Question

Why some metals become dull and lose their shine

Answer 1

It is the effect of air and moisture. Metals like iron, alluminium etc. reacts with moist air and get oxidised and they lose their shine. Parts of air like hydrogen sulphide, sulphur, carbonates reacts with metals like silver (reacts with hydrogen sulphide to form silver sulphide) and copper (reacts with sulphur to form copper sulphate) which causes dullness.