Rochelle salts vs double salt

Rochelle salts and double salt both are nouns.

Word	Noun	Adjective	Verb	Adverb
Rochelle salts	Yes	No	No	No
double salt	Yes	No	No	No

As nouns, double salt is a hypernym of Rochelle salts; that is, double salt is a word with a broader meaning than Rochelle salts:

Rochelle salts: a double salt used in Seidlitz powder; acts as a cathartic
double salt: a solution of two simple salts that forms a single substance on crystallization

Rochelle salts (noun)	double salt (noun)
a double salt used in Seidlitz powder; acts as a cathartic	a solution of two simple salts that forms a single substance on crystallization