What it shows
A balloon hung inside a sealed bottle will not blow up, because the air already in the bottle has nowhere to go. Make a small hole in the bottle and the balloon inflates easily, proving that air is real and takes up space.
The science
Air is a gas made of tiny particles that fill whatever space they are in and push on the walls. The sealed bottle is full of air, and when you try to blow up the balloon the trapped air is squashed and pushes back harder and harder, so the balloon stays small. With a hole, the trapped air can escape and the balloon fills. If you inflate the balloon with the hole open and then cover the hole with a finger, the balloon stays inflated with no knot, because outside air cannot get in to push it back.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two identical plastic bottles (free)
- Two balloons
- A pin or a hot nail to make a small hole, used by an adult
- Tape
Safety
For this project: An adult must make the hole with a heated nail or a drill. Blow gently so the balloon does not pop near your face.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Push a balloon into the first bottle and stretch its neck over the bottle mouth so it hangs inside.
Try to blow the balloon up; it hardly grows at all however hard you blow.
Ask an adult to make a small hole near the bottom of the second bottle.
Fit the second balloon in the same way and blow; this time it fills up easily.
While the balloon is still full, cover the hole with a finger and take your mouth away; the balloon stays inflated.
Lift your finger off the hole and watch the balloon shrink as air comes back into the bottle.
Feel the air coming out of the hole as you blow, by holding the hole near your cheek.
Explain the difference between the two bottles in a simple drawing with arrows showing where the air goes.
What you should see
The balloon in the sealed bottle will not inflate, while the balloon in the bottle with a hole fills easily and even stays inflated when the hole is covered.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why will the balloon not inflate in the sealed bottle?
The bottle is already full of air, which cannot get out, so there is no room for the balloon to grow.
2What does the hole do?
It lets the trapped air escape so the balloon can take its place.
3Why does the balloon stay inflated when you cover the hole?
No air can get into the bottle to push on the balloon, so it cannot shrink.
4Is an "empty" bottle really empty?
No, it is full of air.
Take it further
Fit a straw through the bottle lid next to the balloon neck and use it as a hand pump to blow the balloon up and let it down.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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