Key Concepts
Physics
Chemistry
Solid
Fluid
Rotation
Introduction
Have you ever found an egg in your refrigerator and wondered if it was raw or cooked? Although eggs drastically change inside their shell when cooked, it is still remarkably difficult to distinguish a cooked egg from a raw one without cracking it open. In this activity, you will find out how physics can help you tell the difference!
Background
A bird egg contains a yolk enclosed in a membrane, which is surrounded by a clear fluid (egg white, or albumen)—all packaged together in a hard shell. The egg white primarily consists of proteins floating around in water. The yolk holds protein, some fat, water, and the majority of the vitamin and mineral content of the egg.
Before being cooked, the egg white and yolk are liquid; their particles are not stuck to the particles close to them but instead flow alongside and over one another. The yolk is separated from the white by a membrane, but the particles on either side of this membrane are free to move around.
When the egg gets heated up, these liquids undergo a chemical change. The proteins unravel and bind to one another, which results in a network of proteins that traps water. During this time, the yolk and egg white become gel-like flexible solids. When the egg cools, the bonds become more rigid, and the contents become solid. The shape of those contents and the distribution of their mass within the egg become fixed.
The shell of an egg is primarily made of calcium carbonate. It does not change when placed in boiling water. Although the shell has pores that allow air and other small particles to pass through, almost no exchange of matter occurs during the cooking process. This arrangement means that the mass of the egg does not significantly change.
So if you boil an egg in its shell, its outward appearance does not alter—and neither does its mass. How can you tell whether an egg is raw or cooked without cracking it open? Try this activity to find out!
Materials
- Six raw chicken eggs, similar in size and color (be sure to wash your hands well with soap and warm water after handling raw eggs)
- Saucepan
- Stove (use caution and ask an adult to help you use the stove and handle hot items in this activity)
- Water
- Timer
- Slotted spoon
- Bowl
- Pencil
- Sheet of paper
Preparation
- Pour cold water into your bowl.
- Place three eggs in the saucepan and add enough water so about half an inch of it covers the eggs. Have an adult help you put the saucepan on the stove and turn on the burner.
- Heat the water until it comes to a rapid boil and keep it boiling for seven minutes. How do you think the eggs are changing during this time?
- Turn off the burner.
- Have an adult help you use the slotted spoon to carefully take out each egg, one at a time, from the hot water and place it in the bowl of cold water while being careful to not crack it. Allow the eggs to cool completely.
- Use a pencil to make a small mark on the three eggs that are still raw. Keep the mark subtle because it will make it easier to test your ideas in an unbiased way.
- Store the raw eggs together with the cooked ones. Doing so ensures that all of the eggs will be at the same temperature when you start your tests.
Procedure
- Choose a raw egg and crack it open on a plate. How does the content of the raw egg look? (Be sure to wash your hands well after handling raw egg.)
- Repeat the first step with a cooked egg. How does the content of a cooked egg differ from that of a raw egg?
- The goal of this activity is to find a test that can identify whether an egg is cooked or raw without cracking the shell. What are your ideas?
- Choose one cooked egg and one raw egg from the four uncracked eggs that are left. Put the other two eggs aside for now.
- If you find a difference between the two eggs (besides your mark), note it on your sheet of paper.
- Look at the eggs, smell them and weigh them in your hands. Does one look different, smell different or seem heavier than the other?
- Gently tap your pencil against the cooked egg and listen. Then do the same with the raw egg. Can you hear a difference?
- Shake the eggs, one at a time, close to your ear. Can you hear which one is raw?
- Put one egg on its tip and spin it. Try it a few times before switching to the other egg. Does one spin more easily than the other?
- Perform any other test or look for any other distinguishing characteristics (besides your mark) that you can think of.
- Review your notes. Did you find differences? If so, do you think any of these differences appear because one of the eggs is cooked and the other is not? Why or why not?
- If you found one or more differences between the raw and cooked egg, perform tests to see if they also appear in your last pair of eggs. Try not to look at the little mark on the raw egg while doing the tests. Do the differences distinguish the raw egg from the cooked one in this pair, too?
- If you found a difference that held up for both pairs, do you think it can differentiate all cooked eggs from raw eggs? Why do you think the difference occurs?
Cleanup
Be sure to wash your hands with soap and warm water after handling raw eggs. If you stored and handled your eggs safely, you can eat your hard-boiled ones—and cook your raw ones.
Observations and Results
It was probably impossible to tell the raw eggs apart from the cooked ones without cracking the shell until you tried to spin the eggs on their tip. Even though it is difficult to spin a cooked egg, spinning a raw egg was probably much harder.
When you boil an egg, the inside becomes solid. This transformation does not, however, change the egg’s appearance from the outside, its odor or its sound.
But you can tell the difference between a cooked egg and a raw one by spinning them on their point: a cooked egg is easier to spin. Because the inside of a cooked egg is solid, the particles within it cannot move around relative to one another or the shell. So all of the inside particles move in unison, along with the shell. In a raw egg, however, the inside is still liquid. The particles that make up the liquid can slide and move around separately from one another and the shell. When you spin the shell of the raw egg, the liquid inside does not start spinning right away—it needs some time to “catch up,” and friction between the shell and the liquid slows down the spinning motion. Because it is easier to balance an egg on its tip by spinning it faster, cooked eggs are easier to balance than raw ones. It also helps that the inside of the cooked egg is less wobbly because it does not move around separately, and its center of mass is fixed.
More to Explore
The Physics of Bottle-Flipping, from Scientific American
Hula Hooping with a Rubber Band, from Science Buddies
Shaping Hard-Boiled Eggs, from Science Buddies
Soft-Boiled Science: Egg-cellently Cooked Eggs, from Scientific American
STEM Activities for Kids, from Science Buddies
This activity brought to you in partnership with Science Buddies
