Air Takes Up Space

In this experiment you tape a paper towel inside a plastic cup. When you completely submerge the cup, upside down, you may be shocked to see that the paper towel stays totally dry!

There are no significant safety hazards in this experiment. The optional add-on uses a pin or tack, in which case you should avoid poking yourself or others.

Required

*transparent cup *tape *paper towel *Pot of water (transparent container like tupperware or glass is best)

Optional

*Pin or tack

Basic Experiment

1. Ball up a small piece of paper towel or tissue. Using double sided tape attach the balled up towel or tissue to the inside bottom of the plastic cup so that the whole ball is well inside the cup.
2. Fill a pail of water or beaker approximately half full. Predict what will happen to the paper towel when the cup is dunked upside-down in the water.
3. Hold the bottom of the plastic cup. Push the plastic cup into the water with the open end of the cup pointing down into the water. Completely submerge the cup.

1. Pull the cup out of the water. Inspect the tissue or paper towel (it should be dry).

Optional Add-On

1. Ask students how the experiment could be modified so that the towel got wet. Suggest what if there was a hole in the bottom of the cup.

1. Now do the same experiment, except first poke a hole in the bottom of the cup.

Q: What is in the cup when we put in under water?

A: The paper towel, obviously, but also AIR!

Q: Why does the paper towel stay dry?

A: Air in the cup takes up space and prevents the water from coming in.

Q: Why does the air come out of the cup with a hole in it?

A: Air is less dense than water. When the cup full of air is put under water, the air goes up (through the hole) and water comes in (you might note the level of the water goes down if you have a small pot of water, and quickly submerge the cup and then hold the cup very still as the air slowly exits.

I think the fundamental science is included above. Post comments if you have questions.

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This experiment can be paired with the one below to make a nice combination lesson:

http://experimentopia.org/experiment/air_takes_up_space/