Demonstrate how atmospheric pressure can keep water in a glass even if it is upside down.
Time: about 5 minutes.
- drinking water glass
- small piece of thin cardboard or heavy construction paper
- a sink (or an area that can get very wet)
- Cut a square of cardboard larger than the diameter (width) of the top of the glass
- fill the glass with water to the very top (no air left at top)
- place the cardboard over the glass
- hold the cardboard and turn the glass over
- let go of the cardboard
- What did you observe?
- What do you think is happening?
There are two factors that influence the results of this experiment: air pressure and surface tension. Air pressure is all around us and is caused by the weight of the air molecules above. Now, you may think that molecules don’t weigh that much (and you’re right) but there are billions and trillions (even more) of them weighing down on us and that’s why we have air pressure. This same air pressure helps keep the water in the glass (only if you did the experiment with no air between the cardboard and the surface of the water). The weight of the water in the glass is less than the air pressure pushing on the cardboard. The surface tension of the water helps create a bond between the cardboard and the water surface to help keep the cardboard in place.
- Try it by turning it sideways rather than upside down.
- Try it with a bit less water in the glass (so there is air between the surface of the water and the cardboard).
- Try it with thinner paper.
- Try it with soda water.
- What would happen if you put dish soap around the rim of the glass?