Final answer:
The long, white, needle-like crystals that formed are likely to be zinc carbonate, a compound originating from the zinc present in the brass nut, or they could be sodium carbonate formed by the reaction of sodium from the saltwater with carbonate. This experimental setup mimics aspects of a galvanic cell.
Step-by-step explanation:
The white, needle-like crystals that formed when the saltwater solution dried up in the container with the brass nut are likely to be zinc compounds.
As a component of brass, zinc can react under certain conditions to form various compounds, one of which is zinc hydroxide, which can further react with carbon dioxide in the air to form zinc carbonate, a white, crystalline compound.
The blue copper carbonate that also formed is indicative of the presence of copper from the brass nut reacting with carbonate from the solution or the surrounding air.
The experiment you conducted is similar to the setup of a galvanic cell, where a spontaneous redox reaction occurs between two different metals immersed in their respective solutions and connected by a salt bridge.
In this case, the absence of a strict salt bridge and distinct separation of solutions might have allowed for the formation of unintended byproducts, such as the white zinc carbonate crystals, alongside the expected formation of the blue copper carbonate.
The description of having 'a bunch of long, white, needle-like crystals' could also indicate the formation of a salt crystal, particularly if sodium from the saltwater reacts with carbonate to form sodium carbonate, which is known to crystallize in needle-like forms and could be what you observed.