Do you enjoy being able to explain certain phenomena with scientific lingo? This month you will learn a little about the properties of air pressure and how it pertains to water leaking out of a bottle. You’ll use terms like air pressure and vacuums while you’re at it.
- Plastic water bottle with a tight fitting lid, small nail, water
- Put a small hole near the bottom of the water bottle using the nail. Be careful! Push it through or tap it in with a hammer
- Put a small hole through the lid of the bottle with the nail (you will probably have to tap this through with a hammer, the plastic is thicker)
- Fill the bottle to the top with water, holding your finger on the hole on the side of the bottle
- Tighten the lid
- Do this next part over a sink!
- Cover the top with your thumb
- Remove your finger from the side of the bottle and wait a second
- Notice the water is not pouring out!
- Take your thumb off the top now so the water pours out
- Alternate putting your thumb back on the top of the lid hole and removing it
Did you notice the only time the water poured out of the bottle was when you weren’t covering the hole on the top? Isn’t that strange? There’s a hole there, why doesn’t the water come out? Shouldn’t gravity be pulling it down? There are different forces involved besides gravity. Air pressure is a strong force and it always acts in a certain way: High Pressure moves towards Low Pressure.
This basically means “more air” moves to “less air.” It is how wings cause flight, how spray paint cans works, how a plunger works, and much more! If there is a “vacuum” (a place with no air or less air than normal) then air will rush in to fill that void. So where does that happen in the Drip Drop Bottle?
When you cover up the lid to the bottle, the water is pulling down but that air up in the top of the bottle is becoming a vacuum. That means “low pressure.” The air pressure outside the bottle is higher, so it actually tries to push into the bottle through the bottom hole. In doing so, it keeps the water in!
When you release the hole on the top, there is no vacuum, so the water is free to flow out the hole into your sink.
If you’ve ever tried to squeeze all the water out of a water bottle you might notice that if you hold it straight upside down and squeeze it doesn’t all come out. You have to let the bottle fill back up with air before it starts up. Where else have you seen this phenomenon take place? Some people use canned juice or evaporated milk and have to put a hole in the top to empty it. Do you know what else they do? They put another hole up top to make it pour better. Why? This stops a vacuum from forming and allows the liquid to pour out. Keep your eyes open for other instances of this happening. Now you’ll be able to explain this as a scientist.
Steve Davala is a high school chemistry teacher who likes to write. He’s got two kids of his own and subjects them to these science activities as guinea pigs.