Though it’s all around all of us all the time, it can be hard to really get a good idea of air pressure. It’s a thing, obviously, and they talk about it during weather reports sometimes, but it’s not something you can really “feel,” so it’s hard to explain to kids what air pressure can really do. This experiment will help you demonstrate how air pressure can affect other elements around us.
Using Water to Demonstrate Air Pressure?
All you need for this one is a glass of water and a paper index card (one that’s large enough to cover the mouth of the glass). And a sink. Also, you may want to practice this one a few times before your presentation, as it can be tricky the first few times.
Fill the glass with water to the top. The very tippy-tip-top. Like almost spilling over. Then, place the index card over the glass—make sure the mouth of the glass is completely covered by the card. Gently press the card down a few times atop the glass to make sure any pockets of air are pushed out; you want direct index card-on-water contact and a tight “seal.”
Hold the card tightly on top of the glass and slowly turn it upside down over the sink. Slowly and smoothly take your hand off the index card. If all goes as planned (and after you’ve practiced, it will surely go swimmingly), the card will remain in place, holding the water in the glass—even without anything supporting it.
What Sorcery is This?!?
It’s not sorcery, silly; it’s science! The air pressure outside the glass is greater than the pressure of the water inside the glass, so it (the air pressure) will hold the water in the glass. The air pressure is pushing upward against the index card harder than the water is pushing down.
This is why it’s important that all the air is out of the glass and you’ve established a tight seal. Air shifting upward through the glass as you turn it over will make the water bubble and likely break the seal. Similarly, if the seal isn’t tight enough to keep air out, air sneaking into the glass will cause the pressure to change and… sploosh!
- 365 Simple Science Experiments with Everyday Materials, E. Richard Churchill, Louis V. Loeschnig, Muriel Mandell, and Frances Zweifel, 1997. ISBN 978-1884822674