national capital vs capital

national capital and capital both are nouns.

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

As nouns, capital is a hypernym of national capital; that is, capital is a word with a broader meaning than national capital:

Other hypernyms of national capital include city, metropolis, urban center.

national capital (noun)	capital (noun)
the capital city of a nation	the upper part of a column that supports the entablature
	one of the large alphabetic characters used as the first letter in writing or printing proper names and sometimes for emphasis
	a seat of government
	a center that is associated more than any other with some activity or product
	wealth in the form of money or property owned by a person or business and human resources of economic value
	assets available for use in the production of further assets