Balloons make an excellent tool to teach middle school students about air pressure. Gather two identical empty plastic water bottles and poke a 1/8-inch hole in the bottom of one of the bottles. Place a balloon inside each bottle with the opening of the balloon stretched over the mouth of the bottle and the rest of the balloon hanging down into the bottle. Ask for two volunteers to blow up the balloons. The balloon inside the bottle with the hole will inflate, but the other one won't. Explain that the bottle without the hole has too much air pressure for the balloon to inflate, but when there's a hole in the bottom, compressed air can escape, making room for the balloon to expand.
You can illustrate the effect of air pressure on water inside a glass for a lesson that will amaze your students. Fill a glass with water all the way to the top. Place an index card on top of the glass and place your hand over the index card. Turn the glass over while holding the index card in place. Slowly take your hand away. The index card will stay in place and keep the water from spilling. Explain that the air pressure outside of the glass is greater than the air pressure inside of the glass, so the water stays inside.
The amount of air pressure will differ depending on the elevation level. Ask students about pressure on their ears when they dive to the bottom of a swimming pool. They should tell you that they feel more pressure the deeper they go. Likewise, the deeper you are in the sea of air, the more pressure you feel. Explain that there is more air pressure at sea level than there is high in the mountains. Use an atlas to compare elevations for different cities in North America, and have the students make a chart. At sea level, the average air pressure on a middle school student is 24,000 lbs. Ask students why they are not crushed by this air pressure. Explain that the air inside their bodies balances out the pressure outside their bodies.
For this lesson, you'll need a shallow dish, a candle, matches and a tall glass. Light the candle and stand it upright in the middle of the dish. If it falls to the side, secure it with candle drippings. Fill the dish half full with water. While the candle burns, place the tall glass over the top of the candle to cut off the candle's oxygen supply. As the candle continues to burn, the flame will consume any remaining oxygen until it goes out. When the flame goes out, the water will rise in the glass to balance out the pressure. If you are demonstrating this experiment in front of a class full of students, add food coloring to the water so they can better see the change in the water level.