Squidy - The Cartesian Diver

Squidy is a good role model. Sometimes he's up and sometimes he's down, but he always has a smile on his face.

Australian Curriculum links

  • Science > Chemical Sciences > Year 5 > ACSSU077

You'll need

  • glass eye-dropper
  • permanent marker
  • balloon
  • scissors
  • cup
  • water
  • clear plastic bottle with lid

Try this

  1. Decorate the eye-dropper to look like a squid. Draw some eyes on the top of the dropper. Stick or wrap some tentacles made from balloons around the body.
  2. Fill the cup with water. Fill the eye-dropper ½ - ¾ full with water from the cup, leaving room for a small air bubble. Place your Squidy in the glass of water to test that it floats. It should float with only a small section of the top of the eye dropper floating above the water line. If it doesn't float or it sits too high in the water, change the amount of water in the eye dropper and test it again.
  3. Fill the clear plastic bottle with water. Place the eye-dropper in the bottle, fill the bottle with water right up to the brim and then screw the lid on.
  4. Squeeze the sides of the bottle and watch your Squidy sink. Release the sides of the bottle and watch Squidy rise to the top.

    Note: If you turn the bottle up-side down the air bubble might escape and Squidy will not be able to rise again. If this happens, remove Squidy from the bottle and repeat Steps 2 to 4.

What's happening?

Squidy is made up of the eye-dropper, the bubble of air inside the eye-dropper and the small amount of water in the eye-dropper. It is the bubble of air that gives Squidy its buoyancy and makes it float. When the bottle is squeezed, the increase in pressure causes the air bubble inside Squidy to be compressed and get smaller. This allows more water to enter the eye-dropper which makes Squidy denser and it sinks. When the sides of the bottle are released the pressure acting on the air bubble inside Squidy reduces and the air bubble expands. As the air bubble gets bigger it pushes some water out of the eye-dropper. This makes Squidy less dense so it floats to the top of the bottle.

Real world links

Squidy works in a similar manner to how a submarine dives and rises. Submarines have ballast tanks that can be filled with water to increase the overall weight of the submarine to make it sink. A submarine rises to the surface when compressed air is released into the ballast tanks. This pushes the water out of the tanks which decreases the weight of the submarine, making it more buoyant.