Here’s a classic activity with a showy twist. Surprise your friends by inverting a water bottle without spilling its contents!
* Hint: Smaller bottles with a narrow neck are easier to handle
Carefully cut out a circle of flyscreen about 10 centimetres across.
Put the flyscreen over the bottle’s mouth. Gently bend the flyscreen down the neck of the bottle.
Put a rubber band around the bottle’s neck to hold the flyscreen in place.
Put the bottle under a tap and fill it up with water.
Time to play! Try pouring some water out of the bottle and into the sink. When you want to stop the water flowing out, keep tipping until the bottle is completely upside down.
When water pours out of a bottle, air comes in! If one of these stops, the other will too.
When the bottle is completely upside down, the water stops pouring. Water tends to stick to itself, and this property is known as surface tension. The air can’t stretch the surface of the water from a tiny flyscreen square to a huge bubble. The water molecules don’t stick together particularly strongly, but it’s enough to keep air out and water in.
You can see how weak the force between water molecules is by tipping the bottle sideways. Suddenly, air can get in, even if the bottle is completely full! The trick here is a small difference in pressure.
When the bottle is on an angle, the water at the bottom of the mouth is being squeezed by all the water on top of it. This small pressure difference between the bottom and the top is enough to overcome surface tension, letting air into the bottle. The water then flows freely out of the bottle!
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27 February, 2019 at 9:14 pm
I don’t have flyscreen, would any other materials work?
28 February, 2019 at 9:08 am
What you want is something with lots of small holes – you don’t want the holes to be too big or too small.
You might be able to use a very light cheesecloth or muslin – I haven’t tried it though, and the holes might be too small. maybe a gauze bandage could work?
For a more creative solution, maybe you could blu-tac a tea strainer over the mouth of the bottle? Once again, I haven’t tried it, but it could be worth a try.
If you do try something, please let us know how it went – we’d love to know!