In this demonstration, students will observe two situations. First a student will be lifted off the desk as other students blow air into straws connected to a garbage bag in order to inflate it. Secondly, the class will observe a garbage bag shrink wrapping a student as a vacuum removes air from the bag.
This demonstration will help prepare your students to meet the performance expectations in the following standards:
- MS-PS1-4: Develop a model that predicts and describes changes in particle motion
By the end of this demonstration, students should be able to
- Explain how gas particles collide with the walls of a container, and identify these collisions as pressure.
- Describe how particles pushed the walls of the garbage bag in both demonstrations.
- Explain the difference between the behavior of particles inside the bag and outside the bag. Understanding what it means if the number of collisions inside and outside are not equal.
- Make connections between atmospheric pressure and the role that pressure has in the behavior of gas particles based on the observed phenomenon in each demonstration.
This demonstration supports students’ understanding of
- Kinetic Molecular Theory
- Behavior of Gases
- Gas Laws
Teacher Preparation: 30 minutes
Lesson: 55 minutes
Demo 1: Inflate a Student
- One big contractor trash bag or two large black garbage bags, like the type used in the big cafeteria barrels. (Tip: You can ask the maintenance staff for these.)
- 6 drinking straws
- Duct tape
Demo 2: Shrink Wrap a Student
- One large black garbage bag, like the type used in the cafeteria barrels.
- Vacuum cleaner with a hose extension
- No specific safety precautions need to be observed for this activity.
- Inflate a Student:
- This demonstration involves a modified garbage bag. Use a large contractor bag sealed shut with duct tape or two heavy weight black garbage bags with the open ends taped together. Poke 6 holes and attach 6 straws with duct tape. Arrange the holes so that there is one straw in each of the four corners and one on either side of the middle of the bag. The bag must be air tight.
- Ask for 7 volunteers. One student will sit on top they bag (the lighter the student is the better) and six students will each blow air into the bag.
- Place the bag on a large table or on the floor, and have the student volunteer lay on the bag. The six students who will be blowing air should arrange themselves near a straw. Tell students the object is to lift the student off the desk/floor.
- As students begin to blow air into the bag, the bag will begin to inflate like an air mattress. Instruct students to cover their straw with their thumb if they need to take a break to avoid air escaping.
- The bag can fill unevenly so keep an eye on the student on the bag and help to reposition so he/she does not roll off. Once the student has been lifted or it seems no more air can be added to the bag, end the demonstration.
- Students will record their observations on their student sheet. They will also create a “before, during and after” particle diagram, attempting to focus on why the bag was inflating and why the student was being lifted.
- Shrink Wrap a Student:
- Ask for a volunteer to climb inside the bag (of course there head will not be placed in the bag!) Have the student removes shoes if it looks like they might tear the bag and ask them to sit with legs crossed “crisscross-applesauce”. Refer to photo for guidance.
- Arrange the opening around the student’s shoulders, being careful to not restrict their breathing by placing it tightly around their neck. Put the vacuum hose inside the bag with the student and ask them to hold it away from clothing or the sides of the bag.
- Holding the bag in place, creating a “seal”, turn on the vacuum and watch the student be “shrink wrapped”.
- You may want to shrink a few more kids if time permits, they usually really like this!
- Students should record their observations on their student sheets. They will also create a “before, during and after” particle diagram, attempting to focus on why the bag was inflating why the bag “squished” the student.
- *Note: Student’s face has been purposely concealed in photos.
- After the two demonstrations, students work in small groups to create a model on a whiteboard that uses a particle drawing to explain what was observed in both demonstrations. While they are working, the teacher is asking questions:
- What caused the bag to shrink to the student? How can you show that?
- Is the bag being “sucked” to the student? Why not?
- Is there evidence to support air is “pushing”? How can we show pushing?
- What lifted the student?
- What else did you notice happening?
- The teacher is being careful not to answer student questions and to challenge their thinking. “Why do you think the bag was “sucked” to the student? What do you mean when you say the vacuum “sucked” the air out of the bag? Let’s say “science never sucks”, can you explain what is happening without suction?”
- Students present their ideas to the class and discuss the similarities and differences in the models. Students question each other and provide evidence based answers.
- On the whiteboards students should use arrows to show the force of the particles pushing inside and outside of the bag.
- Example 1: The student descriptions in the photo below say:
- Blow Up: Particles are being added to the system through the straws.
- Shrink Wrap: Particles left the system through the vacuum.
- Example 2:
- Blow Up: Something entered the system from the surroundings.
- Shrink Wrap: Something left the system into the surroundings.
- Notice, neither example focuses on what is outside the system. Students present their ideas to the class and discuss the similarities and differences in the models. Students question each other and provide evidence based answers. The teacher pushes to have them explain WHY. They know from previous lessons that there are air particles present outside of the system. The teacher must get them to consider what evidence we have to support that claim. Allow students to revise their models to include the particles outside of the system.
- Students decide that there is air particles pushing everywhere. Is there a name for this? Students agree it is called pressure. The teacher introduces the pressure bar. At sea level, for every one square inch of surface, 14.7 pounds of pressure is exerted on everything.
- Teachers can purchase an Atmosphere bar from Flinn Scientific.
- Students generate a new question to investigate, “We have evidence to support particles around us exerting pressure, but what causes pressure?”
- Teacher Resources:
For the Student
Part 1: Inflate the Student
Describe in words what you saw happen as students blew air into the straws:
WHY did this happen? Draw a particle diagram representing the bag before students began to blow into the straws, during, and after the student on the bag was blown up off the desks.
Explain the particle diagram drawn above.
After the class discussion, revise your model. Using a particle diagram, explain WHY the student was blown up off the desks.
What conclusions can you draw from your revised model? What new questions do you have?
Part 2: Shrink Wrap the Student
Describe in words what you saw happen as the vacuum was turned on with the hose in the bag with the student:
WHY did this happen? Draw a particle diagram representing the bag before the vacuum was turned on, during, and after the student was shrink wrapped inside the bag.
Explain the particle diagram drawn above.
After the class discussion, revise your model.Using a particle diagram, explain WHY the student was shrink wrapped.
What conclusions can you draw from your revised model?What new questions do you have?