national capital vs Riyadh

national capital and Riyadh both are nouns.

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

national capital (noun)	Riyadh (noun)
the capital city of a nation	joint capital (with Mecca) of Saudi Arabia located in the central oasis; largest city in Saudi Arabia