how2GASES Experiments 

Progression #3:  Air Pressure.   Because we are unable to feel the crushing weight of air against our bodies, it is difficult for children to grasp the idea that air exerts pressure. The balloon, beach ball and bubble experiments described previously begin to illuminate this effect (from the inside out), but the topic warrants further exploration.  A few simple experiments can be used to demonstrate air pressure.

Crush the Bottle. 

- Place a small amount of hot water inside a plastic water bottle (½ liter size).

- Cap the bottle and swirl the hot water around.

- Place the bottle either on its side or standing right side up in a shallow pan filled with very cold water.

The effect of air pressure on the bottle will take a few minutes. The "crushing" of the bottle is in part due to the temperature and pressure changes occurring inside the plastic bottle, relative to the outside air pressure. As the temperature drops inside the closed bottle, the pressure also decreases. The pressure inside the bottle becomes less than the air pressure on the outside of the bottle causing it to collapse.

Why aren't we crushed by air pressure? The pressure inside our bodies is approximately equal to the air pressure pushing against the outside of our bodies. This is the situation on Earth at or around sea level. Deep sea divers would be crushed by the pressure of the water without protective suits or a submersible vehicle. Similarly, astronauts in space must also wear protective gear because there is no air pressure in space. Nothing to keep the pressure inside our bodies from continuing to push outward!

Air Push, #1
Place a baby food jar on a plastic tray and fill it with water, overflowing it. Carefully, place a plastic or plastic-coated card on top of the jar and gently tap it down. Carefully lift the baby food jar without disturbing the card and quickly invert the setup. The card should stay in place.

Air Push, #2
Use a 5-6" inch long plastic tube that is open on both ends. (Flower stem supports, available from a florist, work well.) Dip the tube in a tall glass container to fill it partly. The water will fill the tube up to the level of the water in the glass. Place your hand (or finger) over the top opening. The water will remain inside the tube. Remove your hand and the water will flow out. In each case air is exerting pressure to hold the water or the card in place within these closed systems.

In-Class Activity. You can give your class an opportunity to experiment with the phenomenon by providing them with straws, water and a bucket or a deep bowl filled with water. They will be able to fill the straws by dipping them in the bucket and then placing their finger over the top of the straw. They can release the water by simply lifting their finger away from the top of the straw. You can use the straw-water drop to create a watercolor picture. Have your class dot, spot or make a series of lines using water-based markers on a white coffee filter. Have them release the water droplets over the "canvas" to spread the colors around.

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