A spinning science activity from Science Buddies

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!

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

Camille Bond and E&E News

Lungelo Ndhlovu, Kata Karath and Michael Forster Rothbart

Vanessa LoBue and The Conversation US

Lesley Clark and E&E News

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