Give reason why copper is used to make hot water tanks but steel (an alloy of iron) is not. (b) Explain why, the surface of some metals acquires a dull appearance when exposed to air for a long time.

 Copper is used to make hot water tanks because it does not corrode easily but steel rust in the presence of water.

(b) The surface of some metals acquires a dull appearance when exposed to air for a long time because of the formation of an oxide layer on the surface of the metal.