The white of an egg consists mostly of a protein called ovalbumin. Under normal circumstances, ovalbumin is a sphere-shaped molecule, and the egg white is clear and runny. Why does the cooked egg appear solid and white?

Because ovalbumin is denaturated.

Explanation:

When a protein is heated, denaturation occurs. This is a process in which the protein loses its conformation and structure, so we can observe changes in color and viscosity. Before heating, egg white is clear, traslucent and runny. When heated, it turned opaque, in a solid state. This changes indicate that the protein ovalbumin has been denaturated, so it changes its structure and properties.