Final answer:
The black tarnish on silver is silver sulfide (Ag₂S), resulting from the reaction of silver with hydrogen sulfide gas.
Step-by-step explanation:
The black tarnish on silver is caused by a chemical reaction between silver and sulfur compounds in the air, particularly hydrogen sulfide gas (H₂S). When silver comes into contact with hydrogen sulfide, the silver reacts to form silver sulfide (Ag₂S), which is responsible for the dark tarnish that can be observed on silver objects.
This reaction can be represented by the equation 2 Ag(s) + H₂S(g) → Ag₂S(s) + H₂(g). Therefore, the correct answer to the question 'The black tarnish on silver is?' is 4) Ag₂S.