Dry Tissue Dunk

A Tissue Diving Bell.

 

 

 

Category - Solids, Liquids & Gases

Key Idea - Compressed Gases

 

Purpose - This simple demonstration proved the fact that gases are matter; that they take up space.
Nature -  Demonstration
Materials -  a small aquarium (15 L), a clear plastic glass, a box of tissues
Method - 
  1. Fill the aquarium to about the three quarters point.
  2. Gently wedge enough tissues into the bottom of the glass so that they are wedged in, and wont fall out when you invert the glass they won't fall out.
  3. Turn the glass upside down and push it under water in the aquarium.
  4. Pull the glass out and note that the tissues are dry.
  5. Try again watching carefully.  You should note that a small amount of water does of water does push up into the glass, the the deeper the glass goes, the more is pushed in.
Safety - Nil.
Explanation - As well as tissue, the glass also contains air.  As it is pushed into the water the air resists the push of water into the glass, keeping the tissue dry.  Because air (a gas) is compressible, and the water exerts more pressure the deep it is; some water is pushed into the gas, and this amount gets greater with greater depth. 
Notes - Some of the earliest underwater explorers actually made use of this principle by using diving bells.  These were basically larger versions of this upturned glass that could hold a person on the sea floor.  Of course they had to be very careful about not using up all of their oxygen before returning to the surface! (See Suffocating Flame)