Air Pressure Curriculum

Section 3—Lesson 7: Reasoning About Air Pressure Differentials Using Relational Causality

Lesson Plan

Materials

Erlenmeyer flasks, 250 ml, 3 per group
Juice (a color that is easy to see such as grape, cranberry, or orange juice), 450 ml per group
Petroleum jelly, 1 jar per multiple classes
Pin, 1 per class
Single-holed rubber stopper (size #6 1/2), 1 per group
Straws (chosen to fit tightly through the size #6 1/2 stopper hole), 3 per group
Drinking from a Straw: Models to Reveal What Happens activity sheet, 1 per student (PDF 9 KB)

Prep Step

Review the lesson plan, background information, and understanding goals.
To prepare for the activity, do the following for each group:
- Label the Erlenmeyer flasks, A, B, and C.
- Fill the Erlenmeyer flasks to the 150 ml line with juice.
- Place a straw in Flask A.
- Make a small non-obvious hole in another straw, so that when the straw is in the flask, the hole will be just below the top of the flask. Place the straw in Flask B.
- Insert a third straw through the hole of the stopper. Place petroleum jelly around the hole to make an airtight seal between the straw and the stopper. Place the stopper into Flask C. If the seal is not airtight, place petroleum jelly around the stopper to seal it.
Photocopy the activity sheet, Drinking From a Straw: Models to Reveal What Happens for each student.
Read the Picture of Practice.

Analyze Thinking

Step 1: Reflect on What Happens When You Drink From a Straw

Pass out the activity sheet, Drinking From a Straw: Models to Reveal What Happens. Instruct students to draw a model of what they think causes the liquid to rise in a straw when you drink from it. Give students at least 10 minutes to think through and illustrate their ideas.

Ask for several volunteers to put their models on the board (try to get a variety of models). Discuss the models and encourage the class to either agree or disagree with them. When students say things like, "a vacuum is formed," ask what a vacuum is.

Note to Teacher: At this point, it is likely that the models will be quite similar. Most students tend to construct one-way, linear causal models to explain the event.

RECAST Thinking

Step 2: Demonstrate Why Students Might Need to RECAST Their Models

Explain to the students that they will be doing an activity to help them understand how drinking from a straw works.

Select three students to participate in a mini-competition.
It will begin with a demonstration. Say: "Here I have three clean flasks and straws with juice in them. You will be participating in a little competition to see who can drink the juice from their straw the fastest. You can see that there are some differences between the flasks. We'll discuss those differences in detail later. For the student drinking from Flask C, you'll notice something around the stopper. That is just a little petroleum jelly."


Flask A	Flask B	Flask C

Say, "When I say "go," you are going to put your lips to the straw and begin drinking continuously. Once you have sealed your lips around the straw do not take them off or open them for air. Just breathe through your nose and drink as fast as you can. The student with Flask C should hold the stopper in place gently but not push down on it."
Make sure the students understand. Ask if they have any questions.
Make sure that the rest of the class is positioned well to see the demonstration.
Say, "Ready, Set, Go!"

Note to Teacher: The student drinking from Flask A should finish first. The student drinking from Flask B often cannot remove much liquid at all, and if the student drinking from Flask C seals his or her lips on the straw, little liquid will come up the straw.

The students often laugh or are puzzled at the outcome. Ask each student to tell the class what it was like to drink from the straw, and to explain why he or she is laughing.
Typically, the student drinking from Flask B says something like, "there's a hole in my straw—no fair!" If he or she does, ask why that would matter.
Explain the modifications to the flasks/straws. Say, "The first flask and straw setup, Flask A, has no changes and is just like when you drink from a normal straw. In the second flask and straw setup, Flask B, there is a hole in the straw just beneath the top of the flask. (Point to it.) The third flask and straw setup, Flask C, has a stopper and some petroleum jelly to seal off the inside of the flask. (Point to it.)
Gather one or two ideas from the class about what might be happening.

Step 3: Figuring Out What is Going On

Explain to the students that they will be experimenting with and thinking about what happens with each straw and flask set-up, and analyzing what it tells them about how drinking from a straw works.

Give each trio of students a set of three flasks as described above so that each student has one.
Say, "Instead of having a competition, you will drink from the straws one at a time, starting with Flask A, and observe what happens. Your activity sheet asks you to draw a model of what you think is going on with Flasks B and C and to think about what evidence fits and doesn't fit with it. Don't forget to use each other's minds well¹ and think together about what happens."
Have students work step by step through the flasks, observing, discussing, and modeling their ideas. Circulate while students are working, and ask them about their interpretations and how what they are finding out from the different flasks impacts what they think.

Note to Teacher: Some students might accidentally blow into the straw of the stoppered flask. The additional air in the flask increases the air pressure. The greater pressure causes the liquid to rise in the straw, much to the amazement of the students! In fact, it sometime shoots out and students may end up wearing the liquid. If this happens with any of the groups, make sure they think about what is going on to cause this to happen, and share their findings with the class.

Explore Causality

Step 4: Contrasting How Linear and Relational Causal Models Explain What Happens

Consider as a group what the outcomes suggest about the nature of the causality involved when drinking from a straw:

Ask some students to put their models on the board and to explain them.
Consider for each set of models: "What puzzling aspects do you feel that you can explain? What puzzling aspects remain?" As a group, consider some of the variables that may have been left out. "How might these affect the behavior of the liquid?"
Draw the students' attention to models that use relational causality as compared to linear causality. Review the definition of relational causality, "In a relational causal model, what happens is due to a relationship between two variables—often a relationship of balance or imbalance."
Ask, "Does one form of causality do a better job than the other of explaining what is going on?"
Make sure that the following explanations in terms of relational causality are clear to the students:

Flask A: The student draws some air into his or her mouth, thereby lowering the air pressure in the straw. The outside air pressure (atmospheric air pressure) remains constant, therefore there is a differential where the liquid moves from areas of greater pressure to areas of lesser pressure.

Flask B: The hole in the straw enables outside air to enter the straw when the student is 'sucking.' This makes it impossible to achieve a pressure differential between the air outside the straw and the air inside the straw.

Flask C: With the rubber stopper, there is no way for the outside air pressure to come in contact with the liquid's surface. The student should be able to draw up some of the liquid, which lowers the pressure of the air in the stoppered flask. So while the student lowers air pressure in the straw, the air pressure in the flask is also lowered (both are lower than the outside air pressure, but are equal to each other). This makes it impossible to achieve a pressure differential.

Say, "What else could you do to further test or provide evidence for the relational causal interpretations?" Gather ideas and test them. For instance, what do the students think will happen if they blow gently into the stoppered flask? Can they find a way to get a pressure differential in this case? If they used a straw with a hole above the stopper on Flask C, what do they predict would happen? What other variations can they think of?

Review, Extend, Apply

Step 5: Making Connections to Other Pressure-related Phenomena

Ask students to think about other experiences that they have had when drinking from a straw, and to analyze what is going on. For instance, what happens when you drink from a juice box? Why do you need to take your lips off every so often? Do you ever get juice splashed on you? What might be going on? What other instances of drinking from a straw can you now explain?

Next, broaden the discussion beyond drinking from a straw and ask students to think about other experiences that involve pressure differentials—instances involving higher and lower pressures. Some examples include the pressure changes involved when going up in an airplane, or the pressure differences inside and outside of a house during a hurricane (when there is lower pressure outside and higher pressure inside).

Revisit some of the activities that students did earlier in the module. What do they think is going on with a barometer? What about when we inverted a cup full of water?

Introduce and discuss the following puzzle: "If wind is the result of a relationship between higher and lower pressure, and pressure is not forceful in the sense of having a direction, why does it feel as though it actively pushes against your face?" (Because wind is air moving from areas of higher pressure to areas of lower pressure, it does have a direction. In a sense, it is pushed towards areas of lower pressure, and so you can feel the force of the wind on your face. Just like the liquid in the straw that gets pushed into your mouth, the air gets pushed towards areas of lower pressure.)