Key Concepts
Physics
Air pressure
Water pressure
Surface tension
Introduction
Did you know that at sea level there are about 15 pounds of air pressing on each square inch of your body? This air is very helpful in our daily lives. For example, the layer of air around the earth helps to keep it from getting too cold or hot. It can even help keep a bottle with holes in it leak-free! Try the activity to find out how.
Background
Earth is covered by the atmosphere, which is a blanket of gas that is 60 miles thick. Although we usually think of air as not being full of anything, all air is made up of tiny particles, which have a small amount of mass. We—and anything else around us—experience the weight of this layer of gas as pressure; this is called atmospheric pressure. We are so used to this pressure, however, that we rarely notice its existence. But if you have ever felt your ears "pop" while driving up a mountain, you noticed it changing. As you drive up in elevation, fewer layers of air press on you. The air inside your eardrums remained at the air pressure from a lower elevation—at least, until they "popped."
Water is also made up of tiny particles that have mass. When something is underwater it feels the pressure of all the layers of water above it. Because water is much denser than air it is also much heavier. A layer of about 10 meters of water creates approximately the same pressure as the 60-mile-thick layer of air surrounding the earth. You might have felt water pressure while diving in a deep pool; the deeper you dive, the more you feel the water press against your eardrums.
Water has another interesting quality: its particles like to stay together. It is as if there is a thin film around a body of water. Scientists call this surface tension. You can see surface tension at work when looking closely at water droplets on a solid surface; they tend to clump together in round dots or small puddles rather than spreading out completely flat and evenly.
In this activity you will find out how you can use air pressure and surface tension to keep a bottle with holes in it from leaking.
Materials
- Sturdy plastic bottle with tight-fitting lid (750 milliliters to 2 liters works well)
- Water (enough to fill the bottle)
- At least four pushpins
- Work area that can get wet
- Baking pan with a rim
- Sink
- Towel or cloth for cleanup
- More sturdy plastic bottles (optional)
- More water (optional)
Preparation
- Fill the bottle with water, and close the lid tightly.
- Place the bottle on the baking pan. The pan will catch any water that might flow out.
Procedure
- Push at least four pushpins into the body of the bottle, about one inch from its bottom. Does water leak out? Why do you think this is so? What do you think will happen if you pull the pins out?
- Carefully pull the first pin straight out so you leave a small round hole in the bottle. Do you see a stream of water flowing out or just a trickle? Why do you think that is so? What do you think will happen when you pull the other pins out?
- Carefully pull the other pins out of the bottle. Try not to press on the bottle while you pull the pins out. What happens? Is this what you expected? Why do you think this happens?
- Press on the bottle and release it. Repeat this a few times. What happens when you press on the bottle? What happens when you release it? Why do you think this would happen?
- Carefully move the bottle to a sink. Hold a towel underneath to catch any drips. Hold the bottle over the sink, and open it. What happens? Why would this happen? Can you find a way to stop the streams of water?
- Extra: In another bottle make holes at different heights in the body of the bottle. All of the holes should be well below the waterline. Do you think that making holes at different heights will alter the results?
- Extra: Try again but make your last hole near the bottle's spout so air can flow in and out of the bottle. How does this small hole change the outcome? Why do you think that is?
- Extra: This activity asks to make small round holes in the bottle. Try repeating the activity with larger holes or irregularly shaped holes, and see if the results change. Why would the size or shape of the hole matter?
Observations and Results
It is likely that only a trickle dripped from the holes when the bottle remained closed and no pressure was applied—and that streams poured out when the bottle was opened or pushed in.
Water likes to stick together, so it takes effort to separate a stream of water from a body of water. Unless you press the bottle—or unless the air in the atmosphere pushes on the top surface of the water—no streams will flow out from tiny holes.
Air outside the bottle also presses against the water near the holes. If there is a place for air to flow in (such as an opening at the top of the bottle), the entering air could allow water drip out of the holes. But in an airtight bottle no air can enter to fill the space of any water that dripped out. So if any bit of water leaves the bottle, it reduces the air pressure inside the bottle, keeping the water from pouring out. If you squeeze the bottle, however, you increase the air pressure, and water can squirt out.
If the holes are small enough, the water sticks together just enough to stop air from bubbling in. If you tried making larger holes, you probably noticed that air bubbles manage to creep in as water flowed out of them. If you made a small hole near the spout of the bottle, you undoubtedly noticed that even a tiny hole can allow enough airflow in to get the water flowing.
More to Explore
Physics for Kids: Pressure, from Ducksters
Plug a Leaky Bottle with the Power of Air, from Scientific American
No Wonder My Ears Hurt When Flying, from Scientific American
Puffing Up Marshmallows, from Scientific American
Build a Water Strider, from Science Buddies
STEM Activities for Kids, from Science Buddies
This activity brought to you in partnership with Science Buddies
