national capital vs Leopoldville

national capital and Leopoldville both are nouns.

Word	Noun	Adjective	Verb	Adverb
national capital	Yes	No	No	No
Leopoldville	Yes	No	No	No

national capital (noun)	Leopoldville (noun)
the capital city of a nation	the capital of the Democratic Republic of the Congo on the Congo river opposite Brazzaville