herbicide vs chemical substance

herbicide and chemical substance both are nouns.

Word	Noun	Adjective	Verb	Adverb
herbicide	Yes	No	No	No
chemical substance	Yes	No	No	No

As nouns, chemical substance is a hypernym of herbicide; that is, chemical substance is a word with a broader meaning than herbicide:

herbicide: a chemical agent that destroys plants or inhibits their growth
chemical substance: material produced by or used in a reaction involving changes in atoms or molecules

Other hypernyms of herbicide include chemical.

herbicide (noun)	chemical substance (noun)
a chemical agent that destroys plants or inhibits their growth	material produced by or used in a reaction involving changes in atoms or molecules