Why do eggs turn hard after they're boiled?

Dairy & Protein

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by Mya Kagan (whyzz writer) >> more about the author

Have you ever helped crack an egg, or watched someone else do it? If so, then you’ve seen that what’s inside the egg is gooey and soupy -- an egg white (which is usually clear) and a squishy round yellow yolk in the middle. So why does an egg that has boiled in hot water form into something solid (even if a bit rubbery) that you can hold in your hand!?

The reason why eggs transform shape when they’re heated up (boiled in their shell or even just fried or scrambled in a pan!) has to do with what they’re made up of: Lots of proteins! Proteins are units that make up a large part of most living things, and it’s important for us to eat proteins as part of our diet.

When the proteins of a raw, liquid egg come into contact with heat, it causes them to change form! The reason they change is that the heat makes the individual units of protein link together differently into what are ultimately stronger chains. This leaves them cooked and ready for you to eat!

Egging on!

Have you ever come across an egg and been unsure if it was raw or hard-boiled? Here’s an experiment you can use to find out which is which!

Gather together one hard-boiled egg and one raw, uncooked egg still in its shell. Place both down on a flat table and give them a little spin!

Do you notice that both eggs don’t spin equally? The egg that spins easily is the hard-boiled one, and the one that resists spinning is the raw one! The reason for this is that the gooey, watery interior of the raw egg prevents it from spinning because its liquid-like insides are too busy swishing around! The hard-boiled egg spins easily because it’s solid all the way through, so everything moves at the same time when you spin it!