What it shows
A sachet with a small air bubble just floats in a bottle of water. Squeezing the bottle raises the pressure, squashes the bubble, and the sachet becomes denser and sinks; let go and it rises. Submarines and fish control their depth the same way.
The science
Whether the sachet floats depends on its density compared with water, and the air bubble inside makes it only just lighter than water. Squeezing a sealed bottle of water raises the pressure everywhere inside it, and that pressure passes through the water to the sachet. Water cannot be squashed but the air bubble can, so the bubble shrinks, the sachet takes up less space while weighing the same, and it becomes denser than water and sinks. Release the squeeze and the bubble springs back, the sachet grows lighter for its size and floats up.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A clear plastic bottle with a cap, 1 litre or more (free)
- A sealed ketchup or soy sauce sachet with a small air bubble inside
- Water
- A bowl to test the sachet first
Safety
This project uses safe, everyday materials. Still, read the rules below with a parent before you start.
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.
Drop the sachet in a bowl of water and check that it just floats with its top barely above the surface; try another sachet if it sinks or floats high.
Fill the bottle to the very brim with water so that there is no air at the top.
Push the sachet into the bottle, top up the water and screw the cap on tightly.
Squeeze the sides of the bottle firmly and watch the sachet sink to the bottom.
Let go and see it rise back to the top.
Practise squeezing gently until you can make the sachet hover halfway down.
Hold the bottle up to the light while squeezing and look for the bubble getting smaller.
Explain in a drawing what happens to the bubble, the volume and the density of the sachet.
What you should see
The sachet dives on every squeeze and rises when released, and with a gentle grip it can be made to hover at any depth.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What happens to the air bubble when you squeeze?
The pressure squashes it smaller, because air can be compressed but water cannot.
2Why does a smaller bubble make the sachet sink?
The sachet takes up less space but weighs the same, so its density becomes greater than water.
3How does a submarine dive and surface?
It lets water into tanks to become denser and dive, and pumps it out to rise.
4Why must the bottle be full with no air at the top?
Any air at the top would squash instead of the bubble and the squeeze would not reach the sachet.
Take it further
Make a diver from the cut-off top of a dropper or a pen cap weighted with a blob of clay, and tune its air bubble until it just floats.
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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