Density is related to Mass. Mass is related to Size. Size is related to Medium. Medium is a type of Substance. 
Describe is related to Broad. Broad is a different concept from Narrow. 
Density <--RelatedTo--> Mass <--RelatedTo--> Size <--RelatedTo--> Medium --IsA--> Substance 
Describe <--RelatedTo--> Broad <--DistinctFrom--> Narrow 