poison gas vs chemical weapon

poison gas and chemical weapon both are nouns.

Word	Noun	Adjective	Verb	Adverb
poison gas	Yes	No	No	No
chemical weapon	Yes	No	No	No

As nouns, chemical weapon is a hypernym of poison gas; that is, chemical weapon is a word with a broader meaning than poison gas:

poison gas: a gas that is poisonous to breath or contact; used in chemical warfare
chemical weapon: chemical substances that can be delivered using munitions and dispersal devices to cause death or severe harm to people and animals and plants

Other hypernyms of poison gas include gas.

poison gas (noun)	chemical weapon (noun)
a gas that is poisonous to breath or contact; used in chemical warfare	chemical substances that can be delivered using munitions and dispersal devices to cause death or severe harm to people and animals and plants