Cool Science in Seconds - Activity 1: Cartesian Diver

Cool Science in Seconds - Activity 1: Cartesian Diver 

Materials / Apparatus:
- A packet of condiment (ketchup / chilli sauce)
- A disposable mineral water bottle
- A glass of water

Steps:
(1) Test if your condiment packet is a good Cartesian diver. Fill a glass with water and drop the condiment packet into the glass. The best packets are ones that just barely float.

(2) Fill an empty, clear plastic mineral water bottle to the top with water. Drop your unopened condiment packet into the bottle. Cap the bottle.

(3) Squeeze the mineral water bottle to make the diver sink.  Release to make it rise.

Teacher Serena's Explanations:
This experiment demonstrates the property of buoyancy. Density is what determines whether an object sinks or floats. Density = Mass / Volume. The density of water is 1g /cm³

Many types of sauce are denser than water but it is the air bubble at the top of the sauce that determines whether the packet will sink or swim.

As the bottle is gently squeezed,  there is an increase in air pressure inside the bottle. The increase in air pressure is transferred throughout the bottle and causes the air inside the floating object to be compressed. In short, squeezing the bottle causes the bubble to shrink. This makes the floating object  denser and less buoyant. Hence it sinks.

Releasing the pressure on the bottle and the condiment packet begins to rise back to the top.

Applications in Real Life:

Fish keep themselves from either sinking or floating to the surface by using muscles to squeeze or relax a small sac (with a small air bubble inside) in their bodies. By squeezing the sac smaller, the fish will sink. By relaxing their muscles, the sac increases in size, displaces more water, and a fish will begin to rise to the surface.

Man use this same principle to control the buoyancy of a submarine. By pumping water in and out of tanks stored in the submarine, a submarine can be made to rise and sink. 

~ Serena's Greenhouse