Metallic silver tarnishes due to presence of sulfides (e.g., H2S) in the atmosphere. This results in a surface layer of silver sulfide, which appears brown to black depending on the extent of tarnish. Tarnish will dissolve in acidic solution as well, so really it should not matter if the spoon is tarnished, but it is easy enough to polish away with commercially available silver cloth. Often you can do this dry - rubbing action alone can remove the surface tarnish leaving metallic silver beneath.
Silver salts have variable solubility in water, so depending on what you use to dissolve your silver, you may find various precipitates.
Silverware and other silver items should be stamped with the exact alloy designation, so you should be able to look up the purity using online resources.