Final answer:
Egg white is primarily composed of a protein called albumin, which changes from a liquid to a solid state when boiled due to the breaking of hydrogen bonds.
Step-by-step explanation:
The specific molecule that egg white is mostly made of is protein, specifically a protein known as albumin. When an egg is boiled, the albumin in the egg white undergoes a process where it changes from a compact, ordered structure into a highly disordered structure through the breaking of hydrogen bonds, an event that requires an input of energy. This disorganization causes the molecules to aggregate and forms a disorganized solid, which is an irreversible and spontaneous process at temperatures higher than 373 K (100°C).