Is Gas Matter?
by John Williams and Linda Woodward
Do gases have mass and occupy space? Are gases matter? Can students answer these questions through simple observations? The answer to all of these questions is, "Yes!"
Matter is anything that has mass and occupies space. While both of these criteria are relatively easy to observe for liquid and solid samples, they can be much more difficult to observe for gaseous samples. These activities demonstrate these characteristics for gases using simple methods and materials.
Air has mass
In this demonstration, you can use a soccer ball as a container for air (a mixture of gases). When extra air is added to the ball, comparison of mass measurements will clearly show an increase in mass.
You will need these materials: soccer ball or other semi-rigid, constant-volume, yet inflatable ball. Before beginning the demonstration, the ball should be at its normal volume (sides not pushed in) and the pressures inside and outside of the ball should be equalized by inserting the needle, not connected to the pump, to allow for air flow. Remove the needle before starting the procedure. You need: air pump, pump needle, base cut from a plastic 2-liter soft drink bottle or other object to hold the ball in place on the balance pan, balance (with capacity to measure the mass of the soccer ball, about 320 grams, to at least the nearest 0.1 gram). If an accurate balance is not available, a double pan balance can be used.*
Here is the procedure for this investigation. The intent is for students to investigate whether or not air (a mixture of gases) has mass. Begin by giving them opportunities to suggest ways they might be able to measure the mass of a gas. Discuss their ideas as a class and, if possible, try any that seem reasonable.
Whether or not the students suggestions work, tell them that you have a demonstration that will further help them to decide if air has mass or not. Hold up the soccer ball and ask the students what is inside. Air. Place the soft-drink bottle base on the balance pan and then the soccer ball into the base. Use the balance to determine the mass of the combined ball, base, and air, and record this mass.
Use the air pump and needle to inflate the ball to normal playing pressure. Ask students what has been added to the ball and what effect they predict this will have on its mass? Air has been added to the ball. The students predictions may vary. As a class, discuss the predictions. Determine the mass of the combined ball, base, and air again. Have the students compare the masses found before and after air was added to the ball. The mass after the air has been added will be greater.
Ask the students if air has mass and how this demonstration helps them to answer that question. The demonstration is intended to show that air has mass because the mass increases when air is added to the soccer ball. This air is responsible for the increase in mass.
Air occupies space
In this activity, students try to blow up a balloon within a sealed space which already contains air. As long as the air is trapped in the space, the balloon cannot be blown up.
A group working on this investigation will need these materials: one or two clear plastic, 1 liter soft-drink bottles (or other size bottles if 1 liter bottles are not readily available), and one or more balloons; (for health reasons, only one student should blow into a balloon) and one or more additional clear plastic 1 liter soft drink bottles in which a hole can be cut.
You will also need a pushpin or thumbtack in order to punch a hole in some of the bottles.
Here is the procedure: Students are going to investigate whether or not air (a mixture of gases) occupies space. Begin by giving them the opportunity to suggest ways they might be able to demonstrate this property of air. Discuss their ideas as a class and, if possible, try any which seem reasonable. Whether or not the students suggestions work, tell them that they are going to do an activity which will further help them to decide if air occupies space. Give each group a clear plastic bottle (without a hole in the side) and a balloon. Show the students how to push the deflated balloon into the mouth of the bottle and stretch the open end of the balloon back over the bottles mouth to seal the bottle. With younger students, the teacher may wish to do this in advance.
Ask each group to predict what will happen if one of them tries to blow up the balloon without removing it from the bottle. Answers will vary. Now have one student in each group try to blow up their balloon in the bottle while the other students observe. Then all the group members should share their observations. The student trying to blow up the balloon will find that after the initial, small amount of air is blown into the balloon, efforts to add more are not very successful. No matter how hard the student tries, the balloon will not inflate very much.
Have the groups suggest reasons why they are unable to inflate the balloon in a bottle. Answers will vary. Tell the students that you are going to give them a bottle with a hole in it and that they should repeat the experiment with that bottle. Hand out the bottles with the holes and give the groups time to try them. They will find that the balloon in the bottle with the hole can be blown up.
Challenge the students to propose explanations for the two behaviors they have observed. Have them share their ideas. The air trapped in the bottle without the hole cannot escape so there is no room for the balloon to be blown up. The air in the bottle with the hole is pushed out through the hole when the balloon is blown up. Students can feel the air coming out of the hole by putting their finger just outside the hole while the balloon is being blown up.
Ask students if air occupies space and how this demonstration helps them to answer that question. Yes, air occupies space. The demonstration shows this because the balloon can only be blown up in the bottle if the air which occupies the space in the bottle can escape. Matter is anything that has mass and occupies space. If your students have carried out these investigations, they should be able to answer these questions: Do gases have mass and occupy space? Yes! Are gases matter? Yes!
*If an accurate balance is not available, use a simple double pan balance: carefully balance the soccer ball (in one pan) against some other items in the second pan. Remove the ball but leave the items in the second pan in place. After more air has been added to the ball, return the ball to the same balance pan. The ball side of the balance will move down, showing that it has more mass after the air has been added.
- John Williams is an Associate Professor of Chemistry and Coordinator for Mathematics & Natural Sciences at the Hamilton Campus of Miami University, Ohio
- Linda Woodward is a Research Associate for the Center for Chemical Education, Miami University, Middletown, Ohio. She has extensive background as a classroom instructor, including workshops for teachers. For more than ten years Linda has developed pre-college classroom science activities.