Chemical X is a Shop Relic that makes cost X cards act as though the player spent 2 more energy on the card then they actually did. For example, playing Whirlwind with 3 energy normally causes 5 damage 3 times, but with chemical X it would cause 5 damage 5 times. Chemical X also allows cost X cards to be played with 0 energy and still have an effect of 2.
This item is a reference to the animated series "The Powerpuff Girls". Its flavor text references the cartoon's opening scene, where the titular characters are created when a "chemical X" is accidentally added to a mixture of "sugar, spice, and everything nice".