Their 'Periodic' Table is organized into their 5 Cardinal Elements: Air, Earth, Fire, Water and Metal
An Air Element is any element that naturally occurs as a Gas. Hydrogen in Pandemonia is classified as Air 2 (Pandemonians classify what we know as Oxygen as Air 1), Helium is Air 3. And so on.
Water, therefore, is any element that naturally occurs as a liquid. The Alchemists of Pandemonia, while knowing of Hydrogen, don't yet know that Water is made up of Hydrogen and Oxygen. Water is Water 1. Mercury is also a Water Element.
Earth, being somewhat obvious, is any non-metallic solid. That makes Metal even more obvious.