Question

Give reason surface of some metal acquire dull appearence when expose to air for long time

Answer 1

this is because of the phenomena known as corrosion i.e. slow destruction of metals by the action of damp air ( CO2+ H2O) . Under this process metals acquire a thin layer of oxides,carbonates, sulphides on their surface making their appearance dull when exposed to air(damp air)for a long time.