Question

why the liquid in egg change into solid when it is heated

Answer 1

when an egg is boiled or heated, the heat causes to make a longer and stronger chain of protein molecules and becomes unfolded. Once these proteins become unfolded the protein molecules get trapped among each other and become hard. This is because of heat an egg becomes solid after boiling.

Answer 2

it is so due to denaturing of protien bonds in the egg and existing bond are broken to  form longer and stronger ones