Air Pressure: Air pressure is the force exerted by air on any surface in contact with it.
It is sometimes hard for children to understand the immense pressure that air can exert. These simple demonstrations can help illustrate this.
Egg in The Bottle
This is a great, classic science demonstration. You will need:
A boiled egg
A glass bottle or jar with a neck a little smaller than an egg
Unfortunately, I couldn't find a bottle with the correct sized opening around the house. This was the best one I could find, but the opening is a little too small and the bottle itself is a little too small as well to work optimally. But, as you will, sometimes an experiment doesn't have to work perfectly in order for you to get learning benefits.
Light two matches and drop them inside the glass jar, quickly place the egg back on top.
Once the matches go out, the air starts to cool, making the pressure drop. The air outside the jar now has a higher pressure than the inside, forcing the egg downward.
Our poor egg gets sucked down by the air pressure, but then it gets stuck and remains there for a short time before...
The pressure finally becomes too great for the egg, and it splits open by it. Perhaps our egg didn't plop down into the jar like it was supposed to, but the fact that the egg was torn in two by the pressure was also a dramatic conclusion to the demonstration.
(This demonstration can be found many places, but I most recently was reminded of it at Science Sparks.)
Air Pressure vs. Gravity
This is another great, classic science demonstration. You will need:
a glass or jar
index card or cardstock, large enough to cover the top of the glass or jar
sink or bowl to catch the water
Fill the jar or glass to the brim with water. You don't want there to be any space for air.
Place the card over the top of the jar. Be sure the top is completely covered.
Standing over the sink or bowl, hold the card against the jar as you turn it upside down.
Take you hand away from the card.
The water stays in the jar because the air pressure against the card is stronger than gravity pulling the card down. If you gently pull on the card, breaking the seal or if you wait long enough for the water to soak into the card, the water will come pouring out of the jar. However, if you do that, make sure you have a larger bowl, or you will have to clean water off your counter, like we did.
(This demonstration can also be found many places, but I was most recently reminded of it when I looked through The Ben Franklin Book of Easy & Incredible Experiments by The Franklin Institute Science Museum.)
All you need for this demonstration is an empty plastic bottle, with its cap, and some hot water. It is safer to use hot tap water, but our water takes so long to get hot, I boiled some water and then let it cool a bit. Fill the bottle with the hot water. Dump the hot water out of the bottle. This heats the bottle, and therefore the air in the bottle.
Place the cap on the bottle as quickly as you can to trap in the hot air.
Set it down (in our case, on the towel we used to mop up the water from the previous experiment) and watch it collapse from the pressure of the air outside the bottle.
You can also do this with a metal can. You used to be able to purchase empty metal gas cans at the hardware store, but they are now pretty much replaced by plastic. If you can get hold of a clean, empty metal gas can, you can put some water in a metal can and heat it right on the stove and then put the cap on. As it cools, it collapses in on itself just as the plastic bottle does, but it is even more dramatic because the metal really creaks and groans as it collapses. However, when I saw the above experiment at Almost Unschoolers, I saw that this was a much easier way to do the demonstration.
When you put hot water in, it is so hot that some of the water molecules are in the gas phase and when the water vapor cools it collapses the can or bottle because no air could rush in and fill the void left behind by the condensing vapor. The condensing vapor was taking up less volume, and this also demonstrates the difference between the phases of matter in terms of the volume they occupy.
source: Exploring Creation with General Science, Jay Wile