The aqueous solution of an unknown sodium salt gives the following...

The aqueous solution of an unknown sodium salt gives the following reactions. I. It decolourises a solution of iodine in potassium iodide. II. It gives white turbidity with dil. HCl solution. III. It gives a white precipitate with $\mathrm{AgNO}_3$ solution which changes colours and finally becomes black on standing. The unknown sodium salt is