What are balloons made of? How do they hold air? What are they used for?

Balloons
Why are balloons stretchy?
Brian pops the question about balloons.
Segment length: 7:30

Insights

We make funny animals out of them. We celebrate birthdays with them. We use them to play games. Balloons are a blast, but they're also super for science.

Meteorologists monitor atmospheric conditions by sending up weather balloons. Doctors open blocked veins and arteries by inserting special medical balloons into them. Scientists are even investigating something called superpressure balloons. Sealed against leakage and strong enough to handle the sun's heat, these helium-filled balloons would float in the upper reaches of Earth's atmosphere. They may replace some satellites because they're so much cheaper to launch-just let them go!

The material that makes balloons stretch is latex, obtained from rubber trees. Latex is collected as it flows out of cuts in the tree bark. At this stage, it is fairly gooey. People make balloons by dipping a balloon mold into the latex and then heating it for a while to make the latex firmer and more elastic.

Latex is a polymer, which means that it has long, chainlike molecules made up of repeating units. When it first comes out of the rubber tree, its molecules are loosely tangled up, so they flow slowly. If you heat the latex, you create chemical cross-links between the molecule strands. Long, tangled polymer molecules that have a few cross-links between them can extend and then regain their original shape. These stretchy materials are called elastomers.

The only problem for the serious balloon collector is that the open mesh structure of latex, particularly when it's stretched tightly, lets helium and other gases escape right through the tangled molecules. Mylar, a type of polyester that can be rolled into thin films, is better at trapping helium because its molecules are more closely packed and Mylar does not stretch out like latex. But even a Mylar balloon can't hold all the tiny helium atoms forever, and eventually it loses lift also.

Balloons are a great example of how the pressure and the volume of a gas are interconnected. When you blow up a balloon, you exert pressure on the inside walls of the balloon. When that pressure exceeds the outside air pressure plus the pressure exerted by the latex itself, the balloon begins to expand. The pressure inside a balloon is always a little higher than the surrounding air pressure, because the latex is pushing back as the air inside pushes out. When a weather balloon rises in the atmosphere, for example, the outside pressure decreases and the balloon expands. Eventually, the inside pressure causes the balloon to burst.

Connections

1. How many different uses for balloons can you think of?
2. What effects do balloons have on the environment?

Key Words

cross-links chemical bonds between the molecules of polymers
elastomer general name for any polymer that stretches and then can regain its shape
helium gas that is less dense than air
latex principal raw material used to make balloons
polymer a long, chainlike molecule
pressure force per unit area that a gas exerts on the walls of a container
volume the space that a particular quantity of a gas occupies at a specific temperature and pressure
weather balloons helium-filled balloons released to lift instruments that record atmospheric conditions

Resources

  1. Dane, A. (1993, Feb) A new balloon. Popular Mechanics, pp. 106-107.
  2. deSeve, K. (1994, Nov 18) He, he, helium. Science World, pp. 12-15.
  3. Mullen, M. (1988, Nov-Dec) The return of scientific ballooning. Technology Review, pp. 8-9.
  4. Walker, J. (1989, Dec) Why are the first few puffs the hardest when you blow up a balloon? Scientific American, pp. 136-139.

Additional resources

  1. Flinn Biological Catalog Reference Manual: Lung volume activity. Catalog #AB1242. (800) 452-1261.
  2. Flinn Chemical Catalog Reference Manual: Dynamic gas behavior. Software for Apple II. Catalog #AP4420. (800) 452-1261.
  3. Flinn Chemical Catalog Reference Manual: Soap bubbles-their colors and forces which mold them. Book. Catalog #AP1852. (800) 452-1261.
  4. Producers Studio: Balloon animal safari. Videotape. (503) 683-1400.

Additional sources of information

  1. The Balloon Council
    Princeton House
    160 West State St.
    Trenton, NJ 08608
    (800) 233-8887
    (Literature on the history of balloons and an inexpensive balloon experiment kit for schools.)

Main Activity

Expand on This!
Inflate balloons with different gases and weigh your results.

Air is mostly a mixture of oxygen and nitrogen, but it also contains smaller amounts of other gases, such as carbon dioxide. That's the gas that is released when you drop a stomach-acid neutralizer like Alka-Seltzer into water. Do you think a balloon full of carbon dioxide will weigh more or less than a balloon full of air?

Materials

  1. Weigh two balloons and record their weights.
  2. Pour about 150 ml of water into the bottle.
  3. Working as quickly as possible, drop in six Alka-Seltzer tablets. Lightly stuff the mouth of the bottle with cotton and stretch the open end of a balloon securely over the top of the bottle. (The cotton allows the carbon dioxide through, but soaks up any splashed water.)
  4. When the balloon is filled with carbon dioxide, or the tablets have stopped fizzing,
    tie off the balloon and measure its circumference with a string or
    tape measure.
  5. Inflate another balloon with air to the same circumference. (Two spherical objects of the same circumference will have the same volume.)
  6. Weigh both balloons again.

Questions
1. Which balloon is heavier-the one with carbon dioxide or the one with an approx-imately equal volume of air? (Be sure to take into account any difference in the initial weights of the balloons.) What is the weight of the gas in each balloon? Why do you think one is heavier than the other? If you repeat the experiment, do you get similar results?
2. Part of the weight of a full balloon is supported by the air around it, a phenomenon called buoyancy. Therefore, the balance measures a lower value than the actual mass of either gas. The size of the buoyancy is the same for two balloons of equal volume. How might you find the value of that buoyancy so you could add its effect to find the actual mass of the gas in either balloon?
3. If you let the balloons sit out for a while, which one loses gas pressure faster? Why do you think that occurs?


Submerge an upside-down test tube or small beaker in a large beaker of water. What do you notice about the level of water in the tube as it is raised and lowered in the beaker? Can you explain why this happens in terms of pressure and volume? Why doesn't the air come out of the tube? Tip the tube slightly. What happens?


Pretend that you and your classmates are gas molecules. How is the inside of your classroom like a balloon? How could your group of gas molecules escape? Is the playground like a balloon? Is it easy or hard to get out? At absolute zero, you're packed all together in a corner of the room. As the temperature goes up, you begin to vibrate and move around. Can you stay close as the temperature increases?


Inflate two balloons to equal size. Place one in a pan of hot water and the other in a pan of cold water. What happens? What will happen to equal-sized balloons in a range of temperatures?


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