Materials

• Plastic syringes, 1 per pair of students
• How Does a Syringe Work? activity sheet, 1 per student
• Optional Video, Investigations in Physics: Pressure. (1997). Princeton, NJ: Films for the Humanities and Sciences.
• Optional Computer Simulation, Atomic Microscope 3-D, available at: http://www.starkdesign.com/products/ or Virtual Molecular Dynamics Laboratory available at http://polymer.bu.edu/vmdl

Prep Step

• Review the lesson plan, background information, and understanding goals.
• Photocopy the activity sheet, How Does a Syringe Work?.
• If you decide to do the Reinforcement Activities, be sure to order the video and download the computer simulation well before the day of the lesson.

Analyze Thinking

Step 1: Thinking About Volume and Pressure

Ask your students to recall what they discovered during the Modeling the Jar and the Bag activity, when they tried to pull the bag out of the jar and to push the bag into the jar.

Next, ask them to do a "thought experiment." When the bag was sealed inside the jar, if they were strong enough (and the bag was strong enough) to overcome the differential in air pressure and they COULD pull the bag out of the jar, what would happen to the air inside the jar? Don't collect ideas at this point; rather, have students consider what they think would happen.

RECAST Thinking

Step 2: Exploring the Relationship Between Pressure and Volume

Explain to your students that they will do an activity to help them think about what happens in situations when volume is increased but no additional air is able to enter the enclosed space (as was the case when trying to pull the bag out of the jar).

• Pass out the activity sheet, How Does a Syringe Work? to each student.
• Have students pair up to complete the syringe activity.

During the activity, students will:

• Draw a model to show what the air looks like in a syringe before compression.
• Draw a second model to show what the air looks like in a syringe after compression.
• Explain their observations using relational causality.
• Describe an example of relational causality from their own lives.

Note to Teacher: Note whether students realize that the amount of air in the syringe is the same as before, only compressed in less space. If students' models contain a different amount of air after compression, challenge them to think about whether more air can enter or if the amount was conserved, and what happened to the air when it was compressed.

Give students about 25 minutes to work on questions 1 through 4 on the activity sheet. They will answer question 5 following the class discussion.

Step 3: Discussing Students' Discoveries

Bring the students back together as a group to discuss what they discovered.

Have two students put their models on the board and explain how their ideas changed as they progressed from one model to the next (see examples of students' models).

Gather feedback on students' models. Ask:

• Why are you able to increase the volume with the syringe but not with the plastic bag? Gather ideas. (The bag would rip; because the syringe is hard plastic, you can force it.)
• What happens to the amount of air in the syringe when you compress it with your finger over the top? Gather ideas. (The amount of air stays the same even though the space it takes up is less.)
• What happens to the air pressure in the syringe when you compress it with your finger over the top? Gather ideas. (The air pressure increases.)
• What happens to the amount of air in the syringe when you decompress it with your finger over the top? Gather ideas. (The amount of air stays the same even though it takes up more space.)
• What happens to the air pressure in the syringe when you decompress it with your finger over the top? Gather ideas. (If you can decompress it, the pressure decreases.)
• If the outside air pressure changed, would it affect the air inside the syringe? Would it be obvious? (If the syringe is made of hard plastic, the shape of the syringe would not change, but there would still be an air pressure differential present. That is what the students feel when they push the plunger back into the syringe—the resistance due to the air pressure differential. The higher pressure inside is trying to equalize with the lower outside pressure.)
• What do you think would happen if a hole was poked in the syringe? (The air would rush out, equalizing with the outside air pressure.)

Explore Causality

Step 4: Introducing Boyle's Law Through Relational Causality

Discuss what happens in terms of relational causality:

• Ask, "What does this tell us about volume and air pressure in terms of relational causality?" (That volume and air pressure are related and that as one increases the other decreases.)
• Explain that scientists have a rule to explain what happened with the syringe activity. It is not often expressed in terms of a model or drawing, as we have seen in other lessons. Rather, the model is expressed as a law that fluids (either liquids or gases) consistently follow.
• Tell the students: Boyle's Law is used to predict the behavior of fluids in a closed container when either the volume or pressure of the fluid is changed (at a constant temperature). The law predicts that if the volume of a fluid is increased, the pressure is decreased and vice versa.

Review, Extend, Apply

Step 5: Thinking About Instances When Boyle's Law Applies

Make sure the students understand that Boyle's Law explains what is going on inside a closed container, not the relationship between the inside and the outside air pressure of the container. However, the two are related because the air pressure differential that is created due to an increase or decrease in volume within the container is in relation to the outside environment.

Ask students to think about other instances where they have observed Boyle's Law in action in everyday life. Encourage them to come with some examples. For instance, if you take a partially filled balloon and tie it halfway down the length of the balloon, it will have more pressure than if the molecules that make up the air are spread out in the entire space. Then, they should choose an example and illustrate it under Question 5 on the activity sheet.