Question

why do surfaces of some metals acquire a dull appearance when exposed to Air for a long time?​

Answer 1

key point -"air reacts with metals"

oxygen forms metal oxide and CO2 forms metal carbonates.

it simply means exposure to air corrodes the metal.

Answer 2

Answer: Surfaces of some metals acquire a dull appearance when exposed to air for a long time and they lose their lustre. This is due to the formation of layer of oxides, hydroxides, carbonates and so on on the surface.

For example: Surface of aluminium metal becomes dull white due to the formation of coating of Aluminium oxide (Al2O3).

Similarly, the surface of copper acquires a greenish colour since a layer of basic copper carbonate with the formula Cu(OH)2.CuCO3 is deposited on the surface of copper metal.