Key concepts
Physics
Air pressure
Volume
Forces

Introduction
You might have heard people talking about pressure before: maybe that there is too much of it, or not enough. Pressure can mean different things in everyday English, but in general when scientists talk about pressure they mean a constant physical force (such as a push or a pull) exerted or applied to an object. For example, if you sit on an inflated balloon, you can see the effect of the pressure on that balloon—at least, until it pops!

In this activity we’re going to use pressure in an unexpected way. Can you keep air in a balloon without tying the bottom? What about keeping water inside a bottle, even if there’s a hole in it? Maybe you can do both—with just the right amount of pressure!

Background
Have you ever used your finger to seal the top of a straw, and noticed that some of your drink stayed inside the straw—even if you lifted it above the surface of the liquid? If so, then you’ve already tested the amazing properties of air pressure!

Even though we can’t see them, air molecules comprise the air all around us. You can feel them moving if you clap your hands close to your face, and feel a small blast of air on your cheek. Those are air molecules! Despite being invisible to us, those molecules still exert a pressure or force on everything in our environment, including you. You can feel their pressure by holding a piece of paper in one hand, facing up toward you. Move the paper up and down quickly. Can you feel the resistance, or force, of the air against the paper? Now try holding the paper with the edge facing you. Move it up and down again, quickly. It was easier the second time because the paper wasn’t facing the direction in which you were moving it—there was less resistance or force from the air molecules.

The force of air molecules is important in this activity. They will act on the air and water inside a bottle in a few different (and surprising) ways. Get ready to turn up the pressure!

Materials

• One personal-size plastic water bottle
• A sharp thumbtack or nail
• Scissors
• Two balloons
• Measuring tape (A ruler will work, too.)
• Sink
• An adult helper (This activity requires two sets of hands as well as the handling of sharp objects.)

Preparation

• Ask your adult helper to use the thumbtack or nail to poke a hole in the bottom of your water bottle. Have your helper use the scissors to carefully widen the hole, so that it is slightly smaller than the circumference of a yellow pencil.

Procedure

• Hold your finger over the hole in the bottom of the bottle to seal it. Keep the bottle over the sink, and fill it with water until there are about two inches of empty space at the top. Keep your finger over the hole so no water leaks out!
• Have your helper blow up the balloon until it is approximately seven inches in diameter. Use the measuring tape or ruler to measure.
• Then have your partner use their fingers to hold the balloon sealed closed, but leave about one inch of space at the bottom. Keeping it sealed, have them use their other hand to put the mouth of the balloon over the top of the water bottle. Note: this can be tricky! Try to do it slowly, easing the balloon over the top of the bottle gently.
• Once the balloon is on the bottle, your helper can let go of it. Measure the diameter of the balloon with your ruler. Keep your finger over the hole! What happens to the balloon when your helper stops holding it? Does it start to lose air when their fingers are no longer sealing it? Is this surprising? What usually happens when you let go of a balloon without tying it? Why do you think something different is happening now? (Note: If your balloon is shrinking in size at this point, it may have a hole in it. Check for holes before continuing with the activity!)
• Have your helper hold the measuring tape or ruler up behind the balloon at its widest point, so you can measure its diameter. Keep the measuring tape or ruler there for the next step.
• With the bottle still over the sink, lift your finger and allow water to flow out of the bottle. Watch the balloon. When approximately three quarters of the water has leaked out of the bottle, put your finger over the hole again.
• Have your helper measure the diameter of the balloon. Has its diameter changed? If so, has it gotten larger or smaller? If not, why do you think the air is staying inside the balloon instead of escaping?
• Wait 30 seconds and have your helper measure the diameter of the balloon again. Has the diameter of the balloon changed? If so, has it gotten larger or smaller? If not, why do you think the air is staying inside the balloon instead of escaping?
• Lift your finger again and allow the water to flow out of the bottle into the sink. As the bottle empties, does the water seem to flow out of the bottle faster, slower or at the same rate as it did when you started? What do you notice about the size of the balloon? Do you notice anything else? For example, does the balloon make any noises as the water bottle empties?
• When all of the water is out of the bottle, measure the diameter of the balloon. Did the balloon get larger or smaller as the water leaked out of the bottle? Why do you think this happened?
• Have your helper remove the balloon from the bottle and get ready to try the same test again—this time without inflating the balloon.
• Hold your finger over the hole in the bottom of the empty bottle to seal it. Keep the bottle over the sink and fill it with water until there are about two inches of empty space at the top. Keep your finger over the hole so no water leaks out!
• Have your helper put the balloon over the top of the water bottle. Notice the appearance of the balloon—does it have any air in it at all? How can you tell?
• With the bottle over the sink, lift your finger and allow water to flow out of the bottle. This time, pay attention to the balloon and the water. When the water level inside the bottle gets down to the halfway point, put your finger over the hole again. What do you notice about the appearance of the balloon? Has it changed? If so, in what way? What do you notice about the water? Is it flowing out of the bottle as quickly as it did during the first test?
• Lift your finger again and allow the water to flow out of the bottle. As it empties, does the water seem to flow out faster, slower or at the same rate as it did when you started? Does the water keep flowing out of the bottle until it is empty? How would you explain the changes you observe between this test and the one with the inflated balloon? What do you notice about the appearance of the balloon? Does it look different than when you started? In what way?
• Extra: Try inflating the balloon to different sizes. Start small and then work your way larger. Does changing the balloon size affect the results of your test? Why do you think that might be?
• Extra: Test taking the deflated balloon off the water bottle at the different point during the second test. What happens when the balloon isn’t on top of the bottle?
• Extra: Ask your adult helper to poke a hole closer to the top of the bottle. How does this change your results? (Save this one until last!)

Observations and results
During the first test, when your finger was covering the hole in the bottom of the water bottle, you should have observed that the balloon stayed the same size and did not leak air. This might have surprised you; usually if you let go of an inflated balloon without tying it, you end up chasing after your balloon as it quickly loses air! What was holding the air inside the balloon this time? The answer is: air (and water)! Most of the room in the bottle was taken up by water, with some air at the top. There might have been a little room at the top of the bottle for more air to squeeze in (so your balloon might have shrunk a small amount in the very beginning), but then it should have stayed the same size. The air in the balloon had nowhere to go, because the bottle was already full!

When you lifted your finger and allowed the water to flow out of the bottle, however, something different happened. You should have seen your balloon start to deflate, or lose air. Why did it lose air when you lifted your finger? Because the water leaving the bottle left room for the air inside the balloon to enter the bottle. Even though you couldn't see it, as the water leaked out, the bottle was slowly filling with air from the balloon. You might have seen this when you covered the hole again. If the balloon was shrinking before you re-covered the hole, it probably stopped when the hole was sealed again. When the water stopped leaving, no more air could fit into the bottle.

In the second part of this activity you might have noticed that the appearance of the balloon changed as the water leaked out of the bottle. It might have become shriveled looking, as the tiny bit of air inside the balloon went into the bottle. The reason for this result is similar to the explanation for the first test. As the water left the bottle, it created space for the air inside the balloon. Even though in this case it was a small amount of air, it still had an effect on the size of the balloon—and as that air left, the balloon shrunk. In addition, in the second part of the activity you probably noticed a big change in the water leaking out of the bottle. At some point, the water should have stopped leaking out of the hole (or significantly slowed), even though there was still water in the bottle! How is this possible? Again, the answer has to do with air pressure. Because the balloon is empty, no air is trying to get into the bottle from the top. In fact, the balloon seals the bottle, so no air can get in through the top of the bottle. This doesn’t mean air isn’t exerting a force on the water in the bottle, however. Air molecules outside the bottle are exerting pressure from all sides of the bottle, including at the hole in its bottom.

Normally, without the balloon on top, the water would leak out of the bottle because the pressure from the air coming in the top of the bottle, combined with the force of gravity pulling on the water, is greater than the force of the air molecules pushing on the bottom of the bottle. When we put the balloon on top, however, the air molecules at the top of the bottle can’t exert any pressure, the balloon is sealing the bottle. Therefore, the only forces acting on the water are gravity and the force of the air molecules pushing up. In this case, the force of the air outside the hole is great enough to help slow the leakage of water from the bottle.

More to explore
How Does a Hovercraft Work?, from Science Buddies
Under Pressure: Launch a Balloon Rocket, from Scientific American
Balloon Morphing: How Gases Contract and Expand, from Science Buddies
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies