Why do eggs become hard on boiling?

Question: Why do eggs become hard on boiling?
- Keywords:
  egg
Asked by emmagrace to Dalya, Derek, Sarah, Tim, Tom on 19 Jun 2011.
- Dalya Soond answered on 17 Jun 2011:
  
  Because the proteins in the eggs denature (lose their 3D structure) and come out of solution, forming a solid.
- Sarah Thomas answered on 17 Jun 2011:
  
  When you heat an egg, the proteins gain energy and literally shake apart. Proteins are made of strings of amino acids joined together. The bonds between the amino-acid strings fall apart, causing the proteins to unfold (lose their 3D structure). As the temperature increases, the proteins gain enough energy to form new, stronger bonds 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. 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!
- Tim Millar answered on 17 Jun 2011:
  
  Changes to proteins. They become all jumbled up, thats why the egg whites go from clear to opaque.
- Derek McKay-Bukowski answered on 19 Jun 2011:
  
  Good answers, Team-Chromium!
- Tom Crick answered on 19 Jun 2011:
  
  Already answered well by Team Chromium — the heat denatures the protein in the egg, which changes its structure and texture.

Question: Why do eggs become hard on boiling?