This experiment is a simple one, but it should make you think a bit. It might also change the way you think about egg shells.
You will need:
- a raw egg
- a clear glass
- hot water
- hot water
- food coloring
Be sure to use a clear glass, so you can see what is happening. Fill the glass about half full of hot water from your kitchen sink. We will put the egg into the water, so leave enough room so that the water does not overflow. Your main goal is to have enough hot water so that the egg will be completely under water.
Let the hot water sit for a few seconds, so that any air bubbles from your faucet can float to the top. Carefully place the egg into the hot water. Watch for a minute. What do you notice? You should see one or more thin streams of bubbles rising from the egg. The bubbles will continue for quite a long time. If you listen carefully, you may also hear a tiny squeaking sound that sounds almost like a baby chick.
What is happening?
Think about it, and when you think you know the answer, then continue.
If you have ever peeled a boiled egg, you know that there is an air bubble inside the shell. As we have seen in past experiments, when you heat air, it expands and takes up more space. The air bubble inside the egg is expanding as it is heated by the water. It is pushing outwards, and it has to go somewhere.
The streams of bubbles were coming from microscopic holes in the egg shell. Those holes are too small for the liquid part of the egg to flow through, but they let air get in, so the developing chick can breathe. As the air inside the shell expands, it flows out through the tiny holes. If the water is too hot, then the air will expand faster than it can flow through the tiny holes, and the shell will crack. That sometimes happens when you are boiling eggs. To prevent that, start with room temperature water and heat the water slowly after you add the eggs. That gives time for the air to expand slowly enough to keep from cracking the shell.
The air forcing its way through the tiny pores sometimes makes a tiny whistling or chirping sound. I tried several eggs, and it did not happen with all of them. The eggs might spoil if you put them back in the refrigerator, so only experiment with eggs that you are ready to eat.
Knowing that the tiny pores were large enough for air to pass through, I wondered if they were large enough for a bigger molecule. How would you test that idea?
Think about the new idea, and when you think you have an answer, then continue.
We want to know if the pores in an egg shell will let larger molecules such as water or food coloring get through. How can we test that? Think about how we got the air bubbles to come through. We caused the air inside the egg to expand, and let the pressure force it through the pores. What if we reversed that process, cooling the air inside the egg?
I filled a glass with cold water. I added a couple of drops of green food coloring and moved the egg from the hot water into the cold, green water. If the pores were large enough for the water to pass through, then as the air bubble inside the egg cools, the air inside will have a lower pressure. If molecules of water and food coloring can fit through the pores, then the higher pressure on the outside should force some of the colored water in through the pores.
I let the egg sit in the cold water for about 10 minutes. Then I cracked the egg. I was hoping to find green color inside the egg, but at first I was disappointed. Then I looked carefully at the inside of the shell. There were quite a few tiny, green dots, showing that the water and color had indeed been able to fit through the pore.
If you want to play with this idea some more, try using a hard boiled egg instead of a raw one. Would it work the same? Would the distribution of the escaping bubbles and the food coloring be the same?