Key Concepts
Air pressure, volume

Introduction

Have you ever been helping in the kitchen and found yourself with a recipe that calls for egg whites? Do you use the egg shell 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.

This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

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!

Although we can’t see them, air molecules still 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 super powers!

Materials

  • 2 water bottles
  • 1-2 eggs (second egg is backup!)
  • 2 plates or bowls (white works best)
  • Scissors
  • An adult helper

Tip: Don’t waste your egg! When this activity is complete, use the egg to make something delicious to share with your adult helper.  

Preparation

  1. Ask your adult helper to use the scissors to cut a small (~1/4inch) wide hole in the bottom of one of the water bottles. This will be Bottle 1. The water bottle with no hole in it will be Bottle 2.

Procedure

  1. 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.
  2. Hold Bottle 2 (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 inwards very slightly.
  3. Lower the bottle to the plate, so that it’s 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?
  4. With the egg yolk inside the bottle, carefully lift the bottle and move it to the 2nd 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?
  5. Repeat steps 2-4, 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!
  6. Put Bottle 2 aside, and pick up Bottle 1 (with hole in bottom).
  7. Hold Bottle 1 (over the plate with the opening of the bottle facing downward. Squeeze and hold the sides of the bottle so that they fold inwards very slightly.
  8. Lower the bottle to the plate, so that it’s 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 2? How would you explain this change?
  9. Place your finger firmly over the hole at the bottom of Bottle 1. While keeping the hole covered, try to pick up the yolk again by squeezing and releasing the sides of the bottle, as you did in steps 7 and 8. What happens to the egg yolk when you release the sides of the bottle? Is this different than 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. How cool! However, when you used a bottle that had a hole in the bottom, 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 it’s amazing ‘egg vacuum’ powers? The simple answer is – vacuums 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 on the bottom – 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!

However, when you cut a hole in the bottle, you made it possible for air to move in and out of the bottle, even when 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. Therefore, the air pressure inside the bottle was always the same as the air pressure outside the bottle, and there was no force to push the yolk. When you covered the hole again, you sealed the bottle, so it behaved as though there was no hole.

More to Explore

Credits

Megan Arnett, PhD, Science Buddies

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Key Concepts
Air pressure, volume
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