There are lots of great air pressure demonstrations around, but a leaky water bottle is one of the simplest. It looks impressive and costs almost nothing.
Air is made up of several different gases. Air pressure is caused by gravity pulling air molecules towards Earth. We don’t really notice air pressure as it’s balanced inside and outside of our body.
The weight of air ( and air pressure ) decreases with altitude. At sea level, air pressure is high. At the top of a mountain, air pressure is lower as there’s less atmosphere to push down. We don’t feel the pressure as it is exerted equally in all directions, but we can sense a sudden change in pressure, like when taking off or descending in a plane.
Leaky Water Bottle Demonstration
You’ll need
An empty water bottle with a lid
Needle, nail or paperclip
Water
Food colouring – optional
Sink or tray
Instructions
Carefully use the needle or paperclip to poke a small hole on each side of the bottle about one-third of the way up from the bottom. Ask an adult to help
Place the bottle in a sink and fill with water. Quickly put the lid in place.
Open the lid, and the bottle should start to leak through the holes.
Close the lid, and the water will stop flowing.
With the lid on tightly squeeze the bottle, water should flow out of the holes as squeezing increases the pressure inside the bottle.
Carefully poke a needle into the bottle above the water line – ask an adult to help. When the needle is in place, blocking the hole, water will not flow out. When the needle is removed, it will start to flow again.
Why does it work?
When the lid is unscrewed, water flows from the holes in the side, reducing the volume of water in the bottle. Air rushes in through the loose lid to replace the lost water.
When the lid is tightened, air can no longer rush in to replace the water, so any water leaving the bottle reduces the pressure inside, which stops the water escaping.
Extension Challenge
With the cap slightly unscrewed, drop the bottle to the floor. Watch what happens to the flow of water.
Try one of my other easy air pressure experiments.
Air pressure fun facts
A barometer is used to measure air pressure.
Atmospheric pressure drops with altitude.
Atmospheric pressure is an indicator of weather. Low pressure leads to clouds, wind and precipitation. High pressure usually results in clear skies and fine weather.
The Earth’s atmosphere is approximately 78% nitrogen.
Why do your ears pop in an aeroplane?
Ears pop in an aeroplane as your ears try to equalise the pressure. When a plane takes off, the atmospheric pressure decreases, and as you come down, it increases again.
Last Updated on February 5, 2025 by Emma Vanstone
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