Key concepts
Physics
Volume
Air pressure
Vacuum

Introduction
Have you ever been helping in the kitchen and found yourself with a recipe that calls for egg whites? Do you use the eggshell to separate the egg yolk from the egg white? Instead of using the shell to separate them, you can use science! In this activity you will use simple tools to explore how physics can help you do everyday tasks—such as separate egg yolks from egg whites.

Background
Although they’re invisible to us, our world is full of air molecules filling the air all around us. Even though you can’t see them, you can feel them moving if you fan your face with your hand. That gentle puff of air on your cheek is caused by air molecules!

Air molecules exert a pressure or force on everything in our world—including you! You can feel the pressure of those air molecules by filling your cheeks with air. Press against your cheeks—notice how firm they feel. Can you push gently against your cheeks and move the air from side to side in your mouth? It’s just air inside your mouth but those air molecules exert a force against the sides of your cheeks. The force of air molecules is important in this activity. Those air molecules will act on the air inside a bottle, giving it some pretty amazing kitchen superpowers!

Materials

• Two disposable water bottles
• One to two eggs (The second egg is a backup!)
• Two plates or bowls (White ones work best.)
• Scissors

Preparation

• Ask your adult helper to use the scissors to cut a small, about one quarter inch–wide hole in the bottom of one of the water bottles. This will be Bottle 2. The water bottle with no hole in it will be Bottle 1.

Procedure

• Gently crack your egg over the plate or bowl making sure to keep the yolk intact. If necessary, ask your adult helper to help with this step.
• Hold Bottle 1 (no hole) over the plate with the opening of the bottle facing downward. Squeeze and hold the sides of the bottle so that they fold inward very slightly.
• Lower the bottle to the plate so that it is gently touching the yolk. Continue to hold the bottle but release your squeezing so that the sides return to their normal position. What happens to the egg yolk when you release the sides of the bottle? What happens to the egg white?
• With the egg yolk inside the bottle, carefully lift the bottle and move it to the second bowl or plate. Place the mouth of the bottle directly over the surface of the bowl/plate and gently squeeze the sides of the bottle. What happens to the egg yolk when you squeeze the bottle?
• Repeat these steps moving the egg yolk back to the plate/bowl where it started. Make sure to do this gently so that your egg yolk stays intact!
• Put Bottle 1 aside and pick up Bottle 2 (with the hole in its bottom).
• Hold Bottle 2 over the plate with the opening of the bottle facing downward. Squeeze and hold the sides of the bottle so that they fold inward very slightly.
• Lower the bottle to the plate so that it is gently touching the yolk. Continue to hold the bottle but release your squeezing so that the sides return to their normal position. What happens to the egg yolk when you release the sides of the bottle? Is this different than what happened with Bottle 1? How would you explain this change?
• Place your finger firmly over the hole at the bottom of Bottle 2. While keeping the hole covered try to pick up the yolk again by squeezing and releasing the sides of the bottle, as you did before. What happens to the egg yolk when you release the sides of the bottle? Is this different from what happened when the hole was uncovered? How would you explain this change?

Observations and results
In this activity you used a plastic bottle to separate and move an egg yolk from one plate to another! When you used a bottle that had a hole in the bottom, however, you likely found it impossible to suck up the egg yolk. When you covered the hole with your finger, you could move the egg yolk again.

Why did adding a tiny hole in the bottle ruin its amazing egg vacuum powers? The simple answer is vacuum containers don’t work if they have holes in them! To understand this, we need to understand why the bottle was able to suck up the egg yolk in the first place.

You were able to use you your water bottle to pick up the egg yolk because of air pressure. Even though it looked empty, your water bottle was full of air. When you squeezed the bottle you pushed out some of that air. Putting the bottle on top of the egg yolk created a seal—just like a lid. No more air could come in or out of the bottle. When you released the sides of the bottle you created more space inside of it, but no more air could get in. Because there was less air inside the bottle but more space, the inside of the bottle had lower pressure than the outside. The higher air pressure on the outside pushed the yolk inside the bottle!

When you cut a hole in Bottle 2’s bottom, however, you made it possible for air to move in and out of the bottle even though the yolk was sealing the bottle top. As a result, when you released the sides of the bottle more air rushed in through the hole to fill that space. The air pressure inside the bottle therefore remained the same as the pressure outside of it, and there was no force to push the yolk. When you covered the hole, you sealed the bottle, so it behaved as though there was no hole and a pressure differential was created like that in Bottle 1.

Clean up
Be sure to wash your hands and anything else that comes into contact with raw egg thoroughly with soap and warm water. Use your egg (or eggs) to make something delicious to share with your adult helper.

More to explore
Fallen Arches: The Surprising Strength of Eggshells, from Science Buddies
Salty Science: Floating Eggs in Water, from Scientific American
A Really Long Straw, from Scientific American
Science Activities for All Ages!, from Science Budd

This activity brought to you in partnership with Science Buddies