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 calledelastomers
. 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.
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
- balloons of equal size
- a small, empty, glass soda bottle or other narrow-mouthed jar or flask that can hold some pressure
- string or tape measure
- a balance that weighs to the nearest .01 gram
- Alka-Seltzer or its generic equivalent, broken into pieces small enough to fit through the mouth of the bottle
- Weigh two balloons and record their weights.
- Pour about 150 ml of water into the bottle.
- 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.)
- 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.
- Inflate another balloon with air to the same circumference. (Two spherical objects of the same circumference will have the same volume.)
- Weigh both balloons again. Questions 1. Which balloon is heavier-the one with carbon dioxide or the one with an approximately 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?