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Air has Weight

This experiment comes from a question sent to me a homeschooling mom named Elaine. It is based on a "classic" experiment often seen in textbooks to show that air has weight. While it starts simple, it takes some twists along the way that often cause people to misunderstand what is actually happening.

To try it, you will need:

  • a yardstick or other long, straight stick
  • some string
  • four balloons
  • two paper clips
  • a bathtub of water
  • two zipper top, plastic, storage bags

The experiment is supposed to show that air has weight. You first make a balance by tying a piece of string around the center point of the yardstick and then tying the other end of the string to something like a shower rod, so that the yardstick balances. This may take some work, so be patient.

Next, use the paper clips to attach an empty balloon to each end of the yardstick. If you put them equal distances from the ends of the yardstick, it should again balance.

Once you get this done, carefully remove one of the balloons and inflate it. Tie the neck of the balloon, so it will stay inflated and reattach it to the paper clip. That end of the yardstick now goes down. A balloon full of air weighs more than an empty balloon. We have just shown that air has weight, right?

Before you make up your mind about that, lets think about it. Both balloons are the same. Both paper clips are the same. The only thing that is different is that one is full of air, and the other is not. But wait a minute. The balloon that is full of air is also larger. It takes up more space and displaces more of the stuff around it, which is.....air. That complicates the experiment.

Why is that a complication? Both balloons are surrounded by air, the same stuff that you put inside. To see this in a different way, you need to make a water balloon. Carefully, stretch the neck of a balloon over the end of the faucet in your sink. Just barely turn on the water and carefully inflate the balloon with water. Don't fill it too full, as it will pop and make a mess. Pinch the neck below the faucet and remove the balloon. Make sure all the air is out and then tie a knot in the balloon to keep the water in.

Hold the water balloon in one hand and an empty balloon in the other. Which is heavier? Easy! The water balloon. The difference is that there is water inside the balloon and air on the outside. Put both balloons into a tub of water. Be sure the water is deep enough so that the water balloon will go all the way under the surface. Under water, hold the water balloon in one hand and the empty balloon in the other. Be sure that there is no air in the empty balloon. Under water, they both seem to weigh the same, and they do. You have two balloons with water on the inside and water on the outside. The only thing you are comparing is the weight of the two balloons.

Now lets go back to our original balloons. Why are the balloons the same with water and different with air? When you blew up the balloon, your lungs and the stretchy rubber compressed the air inside. Because it was compressed, the air inside the balloon is denser than the air outside. You have more molecules of air in each cubic inch of space. What you have shown is not that a balloon full of air is heavier than an empty balloon. Instead, you have shown that a volume of compressed air is heavier than the same volume of air at normal air pressure.

So, our experiment was based on a misconception, right? It does not show that air has weight. Or does it? If compressed air weighs more than noncompressed air, then air must have weight. Compressing a weightless substance would not make it heavier. So the misconception was in the explanation, not the experiment. You are not comparing a balloon of air with an empty one to show that air has weight. Instead, you are comparing the compressed air on the inside of the balloon with the normal pressure air on the outside. The empty balloon is just to balance the weight off the other balloon.

Now, I know what you are thinking. Why is there a difference between the balloons with air, but not with water? Water is not compressible, so the water inside the balloon is no denser than the water on the outside. To see that, try the original experiment using zipper top, plastic bags instead. Leave one of them closed, with no air inside. For the other, open it, expand it, and then seal it again with some air inside. You don't want it stretched tight, just about half full of air. Before you try it, what do you think will happen? Think back to what happened with the water balloon under water. Then try it yourself.

Because the air inside the bag is not compressed, both bags balance. The inflated bag does not weigh any more than the empty bag, because the air inside is displacing an equal amount of air on the outside. Speaking of outside, that is a good place to take care of the water balloon when you are done. Otherwise, you might make a mess in the house. Just don't toss it at your brother, unless you can run VERY fast.