Try This: Floating Water
Air is all around us. We breathe it, we use it to inflate the tires on our car, and we feel it when the wind blows. One of the surprising aspects of air is that we can't see it, we can't taste it, and only in certain cases can we feel, hear, or smell it. Yet it is a critical piece of our lives.
When air is in a container, it exerts pressure on its surroundings. This pressure is what creates wind and weather, it's what makes airplanes fly, it's what makes curve balls curve, and it's what keeps your car tires inflated and able to roll on the road. Simply put, air pressure is a part of everything we do.
Here's an easy magic trick to do that isn't really magic. Once you understand how air pressure works, you can amaze your friends once with the demonstration and then again when you explain how it works!
Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world.
— Albert Einstein
Can you make water float in the air?
- A small cup of water, clear plastic works best
- A sink, bathtub, or tray to catch any water that falls while you practice this demonstration
- A note card or other small piece of paper (the card must be large enough to fully cover the top of the cup)
Fill the cup about three-quarters full of water. The amount of water isn't that important, although you may find it more difficult to do this if the cup is full.
While the cup is held over the sink, slowly turn it over and observe how the water pours out of the cup.
Now refill the cup with water and place the card on top of the cup, making sure that it covers the entire top.
Press down gently but firmly on the card as you slowly rotate the cup upside down.
Keep your hand on the card for a few moments, then remove your hand. The card should stay in place and the water will appear to float inside the cup with nothing holding it up.
When you tipped the cup over the first time, the water ran out because gravity pulled it down toward the sink. The only way to prevent this from happening is to find some force that counteracts gravity. Enter air pressure.
When you add the card, you see the effects of air pressure. The air below the cup actually pushes up on the card, just like it pushes on everything around you. In this case, the upward force of air pressure is enough to cancel out the effect of gravity on the water and keep the water “floating” inside the cup.
Depending on the strength of the card you use, you may notice that eventually some water begins to leak out. As this happens, the seal begins to break and the card is no longer able to prevent gravity from winning the battle. Soon all the water will spill out.
KIDS' LAB LESSONS
QUESTION: Can you push an egg into a bottle without touching it?
EXPERIMENT OVERVIEW: Air has the ability to make objects move into and out of places where they otherwise would not fit. In this experiment, you will force a hard-boiled egg into a bottle without touching the egg.
SCIENCE CONCEPT: Air has an interesting behavior. It always flows from high pressure to low pressure. That is why when you get a hole in a tire on your bicycle the air leaks out.
In this experiment, you'll be placing a hard-boiled egg between high pressure (the outside air) and low pressure (the air inside the bottle). Air wants so badly to get inside the bottle, it will push away anything in its way (the egg)! You'll use this principle to get the egg in.
To make it work, you will have to lower the pressure inside the bottle so much that the outside air forces the egg into the bottle. You will do this by placing lit matches inside the bottle. The matches will burn until the oxygen inside the bottle has been consumed.
At this point, there is less air (since some of it was consumed by the fire) inside the bottle than there previously was, which results in decreased air pressure. As the outer air pushes into the bottle, the egg slides in.
- Wide-mouth bottle (20–32-ounce juice bottles will work but make sure that the egg is just barely too big. If the opening is too small, the egg will probably get stuck.)
- 1 hard-boiled egg with the shell removed
- 3 matches
- A very small piece of paper (1″ × 1″)
Place the hard-boiled egg on the mouth of the bottle. It should sit comfortably without falling off. You may try to push the egg into the bottle to verify that it does not easily fit.
Remove the egg and place three lit matches into the bottle with the paper. Use matches only with adult supervision!
Quickly replace the egg on the mouth of the bottle, effectively sealing the top of the bottle.
Watch as the matches go out and the egg is pulled down into the bottle.
Turn the bottle upside down so the egg falls into the opening without coming out. Blow into the opening. (It is recommended to have an adult do this part.)
As the pressure inside the bottle increases, the egg should be pushed out of the bottle into your mouth.
QUESTIONS FOR THE SCIENTIST
Why did the egg get pushed into the bottle? _________________________________
What did the burning matches have to do with this experiment? _____________________
What are some other examples of air flowing from high pressure to low pressure? _____________________________________________________________________________
Can you fill in the blanks to finish the following common sayings about eggs? Choose words from the Word List found in the bottom of the bottle.