Question

Explain why the surface of some metals acquire a dull appearance when exposed to air for a long time. support it with three examples

Answer 1

It is because they react with various gases present in the air like oxygen and carbon dioxide to form metal oxide and metal carbonate.

Answer 2

the surface of some metals acquire a dull appearance when exposed to air because they corrode.
eg 1: silver articles become black
eg 2 : copper reacts with moist CO2 and acquires a green coating
eg 3 : iron acquires a brown coating of rust