chemical science vs open-chain

chemical science is a noun but open-chain is not a noun.

Word	Noun	Adjective	Verb	Adverb
chemical science	Yes	No	No	No
open-chain	No	Yes	No	No

chemical science (noun)	open-chain (noun)
the science of matter; the branch of the natural sciences dealing with the composition of substances and their properties and reactions

chemical science (adjective)	open-chain (adjective)
	having an open chain structure