chemical smell.
the second biggest problem is cracked shells that leak yolk and white out into the cooking water. this is caused by too-rapid heating. all eggs contain an air pocket at the base (the wide, rather than the pointed, end). when the air pocket is heated, it expands. if it is heated too quickly, it more like explodes, cracking the shell.
so what is the best way to get a hard-boiled egg? the answer: steaming! 