Properties of Matter

Cartesian Ketchup Sachet Diver

Enrichment Activity for 5-11 11-14

What you need

  • 2 litre plastic bottle
  • Blue Tac
  • A bowl of water
  • A ketchup sachet


  1. Put the ketchup sachet into a bowl of water to see if it floats upright - if not then add a little Blue Tac to its bottom
  2. Fill the 2 litre bottle with water right to the top
  3. Push your ketchup diver through the neck of the bottle
  4. Put the lid on tightly, squeeze the bottle hard and watch your diver dive

Results and Explanation

The diver sinks when you squeeze the bottle, and rises when you let go.

Tip: if you diver doesn't dive, or sinks without rising, change the amount of Blue Tac on the sachet.

Squeezing the bottle squeezes everything inside it, including the air bubbles in the ketchup sachet. As the air molecules squash together, the sachet gets more dense than the water and it sinks. What happens when you stop squeezing?

appears in the relation m=ρV
IOP DOMAINS Physics CPD programme

The IOP is proud to present our new Forces CPD Domain

This is the first of a collection of videos for you to view ahead of our live support sessions for teachers and coaches of physics.

Find out more