The next 3 experiments all deal with gas pressure. They are among my kids favorite experiments - but, as boys, anything exploding or seeming like magic will always be a hit!
Experiment 8: Exploding Soda
CAUTION: you'll want to do this outside away from your house, as it makes a fantastic mess.
2 liter bottle of soda unopened
1 package of mint mentos
1) set the soda bottle in a wide open space and carefully open the top. (don't shake up the soda as you want as much of hte carbon dioxide to remain in the bottle as possible)
2) this is the tricky party. You'll want to drop at least 5 mentos into the soda at the same time. I stacked the mentos carefully so they could drop, one right after the other into the bottle.
3) duck and run! The soda will shoot up about 15 feet into the air!
Soda is backed with carbon dioxide, that's what all the fizz is about. A mento may seem smooth on the surface, but in actuality the entire surface is pitted with hundreds of microscopic holes. When they are dropped into the soca much of the carbon zooms through these holes, building up the gas pressure to explosive levels, as the pressure inside the bottle becomes greater than the air pressure outside, the liquid seeks to go where there is less pressure, outside the bottle. This is a similar concept to releasing a blown up balloon - it will zoom around as the pressure within the balloon is rapidly released.
Experiment 9: the Coin Launcher
1 small bottle of soda, empty (16 oz is a good size)
1 pieces of tissue paper
1) put the top on tightly on the empty bottle, also place it in the freezer
2) let sit for About 30 minutes
3) observe what changes to the bottle, but leave it in the freezer so it doesn't warm
4) wrap the quarter in the tissue paper
5) get the paper wet
6) with the bottle still in the freezer, remove the top and place the tissue wrapped quarter over the opening.
7) allow to sit in freezer until tissue is frozen
8) take bottle out of the freezer. Wear the safety glasses!
9) gently warm the bottle in your hands pointing away from you. Watch what happens!
When the bottle is frozen it shrinks as the air molecules pull toward the middle of the boodle. When the frozen tissue is in place of the top, it should maintain this vacuum. (if he coin doesnt shoot off, then the tissue dint form a good enough seal). When the bottle is then warmed the air molecules push back away from the center. This causes a change in pressure that pushes the coin away.
Experiment 10: Magic Card Trick
We actually first did this trick last spring for a local science fair. My youngest did the next two experiments as a part of his display on the awesome capabilities of air & water pressure!
Small glass or plastic cup (it must have a rather sharp or flat rim with no bends)
1 index card
1) fill the glass about 3/4 of the way full of water
2) lay the index card flat across the open end of the glass.
3) support the card with the flat of your hand as you quickly turn the cup upside down
4) remove your hand, amaze your family & friends with the fact that the card stays in place and doesn't fall!! (don't leave it too long upside down though, once the water soaks into the card enough it will destroy the seal and the water will go everywhere!)
The reason the card stays in place is based on 2 laws. First Newtons second law of motion: "every action has an equal and opposite reaction" and air pressure vs water pressure. (or more specifically air pressure vs the force of gravity)
Essentially the water in the cup is pushing down with the force of gravity (9.8 meters/second squared). While this force is pointing toward the ground, the air outside the cup of pushing upwards on the card. For a period of time this creates a seal between the card and the edge of the cup. Furthermore there are way more air molecules outside of the cup than there are water molecules inside the cup. This is the true reason for the 'magic'. All air molecules are trying to raise up. This means there is a pressure imbalance. It's like having a car hit a brick wall. If it is a small wall the wall will be destroyed by the car - because there are too many molecules in the car acting against the fewer molecules in the wall. But if it is a gigantic stone wall (like a concrete bridge support) then the car is the one with more damage - as there are more molecules in the wall than there are in the car.