Understanding Air Pressure (a lesson series)
Grade level(s):Grade 5
Topic:The relationship between high and low pressure and the tendency for high pressure to move toward low pressure
Air has mass and takes up space. Changes in air pressure can be visualized and manipulated using syringes.
BEFORE: syringe, mass, volume, force, increase, decrease, circle, oval, cylinder, molecules, density gravity During: static, equilibrium, compression, depression, high pressure, low pressure, atmosphere, atmospheric pressure, omnidirectional
What you need:
1) Copy of Harvard unit on air pressure "Causal Patterns in Air Pressure Phenomena" (001854) or go to weblink: harvard unit on air pressure
2) Set of 250 ml beakers and ziplock bags;
3) Set of 250ml erlinmeyer flasks with stoppers and two funnels, straws with and without holes;
4) Set of straight sided containers and clear straws,
5) Clear plastic aquarium/terrarium,
6) A digital scale (or an accurate balance for small masses (<1 gram))
From a store:
1) One liter juice or other colored liquid for straw race,
2) Cooking oil for barometers (1/2 cup per setup)
3) several large balloons (10 inch)
From your school:
1) FOSS kit: Water Planet (an extra FOSS kit may be borrowed from the SEP Resource Center). You will need items from this kit as well:
a) 0.5 liter water bottle (soft plastic) with stopper, tubing, syringe (see page 181 of FOSS kit for additional details)
b) 30 ml syringes (2 per group), flexible plastic tubing (1 per group), medium or large binder clips (1 per group)
2) Paper towel
It works best to have students in pairs so students get plenty of opportunities to experience the phenomena, but have the support of at least one other person in the poster making process.
This investigation takes place inside the classroom
This investigation has four distinct parts. Properties of air demonstrations, initial air pressure demonstrations and activities, FOSS syringe activities, and culminating poster activity. Together they could take up to four or five class periods, depending on the time you have to devote to the activity.
The activitites establish the concepts of atmospheric pressure, differences in pressure, how changing volume affects pressure, and a molecular model of how air pressure arises. Modified from the 5th grade FOSS Water Planet Investigation "The Pressure is On" (Investigation 4, part 3)
The lesson opens with some demonstrations and activities to introduce the properties of air. Moving on to air pressure, the teacher demonstrates how one can pick up liquid in a straw using a finger as a stopper. The students make a barometer, experiment with a bag and a jar, and participate in a straw race. For each activity the question of what is causing each phenomenon is asked. Students then do single and double syringe activity from FOSS Water Planet Investigation #4. After discussion of syringe activities students are asked to go back to initial demonstrations/activities and pick one to explain in a poster format.
It is highly beneficial if they have already used syringes prior to this investigation and understand that syringes are used to measure a fluid volume. As you pull the syringe plunger out you are increasing the volume of fluid, and conversely, when you push the plunger in you are decreasing the volume of fluid in the syringe.
a) Students will be able to describe how differences in air pressure between two air masses cause air movement. They will understand that relative high pressure always moves toward relative low pressure and be able to use that understanding to explain the everyday phenomena they saw in the original demonstrations and predict the direction a high pressure system will move on a weather map (toward the low pressure system).
b) Students will be able to describe how the properties of air (mass, volume) allow it to create air pressure.
c) Students will have a better understanding of how atmospheric air pressure arises, and is always present.
d) Students will be able to describe how changing the volume of a closed system compresses air, increasing internal air pressure.
Teachers should already be familiar with the relationship between mass, volume and density. An understanding of the concepts associated with air pressure could be helpful, but the materials are very thorough, building up from simpler principles.
Demonstrations and readings from the textbook should help familiarize teachers with the concept of atmospheric pressure, how it arises, and why it is different at different elevations. Additionally, the lesson works to build an understanding of how differences in air pressure can arise, and how differences in air pressure result in air movements.
N.B.: A VERY in-depth module on air pressure and the challenges of teaching air pressure can be found at:
Get materials from SEP and store. Try all demonstrations yourself to be sure you can easily and quickly do them in front of the students. If possible, secure volunteer help to monitor activities as students are experimenting and making posters.
Prepare the collapsing water bottle to demonstrate how reduced air pressure can cause a change in volume (reproduced from page 181 of FOSS kit):
A hose-and-bottle system is used to demonstrate the effect of air pressure. You will need a discarded plastic water bottle. Make sure the one-hole rubber stopper will fit securely in the mouth of the bottle.
a) Moisten the end of a 45-cm plastic tube. Insert it as far as possible into the hole in the rubber stopper.
b) Press and twist the stopper securely into the mouth of the bottle.
c) Attach a 30 or 50 ml syringe (plunger pushed all the way in) to the free end of the tube
d) Ensure that pulling out the syringe plunger causes the bottle to collapse
Introduce a syringe.
a) Show the students how a syringe works, pointing out features of the syringe:
- the plunger, which can be pushed in or pulled out
- the barrel, which houses the syringe, and includes numbered markings on the side, corresponding to the number of mls (ccs) of volume
b) Remind the students that the syringes and tubing are scientific instruments, and should be handled gently and only following the instructions...
Lesson Implementation / Outline
In order to understand air pressure, we must first understand some properties of air. The teacher does the following demonstrations to introduce some properties:
1) Air takes up space-Cup and Paper Towel Air Volume Demonstration: place a balled up paper towel at the bottom of a clear cup (tape it to secure it to the cup) and ask what will happen to the paper towel when you submerge it in the clear aquarium filled with water? Submerge the cup and have students notice that the paper towel stays dry.
- If the cup is 'right-side up', the cup will fill with water, and the paper towel will be soaked
- If the cup is inverted, the air inside the cup is trapped, and the paper towell will not get wet
Ask: Why is the paper towel able to stay dry? Because the air trapped in the cup takes up space, and does not allow the water to soak the paper towel.
Ask: Why couldn't the water pass through the air in the cup like we pass through the air in the room? (the air in the room can move out of our way, the cup is a closed container and the air has nowhere to go).
2) Demonstrate how water easily flows through a funnel into a 250ml Flask, but is barely, or not at all able to flow through a funnel set in a stopper on the same kind of flask. Ask: Why can the water get into the flask without the stopper, but not into the flask with the stopper? (the air in the flask without a stopper has a way to escape, but the air in the flask with the stopper cannot escape and thus blocks the water from coming in).
3) Attach a balloon on the inside of a clear liter bottle and unsuccessfully try to blow it up, then attach a balloon to the inside of a liter bottle with a hole in it near the base and demonstrate how much easier it is to blow the balloon up. (again the air inside the first bottle has nowhere to escape so it is very hard to add more air into the bottle, while the air in the second bottle can escape leaving room for more air in the balloon.
4) Air can exert a force: tell students that you will show them that air can can exert a force and move heavy objects. Place a balloon under a book and proceed to lift it on one side by blowing in the balloon. give students a balloon and challenge them in pairs or groups to see how much they can lift or move with their balloons.
5) Share results. Review properties of air: post on chart paper.
Now that you have demonstrated that air behaves as other forms of matter. It has mass and volume. Students should know that air is composed of molecules, primarily nitrogen and oxygen. Air is not just 'empty space', we can look at what air is. Use pictures and diagrams to discuss the composition of air and define it as a gas whose molecules are in constant movement. A good source for molecular visuals on the different states of matter, including gas, is www.brainpop.com
Next we're going to try to figure out what is behind the movement of air, or wind. In order to get at an understanding of wind we will do some more activities to get us thinking about how air behaves.
1) First, the teacher demonstrates how one can pick up liquid from a container using a straw and closing off one end of the straw with a finger. Ask: Why doesn't the liquid fall out of the straw? (during these demonstrations/activities, simply take student ideas verbally). Then demonstrate how to make a barometer (do not name it as such at this point) with straw, gum, tape, a cup and oil (see Harvard unit p57-58 on air pressure for instructions), have students make one in groups of four. Ask: Why doesn't the oil just pour right back into the cup?
2) Second, using the 250ml beakers the teacher demonstrates the bag in and out of the jar (again, see the Harvard p69-76 unit for description) Ask: Why can't we push the bag in the jar past a certain point? (a review of air takes up space ideas) Then ask: Why can't we pull the bag out of the jar easily? What is the force that is stopping us from pulling it out?
3) Third, introduce the straw race with the three different straw set ups: one flask without stopper, straw without hole; one flask with stopper, straw without hole; one flask without stopper, straw with hole (see Harvard unit p 85-93 for description).
a) Have three students come up and compete to see who can drink the most fluid in ten seconds. Time the students and then, noticing the results, discuss why the class thinks there are differences. There will be a lot of conflicting ideas about these phenomena, and teacher can introduce the FOSS syringe activities as an investigation that will help us solve the mystery of all the different things we saw happen. The syringe activities and many connections are described in-depth on pages 182-188 of the FOSS Water Planet binder.
4) Syringe activities:(one way to introduce it) Now, in order to get a clearer idea of what might have been happening with these demonstrations we’re going to look at what we can do with syringes, specifically these smaller, 30 ml syringes.
Ask: What do we normally used syringes for? (to measure volume of a liquid) What each of you will get is a syringe, a piece of tubing and a binder clip that you may use to crimp the tubing. I’d like you to take the next few minutes to see what you can find out about the syringes.
Allow the students to explore what might be happening to air pressure as they play with the syringes. Students may find that:
- If the syringe is connected to an open air tube, you can freely push the plunger in and pull it out, which will result in air flowing out or into the syringe
- If the tube is clamped off, then pushing in the plunger wil be difficult. And it will then push itself back out to a preferred size. What is happening?
What is causing it to be more difficult to push in the plunger? It is the air that is inside the syringe barrel. As we saw before, air has volume. If we try to reduce that volume, we are raising the density of the air, compressing the molecules of air. This compressed air pushes back, exerting a force. This force is air pressure.
Discuss and list observations. Focus on the three main ideas: equilibrium, compression and depression. Now is a good time to introduce a molecular model of compression and pressure. Using dots to represent air molecules both inside and outside the syringe, show on a poster what happens to the density of the air molecules as the plunger is pushed in on the closed-tube syringe setup. Your drawings should show the three states using a dot model to show the relative concentration of air molecules inside and outside the syringes.
NOTE: An excellent reading and depiction of the molecular model of air is on pages 211-212 in the FOSS Grade 5 Science Resources book.
Take the time to make the connection between the syringes and air pressure. The static model (when the syringe is not moving and is at equilibrium) is an example of how we mostly experience air pressure- that is to say, we don’t experience it much at all because normally we don’t notice it is there- it is a passive phenomenon when the air pressure is the same in and out of the syringe.
Specifically discuss how, when we pull back on the syringe we are increasing the volume that the same number of air molecules have and thus decreasing the pressure in the syringe relative to outside and so there is more force pushing on the outside than inside (Hi to Lo). When we push on the syringe, we decrease the volume and increase the force/pressure inside relative to outside and the plunger is pushed out from inside (again, Hi to Lo).
Review all the situations and have students describe each syringe situation according to increased or decreased volume/pressure and notice that in every case the movement is from the higher pressure area to the lower pressure area. Sidebar on atmospheric pressure: Ask: In the syringe activity, where did the force come from that changed the pressure? (Our hands) We were providing the outside force, but sometimes the atmosphere itself can do that.
Compressed air has a higher air pressure than atmospheric pressure. If you release the binder clip while holding the plunger, the students will notice that air rushes out of the tube, and the plunger is no longer harder to hold in.
What is happening here? The compressed air, that was at high pressure, has moved (flowed in the the form of wind) to the atmosphere, which was at a lower air pressure. This is a key point.
High pressure air will move into space of lower air pressure. This happens when the compressed air flows out of a syringe, when wind blows from high pressure weather systems to low pressure weather systems. What happens if you create a lower pressure area?
If the students clamp the tube again, and this time try to pull the plunger out, they will realize that it gets difficult to pull the plunger out. This is because you are creating a vacuum, or a space of lower air pressure. Depict this with the molecular dot model (the density of air in the syringe is now much lower). If you release the clamp, you should hear air rushing into the syringe. Again, this is air moving from a region of higher pressure (the atmosphere), to a region of lower pressure (inside the pulled out syringe).
It is important to note that we must think of air pressure in relation to its surroundings. Above, we saw that the atmosphere could be considered a region of low pressure (when compared to the compressed air in the pushed down syringe) or a region of high pressure (when compared to the pulled out syringe). What you are comparing to determines whether something is considered higher or lower pressure. (see 'Relational Causality on http://pzweb.harvard.edu/ucp/curriculum/pressure/i_challenges.htm for more information on teaching this tricky concept).
h) Read pages 208-209 in FOSS text which describes atmospheric pressure using the watermelon stacking metaphor. Mention that this is a good model of air pressure, but it doesn't tell the whole story and can even be misleading. Show the students a balloon blown up inside of a cylinder, such as a frozen orange juice container, with the top of the balloon sticking out an pointing to the bottom. Ask: Thinking about the stacked up watermelon model of air pressure, what would you expect to happen to the part of the balloon that is sticking out if I turned it so it was facing up? Will the air pressure change the shape of the balloon? (air pressure does not change the shape of the balloon no matter what direction I hold it because, once I stop blowing, the shape of the balloon comes from the equilibrium of pressure inside and outside, like when the syringes were still. In fact, if we were to draw the balloon in the cylinder using arrows to show air pressure how would it look?
I) Try it in your journal. Circulate while they are drawing and ask two or three students with different models to share on the board. Discuss the models and what they imply. State that the most accurate model is one that shows air pressure equally pushing in all directions.
4) Air pressure is omnidirectional, it doesn't just push down as we would think with the watermelon model, and the following is a dramatic demonstration of that idea.
a) Demonstration with an inverted cup of water, and a piece of cardstock:
-Fill a clear cup almost full with water, hold a piece of cardstock on top of the cup
-Ask the students: What will happen if I place a piece of cardstock on top of this cup and flip it over? (Many might expect the cardstock to fall off and the water to spill)
- Flip the cup while holding the cardstock, then release the cardstock
- If all goes well, the cardstock should stay pressed against the (now) bottom of the cup, preventing the water from spilling
NOTE: you may want to give this a trial run before performing live in front of the students
The force that is holding up the cardstock and the water is air pressure! This should demonstrate that there is air pressure surrounding us at all times. This kind of pressure is called atmospheric pressure. Atmospheric pressure arises because there is air pushing in all directions. This air is pushing, due to the weight of all the air that is piled up on top of us (an excellent reading is in the Grade 5 Science Resources Book, pages 208-210). Draw a diagram on the board, or show an already drawn diagram of the card/cup experiment showing the omnidirectional high air pressure outside the cup.
k) Return the students' attention to the original demonstrations/activities and ask them to work in pairs or fours to choose one of the demonstrations/activities to make a poster of, explaining and labeling what is happening in terms of high and low pressure. Circulate to check for student understanding and answer questions. Have all students make a presentation of their poster, or choose particular presenters to highlight the big ideas (Increasing or decreasing volume changes pressure and igh pressure always moves toward low pressure). Read section on wind in FOSS text and have students do I Check numbers: 2, 10, 43, 44, 48 Check student answers and discuss orally any misunderstandings.
All student drawings in their journal and on posters can show level of student understanding. I Check answers will also show individual ability to articulate the concepts.
Finally, we go back to the 'barometers' that the students made at the beginning of this investigation. Ask: When the level of oil in the rises or falls what does that indicate? So the movement of fluid in the straw can show changes in air pressure. We have actually created a very simple barometer. A tool that meteorologists use to measure air pressure. Take the time to describe a mercury barometer and how they work, as well as other models if there is interest. Ask: Why are meteorologists interested in air pressure? (because when there are changes there is likely to be movement of air/wind).
Extensions and Reflections
It is very important to do FOSS investigation #3 leading up to this because you will already have discussed molecular movement of a fluid while dealing with evaporation and condensation.
For those of you who did the City Science week, notice that I suggest focusing the students' attention on particular aspects of the double syringe activity. As it is written in the FOSS kit, it isn't very helpful in focusing on Hi to Lo.