Why an egg goes hard when cooked.

When you boil an egg, the heat first breaks (unfolds) the proteins, and then allows the proteins to link to other proteins. As the proteins form these new, strong bonds, the water that surrounded each protein molecule when the egg was liquid is forced out. That’s why the egg turns hard. (Heat affects all of the proteins in an egg, so it’s the best way to cook an egg.)
Chemicals – You can also break the weak bonds between protein molecules with chemicals. According to an experiment done by the BBC Science Shack, if you put vodka on an egg white, the alcohol in it breaks weak hydrogen bonds in the protein. If you put vinegar on an egg white, the acid in it breaks ionic bonds. If you mix the two (vodka and vinegar), you break both types of bonds and quite effectively denature the protein. Lots of other chemicals will also break those weak bonds. This won’t work as well on the yolk, which, while also protein-rich, has lots of fats and other properties that make it more difficult for it to become hard.

You can do something similar when you whisk egg whites: By exerting mechanical energy in the whisking process, you cause the protein bonds to break, and subsequently re-connect. Once these new, strong bonds are formed, the egg stays in that state. The proteins have formed a network of strong, permanent cross-links. A cooked, chemically-altered or well-beaten egg will never go back to its original state.

When cooking eggs, use moderate heat. High heat causes the protein in eggs to become tough and rubbery. When you use high heat to boil an egg, it causes a chemical reaction between the yolk and the white that leaves a green film around the yolk. That film is iron sulfide, caused by iron in the yolk reacting with hydrogen sulfide in the white. It won’t hurt you to eat it, and the egg will taste the same, but it sure looks awful!

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Why an egg, will go hard when boiled