ChemixaLEARN CHEMISTRY

Cartesian diver bottle

Objective: build a Cartesian diver that sinks when you squeeze its bottle and floats when you release. You will assemble the diver, control it with pressure, and explain the motion with Boyle's law and buoyancy.

Safety:

Use a plastic bottle only, never glass, because squeezing glass can shatter it. Do not squeeze so hard that the cap pops off. Use only sealed condiment packets; do not open them. Adult supervision is required for minors.

Materials

  • Clear plastic bottle with a tight cap (1 or 2 liter)
  • Water
  • Small sealed ketchup or soy sauce packet
  • Bowl of water for testing

Procedure

1

Choose a floater

Test candidate packets in a bowl of water and pick one that barely floats.

2

Fill the bottle

Fill the plastic bottle completely to the brim with water.

3

Load and seal

Drop the sealed packet in and screw the cap on tightly with no air gap if possible.

4

Squeeze to sink

Squeeze the sides of the bottle firmly and watch the diver sink.

5

Release to float

Release the pressure slowly and watch the diver rise again.

6

Hover the diver

Practice gentle squeezes to park the diver in the middle of the bottle.

What to observe

  • The packet floats at the top of the sealed bottle at rest.

  • Squeezing the bottle makes the diver sink to the bottom.

  • Releasing the squeeze lets the diver float back up.

  • A gentle squeeze holds the diver hovering mid-bottle.

Why it happens

The diver floats because the air trapped inside it makes its overall density lower than water. Squeezing the bottle raises the pressure throughout the water, and by Boyle's law the trapped air bubble compresses into a smaller volume. Less air volume means less water displaced, so the buoyant force pushing up on the diver decreases while its weight stays the same, and it sinks. Releasing the squeeze lets the bubble expand again, restoring buoyancy so the diver rises. Submarines use the same principle, adjusting their buoyancy with ballast tanks instead of a squeeze.

Try variations

  • Compare packets of different sizes and find which dives easiest.

  • Leave a small air gap under the cap and see how the response changes.

  • Build a diver from a small medicine dropper instead of a packet.