Black precipitate of $$\text{PbS}$$

$$\text{S}^{2-} + \text{Pb}^{2+} \to \text{PbS} \downarrow \text{ (Black)}$$

$$\text{Na}_2\text{S} + (\text{CH}_3\text{COO})_2\text{Pb} \xrightarrow{\text{CH}_3\text{COOH}} \text{PbS} \downarrow \text{ (Black)} + 2\text{CH}_3\text{COONa}$$

Concept: During the Lead Acetate Test for qualitative analysis of sulfur (in Sodium Fusion Extract / Lassaigne's Extract), sulfide ions ($$\text{S}^{2-}$$) react with lead ions ($$\text{Pb}^{2+}$$) from lead acetate under acidic conditions to form an insoluble salt, lead sulfide ($$\text{PbS}$$).