ammonium alum vs double salt

ammonium alum and double salt both are nouns.

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

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

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

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

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