Purpose To test whether air has weight, exerts pressure, and applies force. Process Skills Observe, measure, collect data, interpret data, identify and control variables, form a hypothesis, predict, draw conclusions Background The blanket of air around Earth is called the atmosphere. Air is a mixture of 78% nitrogen, 21% oxygen, and 1% other gases, such as argon, carbon dioxide, water vapor, methane, and helium. These gases consist of small, invisible particles called molecules, which are made up of atoms. The molecules that make up air take up space and have weight. Any matter that has weight can exert pressure and apply force to make things move. The constant movement of air in our atmosphere creates wind. Areas of high and low air pressure affect the weather everywhere on Earth. Air: Weight and Pressure We cannot see air, but we can observe it in action to learn about its properties. Under different conditions, molecules of air can be squeezed together (high pressure) or spread apart (low pressure). Air molecules can also be added to fill a space of any shape, creating higher air pressure. Air can be squeezed into a tire or ball through a tiny opening to make the flexible object expand. In these activities, you will explore two important properties of air: weight and pressure. Time About 45 minutes Grouping Small groups ATMOSPHERE AND CLIMATE Text Materials Part One: Data Sheet 1 2 tables or desks meterstick 3 lengths of string, 30 cm (1 ft.) each ruler, 30 cm (1 ft.) 2 balloons of equal size masking tape thumbtack Part Two: Data Sheet 2 small, clear plastic bottle sharp object to puncture the bottle balloon 1
Atmosphere and Climate Air: Weight and Pressure Procedure Part 1: Does Air Have Weight? 1. Discuss with your group whether or not you think air has weight and how you can tell. Then have a group member inflate one balloon to a large size and tie it. Let each person hold the inflated balloon in one hand and a noninflated balloon in the other hand. Are you able to tell whether one is heavier than the other? Does the fact that one of them has air in it make a difference? After discussing these answers, move on to the rest of the activity in Part 1. 2. Place two tables or desks less than 1 meter (3 ft.) apart. Set a meterstick across the space between the tables. Use tape or weights to hold it in place. 3. Tie a 30 cm (1 ft.) length of string around the middle of the meterstick so some of the string hangs down. Then tie the other end of the string around the middle of a 30 cm (1 ft.) ruler so the ruler is parallel to the meterstick. This ruler will act as a balance scale that will let you compare the weights of two balloons. 4. Tie two strings to the ruler, one at each end, so an equal length of each string hangs down below the ruler. 2 5. Inflate the second balloon from step 1 so it is nearly exactly the same size as the first balloon, and tie it. Then tie each balloon onto one of the strings so one balloon hangs from each end of the ruler. Adjust the position of the strings on the ruler as needed until the balloons balance each other. The ruler should be parallel to the ground (see Figure A). If necessary, tape the strings to the ruler to keep them from sliding. Figure A 6. On Data Sheet 1, write a hypothesis describing what you think will happen if you release the air from one of the balloons. 7. Place a small piece of tape on the outside of each balloon near the knot. Be sure the balloons are still balanced, adjusting their positions on the ruler if necessary. Use a thumbtack to carefully poke a hole through the tape on just one of the balloons. This will allow you to let the air out without popping the balloon.
Atmosphere and Climate Air: Weight and Pressure 8. As the air escapes from one balloon while the other balloon remains inflated, does the ruler lean in one direction or the other? As a group, discuss why this might be so. Then complete the diagram on Data Sheet 1. Part 2: Does Air Exert Pressure? 1. Discuss with your group how air molecules move as you inflate and deflate a balloon. Why does it expand? Why does it shrink back to its starting size? 2. Your group will perform a test to find out whether air has pressure. Use a sharp object to carefully poke a hole into the side of a plastic bottle near the bottom. The hole should be approximately 3 mm (1/8 in.) wide. 3. Remove the bottle s cap. Place an empty balloon inside the bottle while rolling the neck of the balloon over the mouth of the bottle (see Figure B). Hold onto the balloon so it doesn t fall in. 4. For Test 1, have one member of the group attempt to blow up the balloon. Does it inflate? When he or she stopped blowing, what happened to the air in the balloon and why? Discuss these questions as a group. 5. Now discuss with your group what you think will happen when you cover the small hole in the plastic bottle and try to inflate the balloon. Write your first hypothesis on Data Sheet 2. 6. Next, discuss with your group what you think will happen if you inflate the balloon inside the bottle and then cover the small hole. Write your second hypothesis on Data Sheet 2. 7. Have the same group member completely cover the hole in the bottle with one finger and try to inflate the balloon again. Compare this result with your first hypothesis. 8. For Test 2, have the same group member open the small hole, inflate the balloon, and then immediately cover the hole. Does the balloon stay inflated or does it deflate? Compare this result with your second hypothesis. 9. Follow the instructions on Data Sheet 2 to complete the four diagrams. Figure B 3
Atmosphere and Climate Air: Weight and Pressure Data Sheet 1 Part 1: Does Air Have Weight? Hypothesis: What will happen to the balloons hanging from the ruler when you release air from one balloon? Why? Collect Data 1. Draw a diagram of your balloon weight test, showing the position of the balloons after you released the air from one balloon. 2. Draw a star beside the balloon that weighed more. 4
Atmosphere and Climate Air: Weight and Pressure Data Sheet 2 Part 2: Does Air Exert Pressure? Hypotheses: 1. When you cover the small hole in the plastic bottle and blow into the balloon, will it be easier or more difficult to blow up? Why? 2. Once you stop inflating the balloon and cover the small hole in the bottle, will the balloon deflate in the same way it did when the hole was open? Why or why not? Collect Data 1. Draw the balloon inside each bottle to show whether it was inflated or deflated during each situation. 2. In each diagram, draw one arrow to show the direction of air movement in or out of each balloon. Then draw another arrow showing the direction of air movement through the small hole in the bottle, if there was any. 3. On each blank line, write H or L to indicate whether the air pressure was high or low in that space. (Hint: The air pressure rises as more air is added to a space, and the air pressure drops as air leaves a space.) Test 1: Hole Always Open Test 2: Hole Closed After Inflation During Inflation After Inflation During Inflation After Inflation 5
Atmosphere and Climate Air: Weight and Pressure Questions Analyze Data 1. In Part 1, what happened to the balloons on your balance scale when you let the air out of one of the balloons? 2. Does air have weight? How can you tell, based on the results of Part 1? 3. In Part 2, was it more difficult to inflate the balloon when the hole in the bottle was covered or when the hole was open? Why do you think this was so? 4. In Part 2, when the hole was covered, did the balloon stay inflated, or did it deflate? Why do you think this was so? 5. How do areas of high and low pressure inside and outside a balloon determine whether or not it stays inflated? 6
Atmosphere and Climate Air: Weight and Pressure Questions Draw Conclusions 1. Anything that has weight can exert pressure. So why is air pressure much higher near Earth s surface than it is farther up in the atmosphere? 2. Why are the weight and pressure of air so important to Earth s weather and climates? 7