Eggs have a lot of protein, and protein is a chain of amino acids. These amino-acid strings fold back on themselves. The proteins are held in place by weak bonds (non-covalent) between different parts of the amino-acid string. When you heat an egg, the proteins gain energy and literally shake apart the bonds between the parts of the amino-acid strings, causing the proteins to unfold. As the temperature increases, the proteins gain enough energy to form new, stronger bonds (covalent) with other protein molecules. When you boil an egg, the heat first breaks (unfolds) the proteins, and then allows the proteins to link to other proteins. That’s why the egg turns hard.
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