Capitol Hill vs American capital

Capitol Hill and American capital both are nouns.

Word	Noun	Adjective	Verb	Adverb
Capitol Hill	Yes	No	No	No
American capital	Yes	No	No	No

Capitol Hill (noun)	American capital (noun)
a hill in Washington, D.C., where the Capitol Building sits and Congress meets	the capital of the United States in the District of Columbia and a tourist mecca; George Washington commissioned Charles L'Enfant to lay out the city in 1791