Answer: (4) Ag2S
When placed in the air, the silver object turns black because the silver element reacts with sulphur compounds such as H2S (hydrogen sulphide) present in the air. The above mentioned reaction can be expressed as,
Ag + H2S → Ag2S + H2
The phenomenon is known as corrosion. It is called silver tarnishing, particularly for silver. The black substance obtained is known as silver sulphide.