straight chain vs open chain

straight chain and open chain both are nouns.

Word	Noun	Adjective	Verb	Adverb
straight chain	Yes	No	No	No
open chain	Yes	No	No	No

As nouns, open chain is a hypernym of straight chain; that is, open chain is a word with a broader meaning than straight chain:

straight chain: an open chain of atoms with no side chains
open chain: a chain of atoms in a molecule whose ends are not joined to form a ring

straight chain (noun)	open chain (noun)
an open chain of atoms with no side chains	a chain of atoms in a molecule whose ends are not joined to form a ring