Question

A numismatist (coin collector) has been collecting gold coins, silver coins and copper coins for a long time. One day he observed a black coating on silver coins and green coating on copper coins. Which chemical process is responsible for these coatings?

• A. Osmosis
• B. Reverse Osmosis
• C. Erosion
• D. Corrosion

Answer 1

The correct option is D Corrosion

The coating occurred due to a chemical process called corrosion. Black coating is due to deposit of Silver Sulphide (Ag₂S). Green coating is due to formation of Copper Carbonate (CuCO₃).

$2Ag\left(s\right)$ + $H_{2}S\left(g\right)$ $\to$ ${Ag}_{2}S\left(s\right)$ + $H_2\left(g\right)$
(Note: Silver usually is corrosion resistant and does not oxidize. However presence of sulphur gases in atmosphere can react and cause sulphur-corrosion).