potash alum vs double salt

potash alum and double salt both are nouns.

Word	Noun	Adjective	Verb	Adverb
potash alum	Yes	No	No	No
double salt	Yes	No	No	No

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

potash alum: a white crystalline double sulfate of aluminum: the potassium double sulfate of aluminum
double salt: a solution of two simple salts that forms a single substance on crystallization

Other hypernyms of potash alum include Al, aluminium, aluminum, atomic number 13.

potash alum (noun)	double salt (noun)
a white crystalline double sulfate of aluminum: the potassium double sulfate of aluminum	a solution of two simple salts that forms a single substance on crystallization