Potomac vs capital of the United States

Potomac and capital of the United States both are nouns.

Word	Noun	Adjective	Verb	Adverb
Potomac	Yes	No	No	No
capital of the United States	Yes	No	No	No

Potomac (noun)	capital of the United States (noun)
term sometimes used to refer to Washington, D.C.	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
a river in the east central United States; rises in West Virginia in the Appalachian Mountains and flows eastward, forming the boundary between Maryland and Virginia, to the Chesapeake Bay