Unit #1

Activity 16
Floating on Air

Activity Overview

Any type of balloon that floats, does so because it contains a gas that is less dense than the surrounding air.

Part A: A demonstration is done showing the different densities of gases by filling one balloon with CO₂ and another with He. The helium balloon floats and the CO₂ balloon sinks. Students then use a computer simulation to understand different ways to achieve different densities.

Part B: Students use Workbench software to see the effect of heating air inside a balloon and causing it to float away.

They will be asked to use all that they have learned during this unit to describe how a balloon flies and the environment around the balloon as it ascends into the atmosphere as high as the clouds. They will have to explain everything at both a macroscopic and microscopic level.

Learning Objectives

Students will:

• Identify gasses that would cause a balloon to float by using their density relative to that of air.
• Describe at that atomic level how a hot air balloon flies.

Conceptual Prologue

Macro-Micro Connection

The history of hot air ballooning is intimately mingled with all varieties of "lighter than air" ballooning. The technique of using hot air or some gas of very low density to float a balloon were developed at nearly the same time. Using other gasses besides air eliminates the need to constantly heat the inside of the balloon. Blimps work in this way and were one of the first forms of air transportation.

Science Concepts

Mercury is a very dense substance. 10 mL of mercury has a mass of 135g. The same volume (10 mL) of water has a mass of only about 10g. Because of this, every drop of mercury is heavier than every drop of water. If you put mercury and water in a container the mercury will sink to the bottom because gravity pulls more strongly on each part of mercury when compared to its pull on each part of water. This effect of having less dense things float on more dense things is called buoyancy.

Balloons that float do so by having an overall density that is less than the air surrounding them. This can occur in two ways:

• Fill the balloon with a gas that is less dense than air. Helium gas is typically used in blimps and balloons associated with parties.
• Heat the air inside the balloon so that the atoms move faster, bumping into each other harder, and causing them to spread apart more. If the same mass (all the bumping gas atoms) takes up more space, then the density will be lowered.

Naive Conceptions

• Density relates only to the size or mass of an object.

Students almost always interchange the word heavy with dense. They don't understand that a very tiny piece of gold which is not heavy is still one of the densest substances on Earth. Density is the ratio of mass to size (or volume), so something which is very small, but dense should have a high mass when compared with something else of the same volume.

Activity Design and Execution

Major Science Concepts:	• density • buoyancy
Assumed Previous Knowledge:	• That density is the ratio of mass to volume or “the concentration of mass”. • That an increase in temperature results in an increase in kinetic energy [energy of motion]. • That less dense things float on more dense things.
Time:	Part A: approximately 50 minutes Part B: approximately 25 minutes
Materials:	• CO2 gas (either from a canister or from the reaction between baking soda and some acid - HCl or vinegar) • Helium gas (A He filled balloon could be brought to class for this.) • Methane gas* (from the laboratory gas jet if possible) • Butane gas* (from lighter refill containers) • A short candle* • A thistle tube* • A ziplock bag which can be connected to the thistle tube.* • Some rubber tubing for connecting the thistle tube to the gas jet or ziplock bag with butane.* • Bubble solution.* • Wooden splints.* • Matches.* • Computers with Workbench software. * These materials are only necessary for the methane/butane demo.
Advanced Preparation: (if any)	• None

Investigative Question: Why do some gasses float?

Explain to students that this is the last activity in this unit, and that after today, they have learned everything they need to know about why a balloon floats. At the end of this unit they will be asked to put all of their knowledge together in explaining the balloon and its environment.

Part A:

1. Demonstrate that a CO₂ filled balloon sinks and a Helium balloon floats.
2. If you have the materials attach a thistle tube to a gas jet, dip the thistle tube into the bubble solution and blow a methane bubble. Shake the bubble from the tube and have an assistant burst the bubble with a flaming splint. Note: if the bubble bursts before catching on fire, the person with the flame should back away.
3. To demonstrate the butane bubble, prepare a ziplock bag with a nozzle in one corner (the glass part of an eyedropper can be used). Attach the thistle tube to the nozzle and squirt some butane into the ziplock bag. The liquid butane will become gaseous filling the ziplock bag. Dip the thistle tube into the bubble solution and blow a small (3 inch diameter or smaller) butane bubble. Shake this bubble off and let it fall toward a short candle which has been previously ignited and placed on the floor. Make sure any flammable objects have been placed several feet from the candle. Note: If the bubble doesn’t burst into flames quickly, then back away from the candle.
4. Have the students bring up the sealed balloon simulation by using Workbench software, zooming down to the “Atoms” room, and clicking on the balloon sitting on the table.
5. The software will then ask them to visually compare the different gasses and ask them to explain why they have different densities. (They can look at two different gasses at the same temperature and pressure. By seeing that one is moving faster than another, they should be able to tell that one has heavier molecules [displayed as single atoms]. They can also use gauges indicating the mass and volume of each container.)
6. The software will then ask them to make predictions about what will happen to a balloon filled with each gas. Then the software will have the student perform the experiment to confirm or contradict the students’ prediction.

Part B:

1. Have the students zoom down to the "Atoms" room in and bring up the "Hot Air Balloon" simulation by clicking on the burner controls near the window.
2. The software will ask them to experiment with holding down the “Burner On” button and to observe the relationship between temperature, density, and the buoyancy of the balloon.
3. The students will be able to see an atomic level view of the air surrounding the balloon and the air inside the balloon. They will be asked to notice the difference when the balloon begins to fly.
4. The software will then ask the students to explain why a hot air balloon floats.
5. Have a class discussion of what students notice. Ask how this is similar or different from filling a balloon with gasses other than air.

Assessment

Have students write the following in their notebooks:

Explain in as much detail as possible how a hot air balloon works and the environment the balloonist experiences as they ascend to the clouds. Describe on an atomic level what is happening, starting with the balloon on the ground not yet fully inflated and ending with the balloon inside a cloud. What happens when the burner turns on? What happens as the balloon begins to fly? Explain everything on an atomic level. Be sure to include descriptions of atomic motion, thermal equilibrium, density, pressure, phase changes, and van der Waals attractive forces in your answer.

Internal Notes:
•Part A: See computer lab P for the mock up of part a of this activity.
•Part B: See computer lab I for the mock up of part b of this activity.