What it shows
A capped bottle of warm air shrinks and crumples when it is cooled quickly, because the cooled air inside takes up less space and presses less, while the air outside keeps pressing in. It shows that air pressure is a real and strong push.
The science
Air pushes on everything around it; at sea level it presses with about 1 kg on every square centimetre, but we do not notice because the air inside objects pushes back just as hard. Warming the air in the bottle makes its particles move faster and spread out, and some escape before you put the cap on. When the bottle is cooled, the particles inside slow down and press less, so the air outside wins and squashes the bottle inwards until the pressures balance again.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- An empty thin plastic water bottle with its cap (free)
- Warm tap water, not boiling
- A bowl of cold water with ice cubes
- A towel and a tray
Safety
For this project: Use only warm tap water from the hot tap, never boiling water, and an adult must handle the warm water for younger students.
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.
Pour warm tap water into the bottle, cap it and shake it for half a minute to warm the air and the plastic.
Pour the water out, leaving the inside warm and steamy, and screw the cap on tightly straight away.
Stand the bottle in the bowl of iced water, or pour cold water over it, and watch the sides.
Within a minute the bottle creaks and crumples inwards; take a photo or sketch it.
Open the cap and listen for the hiss of air rushing back in as the bottle pops back into shape.
Repeat with the cap left off and notice that nothing happens, because air can flow in freely.
Try the test once with a thick-walled bottle and once with a thin one and compare how much each crushes.
Write an explanation using the words air pressure, cooling and particles.
What you should see
The capped bottle crumples visibly as it cools and pops back out with a hiss when the cap is opened. The uncapped bottle stays the same shape.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What crushed the bottle?
The air outside, pushing inwards harder than the cooled air inside pushed outwards.
2Why did the uncapped bottle stay the same?
Air flowed in through the open neck, so the pressure inside and outside stayed equal.
3What does cooling do to the air inside?
The particles slow down and take up less space, so they press less on the walls.
4Why does the bottle pop back when you open the cap?
Outside air rushes in and the pressure inside becomes equal to the outside again.
Take it further
Repeat with an empty aluminium drink can and warm water, and compare how strongly it dents.
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.
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