Chemical X is a mysterious substance. Nobody really knows what it is, but we do know it's highly dangerous and just breathing in the fumes can have a ridiculous number of deadly side effects. Accordingly, Chemical X is sold in just about every grocery store as a refreshing drink.
We're not sure what exactly Chemical X is made of. Some suggest it may be a radioactive isotope, while others believe it is some form of toxic residue. Po has attempted to market acid from the Teletubby Land Acid Lake as Chemical X, but it was instantly identified as a cheap knockoff and discontinued.
No one knows the history of Chemical X. All we know is that when Professor Utonium was creating the Powerpuff Girls using the blood of ponies, he accidentally knocks out a bottle of Chemical X which he invented.