Why is gold nitrate soluble?

Answer 1

The nitrate anion is a large, monovalent moiety. Gold is also a fairly large cation, so, although the charge is +1, the effective attraction over that distance is somewhat lessened. These two factors make it easy for a polar solvent (like water) to separate gold nitrate into its respective ionic species. For the record, I cannot think of a single non-soluble nitrate compound.

Answer 2

Gold nitrate is soluble in water because it forms a coordination complex with water molecules, where the oxygen atoms of water molecules coordinate with the gold ions, stabilizing them in solution. This coordination interaction disrupts the strong ionic bond between the gold and nitrate ions, allowing the compound to dissolve.