What part of speech is congress?
The word congress is a noun. When referring to the American Congress it is a proper noun.
First amendment. Actually, in no place is Freedom of Speech described. It is mentioned in the 1st Amendment ("Congress shall make no law [...] abridging the freedom of speech..."), but is never defined or described. This is intentional, as Freedom of Speech is a concept that changes over time as to what activities it encompasses, and has no strict interpretation.