Summary
Boiling an egg causes significant chemical and physical transformations. Heat denatures the proteins in both the albumen and yolk, triggering coagulation, which is responsible for the solid texture of a hard-boiled egg. As the egg heats, the contained air expands and may cause the shell to crack if not managed properly. Overcooking can lead to the production of hydrogen sulfide which, reacting with iron from the yolk, forms iron sulfide—the compound that gives overcooked yolks a greenish tint. Understanding these processes helps explain various cooking techniques and offers practical solutions to common problems such as cracked shells and undesirable color changes.