April Altott

An Upside-Down Beaker in Water

Science Concept: Gas Pressure. A difference in air pressure will prevent water from entering an upside-down beaker placed in water.

Materials & Pre-Demo Preparations:

1 - 400 mL beaker

Paper towels

1 - 700 mL beaker filled halfway with water

Directions: Fill a 700 ml beaker halfway with water and then put the 400 ml beaker upside-down into the beaker of water. Question the students and ask them if they think there is water in the upside-down beaker. Next, pull the small beaker out of the water and crumple some paper towels inside the beaker. Then put the beaker with the paper towels upside down into the 700 ml beaker of water. Show the students what it looks like through the clear glass side of the beaker. Then pull the small beaker out, and pull the paper towels out of the beaker. Show the students that the towels are dry.

Introduction & Commentary: When objects are placed in water, they usually get wet, right? Well, I'm going to do a magic trick involving water and these paper towels. Something is going to happen that you wouldn't expect. Are there any guesses as to what might happen?

Explanation: When the beaker with paper towels is placed upside-down in water, the towels stay dry due to the air pressure on the inside of the beaker. The air molecules inside the empty beaker collide into each other and on the sides of the beaker, therefore causing pressure. In addition to this, the air molecules are also colliding against the surface of the water on the bottom, therefore keeping the water out of the beaker by creating enough pressure to prevent the flow of liquid into the empty beaker. The air molecules create an invisible wall as represented by the dotted red line in the graphic on the right.

Safety Precautions: There are no specific safety precautions for this demonstration.

Waste Disposal: Empty the contents of the beaker down the drain and throw the paper towels in the trash.

Reference: Public Domain