the chemical element hydrogen, a colorless, odorless, and tasteless gas that is the lightest of all known substances; usually referred to as 氢气 qīng​qì

(bound form) fluorine; the chemical element with atomic number 9, which is a highly reactive and toxic gas

(bound form) any of a class of organic chemical compounds composed only of carbon and hydrogen; also called 碳氢化合物 tàn​qīng​huà​hé​wù