the graphing and analysis sections of the student page. If they need to collect additional data, they can scroll back up to the gas container.

Summary:

This lesson should give students experience with manipulating variables and describing the relationships between those variables. Collecting and recording data correctly will be emphasized. Analysis of data both qualitatively and quantitatively is a major thrust of the lesson. The importance of controlling certain variables during an experiment can be emphasized.

Students should be encouraged to make connections from this lesson to some common events in their own lives.

• What happens to the pressure in an automobile tire when we overload the car, causing the tire to be compressed?
• What happens to the pressure in our lungs when we increase the volume during inhalation?
• Why does a balloon usually burst if we press on it too hard?
• During scuba diving, what happens to the pressure of the air behind your eardrum as the water pushes on it? What will you experience when this happens?
• As the piston in an automobile engine moves upward causing the gasoline vapor and the air to take up a smaller volume, what happens to the pressure of these gases? Why is this condition of the gases advantageous to the operation of an internal combustion engine?

From a purely mathematical standpoint, it is expected that after completing the activities and analyzing the data, students will discover:

• There is an inverse relationship between gas volume and pressure at a given temperature.
• The product of P x V is always the same number for any particular temperature.
• When a value for P is given, the corresponding value for V can be calculated by using the formula V = constant/P.

Post-Assessment:

Teachers may return to the pre-assessment questions and use these same questions or construct their own post-assessment instrument. Hopefully, teachers can also include more difficult, higher-level questions in their post-assessment.

Extension activities: