Question

State giving reason for the change in appearance observed when each of the following metal is exposed to atmospheric air for some time:
(i) Silver,
(ii) Copper and
(iii) Iron

Answer 1

Silver: Silver tarnishes when exposed to air due to the formation of a layer of silver sulfide, which occurs due to a reaction with hydrogen sulfide present in the air: \[ 4\text{Ag} + 2\text{H}_2\text{S} + \text{O}_2 \rightarrow 2\text{Ag}_2\text{S} + 2\text{H}_2\text{O} \] This reaction causes the shiny metal to look dull and gray.
Copper: Copper develops a green patina when exposed to air for an extended period, especially in the presence of moisture and carbon dioxide, leading to the formation of copper carbonate: \[ 2\text{Cu} + \text{H}_2\text{O} + \text{CO}_2 + \text{O}_2 \rightarrow \text{Cu}_2(\text{CO}_3)(\text{OH})_2 \] This patina acts as a protective layer and gives copper roofs and statues their characteristic green appearance.
Iron: Iron rusts when exposed to moist air, forming iron(III) oxide-hydroxide, commonly known as rust: \[ 4\text{Fe} + 3\text{O}_2 + 6\text{H}_2\text{O} \rightarrow 4\text{Fe}(\text{OH})_3 \] This red-brown layer causes significant damage and weakening of iron structures.