From Wikipedia, it states that:-
Washington, D.C., has been the capital of the United States since 1800. Eight other cities have served as the meeting place for the U.S. Congress and are therefore considered to have once been the capital of the United States.
In addition, each of the fifty U.S. states and the five principal territories of the United States maintains its own capital.
I have visited the white house in washington, the reason to call it as white house which is because of the war, canadian burn this house during the war, the whole building is damage by dark smoke. To recover it, the American paint it into white. After that everyone call it white house. It becomes as the president's office.
It is not open for public, I only can take the photo out of it. I cannot keep close to it because they have the security guys on duty for 24 hours a day.