Simple Science Experiment: The Bag & Bottle Problem - MetroFamily Magazine
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Simple Science Experiment: The Bag & Bottle Problem

by Steve Davala

Reading Time: 3 minutes 

This month we’re going to experiment with air, that fabulous mixture of gases that doesn’t really look like anything and the only time you think you feel it is when the wind blows. It really blows my mind (pun intended) to think that something that exists can be invisible. How much do we take this stuff for granted? 

There’s so much to think about with air but we wouldn’t have nearly enough time in one experiment to talk about it all. You’re going to prove today that air actually takes up space. Indeed, just like you can only get so much sand into a bottle, you will see air follows the same rules. 

Materials: 

A large jar or two (a Mason jar or a big pickle jar would work well)

A plastic bag or two that will fit around the mouth of the jar

Tape

Procedure for bag over the top of jar: 

Put a plastic bag over the mouth of a jar. 

Tape the bag securely to the jar, and before you seal it completely puff a little air into the bag so it fills up. Put the last bit of tape on so it is airtight.

Try to gently push the plastic bag into the jar.

Procedure for bag inside the jar: 

Either get a new jar or just start over with your first jar.

Put a plastic bag inside the jar, pushing it all the way down.

Tape the plastic bag around the edge of the jar lid to make it airtight.

Put your hand in the jar and try to pull the bag out gently (don’t tear it).

Explanation: 

When you do the first experiment, you cannot push the bag into the jar! If you push hard enough you can hear air squeezing through the tape or you might pop the bag. Why is this? The reason is this: air takes up space! You might not think of it when you “look” at air, because, well, there’s nothing to see! But air is all around us, weighing down on us, causing air pressure. Ever feel your ears pop when you go up a mountain? There’s less air pushing down on top of you and your body is reacting!

If you’ve ever sat on an air mattress you are doing the same thing. Air is inside, and can’t be pushed in or compressed anymore. 

But what about the opposite experiment? Why can’t you pull the bag out of the jar? This has to do with another property of air called air pressure. When you pull up on the bag, you are making a “vacuum,” or a space with less air. Nature doesn’t like to have a vacuum, and so it tries to force that bag back in there. That’s the pulling back you feel when you do this. 

Experiment further: 

Good scientists think of questions they can ask and ways to change a system. Can you? What might happen if you change the temperature of the jar with the bag over the top? Here’s something to try out… put that jar into the freezer. Want to see what happens? Check out the author’s website.

I hope you enjoyed this simple experiment and learned a little bit about growing things. If you have more questions about this, or need tips about science fair ideas around this topic (or others), contact the author. 

Steve Davala is a high school science teacher who likes to write. He’s got two kids of his own and subjects them to these science activities as guinea pigs. Follow him on Twitter, on www.stevedavala.blogspot.com or email him at steve.davala@gmail.com.

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