Grades: 5 - 8
Purpose: The students will be able to witness air pressure and how it affects objects.
- a hard cooked egg
- paper towel
- small jar (opening large enough to let the egg barely pass through)
Time: 20 Minutes
Peel the egg just before the experiment. Place the egg over the jar opening to demonstrate that the mouth of the jar is too small for the egg to pass through. Remove the egg and drop a small piece of burning paper towel into the jar. Place the egg gently on the opening of the jar, small end first. The egg may wobble on top of the opening, then the egg will appear to be pulled into the jar. (An egg with a wet surface may enter the jar more readily than an egg with a dry surface. If necessary, immerse the peeled egg in water.
Ask students to explain why the egg went into the jar. Define air pressure: air pushes on all surfaces that it touches. This push is called air pressure. Allow students to explain how you could get the egg out of the jar, keeping in mind the first part of the experiment. Hold the jar upside down with the small end of the egg in the jar neck. Tilt the jar bottom down until there is a small opening between the neck of the jar and the egg. Blow hard into the jar, making a closed seal with your mouth. Before you remove your mouth, tilt the jar upside down (i.e. jar bottom up). The egg should drop out.
CAUTION: An adult should demonstrate this activity to students. Students should be reminded to stay a safe distance away from the burning match, burning paper towel, and hot jar.
Expected results and the reasons they occur: As the air in the jar is heated by the fire, it begins to expand. Some of the air escapes, causing the egg to wobble. When the fire is extinguished, the air begins to cool and contract, and the egg seals the bottle. There is then less air in the jar, causing unequal pressure to occur between the air in the jar and the air outside the jar. The greater air pressure on the outside pushes the egg into the jar. When the egg enters the jar, air pressure is equalized. Blowing into the jar creates greater pressure inside the jar, causing the egg to be pushed out of the jar.
Activities adapted from the Puget Sound Air Pollution Control Agency's "Clean Air Express," and from "Where's the Air," Wisconsin Department of Natural Resources.