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Air vs. Water

Objective

Learn that air is constantly exerting pressure on us, even if we don’t always notice it.

Materials

Small drinking glass, plastic bottle, barbecue skewer or awl, water, index card or cardstock.

Background

The air that surrounds us is called Earth’s atmosphere. It is made up of transparent gasses; about 78% nitrogen, 21% oxygen, 1% argon, and the rest is small amounts of other gasses. We can feel the air when the wind blows, or when we move rapidly. Air also contains solid particles of dust, soot, pollen, salt and even microorganisms. These particles are usually not visible to us unless they are present in large numbers, and when we see them as smoke or dust clouds.

Procedure

Demonstration 1

Poke two holes in the sides of a plastic bottle, using a barbecue skewer or similar sharp tool. Holding your fingers over the holes, fill the bottle completely with water and replace the top. Remove your fingers from the holes and observe that the water remains in the bottle.

Demonstration 2

Completely fill a small glass with water, leaving no space at all at the top. Slide an index card onto the top of the glass so that the opening is completely covered. Put your hand across the index card to hold it sealed against the glass and invert the glass. Remove your hand from the card, being careful not to disturb it. The water will remain in the glass.

Discussion/Extensions

Air pressure caused by the weight of the column of air from the top of our atmosphere to your classroom made the water to remain in the bottle and the glass. The atmosphere exerts a pressure of about 14.7 pounds per square inch at sea level (less at higher elevations).

  • What happens to the water in the bottle if you remove the top?
  • Is the air pressure at the top of the Rocky Mountains the same as it is on a Florida beach?


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