【Room】 , 【chamber】 , 【apartment】 all denote space in a building enclosed or set apart by a partition.
【Room】 is the word in ordinary use.
【Chamber】 is somewhat elevated; it is used chiefly of a private 【room】 , especially of a bedroom on an upper floor.
【Apartment】 is decreasingly frequent in the sense of a single 【room】 .