national capital vs Mexico City

national capital and Mexico City both are nouns.

Word	Noun	Adjective	Verb	Adverb
national capital	Yes	No	No	No
Mexico City	Yes	No	No	No

national capital (noun)	Mexico City (noun)
the capital city of a nation	the capital and largest city of Mexico is a political and cultural and commercial and industrial center; one of the world's largest cities