Air Takes Up Space

It cannot get simpler than this: air is a “thing” that occupies space! Even though we cannot see it, air is made of molecules that exert pressure. In this experiment, the air pressure trapped inside the glass is strong enough to exclude the water and keep the ping pong ball at the bottom.

Using a dry tissue taped to the bottom of the glass is a classic way to prove this. When you submerge the glass vertically, the air has nowhere to go. It forms a protective “bubble” that prevents the water from reaching the tissue. You would have noticed the water level in your main container rise as you pushed the glass down; this is due to displacement, where the volume of the trapped air pushes the water out of the way.

This exact principle is used in deep-sea diving bells and underwater airlocks. As long as the internal air pressure is maintained at a level equal to or greater than the surrounding water pressure, the water cannot enter the cabin. This allows divers to work at great depths while staying relatively dry!

Variables to test

Find out more about variable testing here.

• Liquid Density
  Does this work with all liquids? Try using oil or a sugar-heavy syrup. Does the density of the liquid make it harder to push the glass down?
• Volume and Pressure
  What happens if you change the shape or size of the glass? Does a taller, thinner glass hold more or less air than a short, wide one?
• Depth Perception
  If you push the glass very deep (like in a swimming pool), does the water eventually start to creep up into the glass? (Hint: Air can be compressed under high pressure!)