UPSC CDS 2 2020 GK Paper

Concept:Silver tarnishes (turns black) due to formation of silver sulfide ($\mathrm{Ag}_2\mathrm{S}$) on its surface.Explanation:When silver is exposed to air, it reacts with traces of hydrogen sulfide ($\mathrm{H}_2\mathrm{S}$) present in the atmosphere.This reaction produces a black layer of silver sulfide ($\mathrm{Ag}_2\mathrm{S}$).The chemical equation is: $4\mathrm{Ag} + \mathrm{O}_2 + 2\mathrm{H}_2\mathrm{S} \rightarrow2\mathrm{Ag}_2\mathrm{S} + 2\mathrm{H}_2\mathrm{O}$.The black coating is not silver oxide; it is silver sulfide.This process is called silver tarnishing, a type of corrosion.Answer:$\mathrm{Ag}_2\mathrm{S}$ (silver sulfide)