Objective: To demonstrate air pressure and its effects and influences on surrounding materials.
Standards: Structure of Earth System (atmospheric composition and properties at different elevations) and Structure and Properties of Matter (properties of solids, liquids, and gases).
Materials: large index card, glass or cup, and water.
(1) Fill the glass with water (you will have to adjust amount of water depending on size and shape of glass).
(2) Place the index card over the entire mouth of the glass.
(3) Ensure that no air bubbles enter the glass as you hold the index card tightly against it.
(4) Slowly turn the glass upside down over a sink or basin.
(5) Remove the hand holding the cardboard.
*** May want to practice this demo at home a few times before presenting in front of the class so that water does not dump everywhere. ***
Science Behind It: The index card remains in place and the water remains in the glass. This happens because the pressure of the air outside the glass is greater than the pressure of the water inside the glass. The air pressure therefore keeps or “holds” the water in the glass because the air pressure pushing upward against the index card is stronger than the pressure of the water in the glass pushing downward. This is why it is very important to prohibit air bubbles from entering the glass as you hold the card against it and turn the glass upside down. If bubbles enter the glass, the pressure will increase and become greater than the air pressure outside of the glass resulting in the water spilling. The air pressure may become increased in this way because if the seal is not perfect, air can easily enter the water and add pressure.