To demonstrate that air takes up space, and puts pressure, or pushes, on everything around it.
The world we live in is covered with air, called the atmosphere. Although we can't see it, the air in our atmosphere is made up of a mixture of colorless gases. Earth's gravity pulls the air to the planet's surface. This is why the air in the atmosphere is densest near the ground (where the pull of gravity is greatest). The higher you go, the thinner the air becomes. Air pushes on all surfaces that it touches.
In this activity students will be conducting investigations that will help them develop and refine their ideas about air. To start the lesson students will be asked to write an explanation of what air is. At various points in the lesson they will be asked to revisit their explanations and refine them based on the phenomena they have experienced in the lesson.
This lesson addresses some of the physical properties of air on the substance level in order to help students’ build toward future understanding of how air particles behave on a molecular level. However, such concepts should not be introduced at this time. Research indicates that students of all ages show a wide range of beliefs about the nature and behavior of particles and have difficulty in appreciating the intrinsic motion of particles in solids, liquids and gases. (Benchmarks for Science Literacy, p.337).
Begin the activity by asking students to answer the following question in their science journals: What is air?
Then ask the class to list things that they can think of, or see in the room, that are full of air. Ask students if they have ever felt the force or pressure of air. Discuss experiences they have had filling up balloons or beach balls or bicycle tires.
Do the following demonstration to illustrate that air takes up space and exerts a force.
- Place a plastic jar or bucket on a table so that everyone will be able to stand around the table and see and reach the container.
- Test plastic bags for holes by filling them with water. (Have extra bags handy as the bags may develop holes as you do the activity.)
- Put air into a plastic bag by blowing into it or waving it through the air.
- Clamp the opening of the bag around the mouth of the container and fasten it tightly by either wrapping string around it two or three times or by putting a rubber band around it.
- Say to students, "Now I will try to push the bag into the container. What do you think will happen?" After the class has made some predictions, give everyone a chance to try to push the bag into the container. Ask the students to describe what they feel as they push down on the bag.
- Remove the bag and place it inside the container, like a liner. Drape the top of the bag over the lip of the container, just like the lining in a trash can. Tie string tightly around it two or three times, or fasten it tightly with a rubber band.
- Say to students, "Now I will try to pull the bag up out of the container. What do you think will happen?" What happens? Can the students explain what happened?
Ask students to look at their answer to the question "What is air?" Ask if they have discovered anything new about air. If so, they should write it in their science journal.
Part 1: Feeling Air Pressure
- Place one end of a thin board on a table with slightly less than half the board hanging off the edge.
- Lay a sheet of newspaper over the part of the board on the table.
- Allow students to predict what would happen if the protruding end was struck as hard as possible.
- Strike the protruding board as hard as you can. The paper will not move. If you hit hard enough, the stick will break. This is due to the air pressure that is exerted downward. Since the paper is flat against the board and table, no air is beneath the paper to counteract the pressure from above.
- Calculate the surface area of the newspaper. (length by width)
- Multiply the surface area by 14.5. The result will be the amount of air pressure exerted on the paper. (Note: Air at sea level exerts a pressure of approximately 14 1/4 pounds per square inch. Therefore, on a piece of paper 10 inches square the air exerts a pressure of 1,450 pounds on the top surface. The same pressure is also exerted on the opposite surface, counteracting the pressure on the other side.)
- Have students predict what would happen if you pushed down on the board slowly.
- Place your hand on the protruding piece of the board and slowly push down.
- Ask students to discuss why it was difficult to move the board.
Again, have students look at their answers to the question "What is air?" Did they find out anything about air that they want to add to their answers? If so, have them write it in their science journal, including an explanation of why they changed their answer.
Part 2: Measuring Air Pressure
In this activity students will make a barometer to measure air pressure. Before they begin, have them visit What's happening inside highs and lows on the USA Today Weather website.
- Use scissors to cut a piece of balloon large enough to cover the mouth of a small glass jar and extend over the sides. Attach the balloon securely to the jar with a large rubber band or string, making sure to cover the entire mouth of the jar to ensure that no more air can enter.
- Roll up an index card so that it will fit over a pencil. Secure the ends with a small rubber band or tie them together with thread. Then slide the card over the pencil and place the pencil, lead side down, on a thread spool. The spool should support the pencil so that it stands straight up.
- Use the scissors to cut a straw on the diagonal, creating a point on the cut end. Then glue the uncut end of the straw to the center of the balloon over the mouth of the jar. The pointed end should extend out beyond the jar.
- Put the jar on a shelf with the pencil on the spool alongside it. The pointed end of the straw should be facing the pencil and index card. With a pen, mark the place on the index card to which the straw is pointing. That is your starting point.
- Watch your barometer from day to day for about two weeks. What happens to the straw?
- Students will notice that on some days the straw will be higher than the starting point and on other days the straws will be lower. Ask students to consider what they have learned about air pressure in the previous activities to come up with an explanation of what is happening. (When the air pressure is high, it will press hard on the air in the jar, and the pointed end of the straw will rise. When the air pressure is lower the air pressure in the bottle will push up, and the point of the straw will go down.)
Students could graph the results of the experiment by setting a ruler beside the pencil and measuring how many centimeters plus or minus the starting point the air pressure has changed during the course of the activity.
To assess the ideas in this lesson, have students prepare their "final" answer to the question: What is air? Their answers should include evidence from the investigations conducted in this lesson.
Make Your Own Barometer, on the Franklin Institute website, provides a different way to make a barometer. As a class project, students can make both kinds and compare the results over the course of a few weeks.