Middle school students may think of air as weightless, but air has mass, volume and density. In the general atmosphere on earth's surface, the air pressure is 14.7 psi (pounds per square inch). Air pressure changes in the upper atmosphere, becoming less dense and more sensitive to changes in air pressure. Think of what happens to your ears when riding in an airplane that is taking off or landing. The air pressure encountered by the moving plane is lower than the air pressure inside your ears, causing them to "pop."
Fill a 2-liter soda bottle half-full of water, put a long straw in the opening of the bottle and then use clay to form a seal around the straw. Students can blow into the straw, thus increasing the air pressure in the sealed bottle. This higher air pressure will push down on the water, causing it to be forced up and out of the straw. For another experiment, lay an empty water or soda bottle on its side. Wad up a piece of paper towel into a ball about half the size of the opening of the bottle. Gently set the paper towel ball into the opening of the bottle, but don't let it fall into the bottle. Students can see how the air pressure inside the bottle prevents the paper towel ball from being blown into the bottle.
Take a soda or water bottle and fill it all the way to the top with water. Make sure that there are no air bubbles at the top, and then place a small piece of paper over the opening. Carefully using the palm of your hand to hold the paper in place, flip the water bottle upside down. The water should remain in the bottle with the paper held in place by the air pressure from outside of the bottle. It is best if students do this experiment over the sink, as it may take a few attempts to be successful.
This experiment requires an empty 2-liter soda bottle, a launch system and a bicycle pump. First, students can make their rockets more aerodynamic by attaching tail fins and a nose cone to the soda bottle. These can be made out of paper or thin craft foam. To launch the rocket, a bicycle pump needs to be attached to a launch system, which can be very simple or more complex, using a variety of materials. A simple launch system uses a rubber stopper to seal the air into the bottle and a value through the stopper to pump more air into the bottle. The air pressure increases in the bottle and will eventually reach a point where the rocket will launch several feet into the air. A more complex launch system can be built with the help of an adult using PVC pipe and a bicycle pump. (See pbs.org/kcet/wiredscience/video/330-geekdad_soda_bottle_water_rocket.html).