Question

why some metal loses their shine and apperance dull ?

Answer 1

Some metals often lose their shine and appear dull, because of the action of air and moisture on them. Some metals as a result of oxidation with the atmospheric oxygen form oxides and by which their appearance becomes dull.

Answer 2

If Metals are kept exposed,they undergo air oxidation. This is called corrosion of metals.

Example: iron corrodes and reddish coloured iron oxide layer(rust) forms on it.

Copper combines withCO^2 to form greenish coloured copper carbonate. This is called pagination of copper.

Due to formation of such substances on the outer surface of metals, they lose their shine and appear dull

Hope this helps