Deep Diving Ketchup Sachet

Also known as the Cartesian Diver; here’s a cool little trick which is easy to set up, shows off some science… and can even be used as a fun strength test among friends.

You wi¬ôll need:

  • one ketchup sachet (available in most restaurants)
  • water bottle (2 litre works best)
  • a supply of water


What to do:

Place the ketchup sachet in a bowl or cup of water to see if it floats (for this experiment you will need a packet that only just floats). Put the sealed ketchup packet (complete with ketchup) in the bottle. Completely fill the bottle with water, right to the brim, and close the lid tightly.

Now squeeze the bottle hard! Can you make the ketchup sachet sink? You may have to really crush the bottle with all your might, but you should find the ketchup packet rather rapidly drops right to the bottom. If you’re having trouble doing this, it may be because you’ve not filled your bottle right up with water, so that it’s overflowing. It could also be that your sachet was made too full (great value for money, but not great for science!).

What’s happening?

When you squeeze the bottle, the water inside it cannot be squashed at all, because it is a liquid. Because the liquid can’t be squashed, that means all the pressure you put on the outside of the bottle is directly transferred to the ketchup packet. There is usually a small air bubble trapped inside the ketchup packet. Being a gas, the air can be squashed – in fact, if you look closely at the sachet when you squeeze the bottle, sometimes you can this.

Now, if we’re squashing the air inside a sachet, this means the pressure inside the sachet increases. One way to think of it is that we still have the same amount of ‘stuff’ in our sachet (no air or ketchup escapes), but because the sachet is being squashed, it has a smaller volume. So importantly, we have the same amount of stuff (air) in a smaller space (the sachet), which we refer to as an increase in an object’s density.

In this case the density of the sachet becomes more than the density of the water in the bottle. An easy way to think of it is that the ketchup sachet becomes heavier, and so sinks. When you let go of the bottle, the pressure inside the bottle decreases again so the air in the packet will expand. This causes the packet to rise back to the top, to do the cycle again.

What are the applications of this?

This is a well known scientific ‘trick’ that has been around for decades. But moving beyond simple water bottles, there are actually real uses to the science behind this – we use it to control the depth of submarines! Submarines have big air tanks, which keep it afloat on the water’s surface. When it wants to sink, it squashes the air in the tanks (the scientific word for squashing air is compressing). All this compressed air make the submarine tanks more dense and heavier, so the submarine sinks in the water. The same as our ketchup sachet, but on a much bigger scale!

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