Option 1: Nitrate coating on silver. Nitrate is a compound that typically does not result in black discoloration on silver. Therefore, this option is not the correct explanation for silverware turning black over time.

Option 2: Sulphide coating on silver. This is the correct explanation for silverware turning black. Sulphur compounds in the air react with the silver to form a black layer of silver sulfide on the surface. This process is known as tarnishing.

Option 3: Chloride coating on silver. Chloride compounds can react with silver, but they typically do not result in black discoloration. Instead, silver chloride tends to form as a white or grayish layer on the surface.

Option 4: Oxide coating on silver. While silver can react with oxygen to form silver oxide, this compound does not typically result in black discoloration on silver. Instead, silver oxide tends to be either colorless or slightly yellow.

In summary, the correct explanation for silverware turning black over time is the formation of a sulphide coating on the silver.