Sulfur

"Hydrogen sulfide is the chemical compound with the formula H. S. It is a colorless chalcogen hydride gas with the characteristic foul odor of rotten eggs. It is very poisonous, corrosive, and flammable." (Wikipedia, 2020)

The British English spelling of this compound is hydrogen sulph ide, but this spelling is whack. Not even International Union of Pure and Applied Chemistry (IUPAC) recommends spelling it like those tea drinking, biscuit eaters.

"Old Faithful" in Yellowstone and molten volcanoes give off a huge amount of energy as well as large amounts of sulfur gas.

Pure sulfur gas and hydrogen sulfide are responsible for lots of stinky smells. Skunk spray smells because of sulfur, and so do rotten eggs. If something stinks in our classroom, it's probably because of sulfur.

"Gas" smells because of a small sulfur molecule

Mercaptan is the compound added to natural gas that gives it its unique and distinct smell. Without this sulfur compound, we would not be able to smell the odorless methane gas, and more preventable fires would cause a lot more damage.

Some cultures preserve eggs for long periods of time. These black "century eggs" or "millennium eggs" are a delicacy.

"After admiring the unique pattern, you can halve the egg with a knife to find a runny, gooey green egg yolk and a translucent brown, rubber-like white. I find the egg yolk mesmerising, as it’s surrounded by concentric rings of brown, dark blue, and green. By now you may have also caught a whiff of a slight odour that is a mixture of sulphur and ammonia. Here’s a good reason to pay good money for your century eggs, as some say the odour of good quality century eggs is less pungent." - (Travel Gluttons, 2020)

Is cooking an egg a chemical or physical process?

Cooking an egg is a chemical process, because old chemical bonds are broken when you heat up protein, and new chemical bonds form. Sulfur from the amino acid Cysteine bonds with sulfur from other nearby Cysteines, causing crosslinking. These linked S-S bonds (disulfide bonds) are also responsible for curly hair and the structure of feathers. This is why chicken eggs are so high in sulfur; it's a key building block to making feathers.

If boiling an egg is a chemical process, can you reverse the process and unboil an egg?

Well, sort of. Unfortunately, this would require some less appetizing chemical compounds (like urea, the chemical that gives urine its look and smell) and a super fast centrifuge to spin the proteins apart. It would probably cost more to reverse the cooking process than to start with a brand new egg. While most chemical processes are reversible, sometimes they require lots of energy and undoing the reaction would not be very practical.