carbonic acid; a weak inorganic acid formed by dissolving 二氧化碳 èr​yǎng​huà​tàn in water, used in making soft drinks and chemicals

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

(chemistry) a group of atoms that behave as a single unit; a radical