on-line vs computer science

on-line is not a noun while computer science is a noun.

Word	Noun	Adjective	Verb	Adverb
on-line	No	Yes	No	No
computer science	Yes	No	No	No

on-line (noun)	computer science (noun)
	the branch of engineering science that studies (with the aid of computers) computable processes and structures

on-line (adjective)	computer science (adjective)
connected to a computer network or accessible by computer
on a regular route of a railroad or bus or airline system
being in progress now