If earth was like a hard-boiled egg which part of earth would the egg shell represent and why

Earth's crust.

Explanation:

A boiled egg can be used to represent the layers of the earth. Because eggshell is the outermost layer of the egg, it can be compared to the earth's crust. Following, we can consider that the egg white is the earthly mantle and the egg yolk is the nucleus.

The Earth's crust is the surface layer of the earth, the outer manifestation of rocks and minerals. Another possible definition also for the earth's crust is to consider it the solid layer of the planet, although many geologists believe that the inner core portion is also solidified.

the answer is:

carbon dioxide