open-chain vs chemical science

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

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

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

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