Why do eggs become hard on boiling?

  Egg contain 67 per cent proteins (in egg white) and 33 per cent fats and proteins (in egg yolk). Egg white protein is mostly albumin (ovalbumin and Conalbumin) All proteins have primary,secondary and tertiary structures. Tertiary structure of egg white protein is due to hydrophobic interactions and hydrogen bonding. They also contain Cysteine amino acid, which has sulfhydryl group (SH).

  These sulfhydryl groups form covalent disulfide bonds and hold 2 distant sections of proteins in close proximity. Disruption of the tertiary structure (or 3 dimensional structure) due to heat, chemicals or acidity is called "denaturation". When we heat an egg, the, heat breaks the intermolecular forces and the tertiary structure gets broken. The proteins unfold from their nature folded structure and precipitate forming a white solid mass. Courtesy : The Hindu