(a) 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.

(a) Copper is a less reactive metal compared to iron. It reacts very slowly with water. However, steel has less resistance to corrosion, even though it is an alloy of iron. Thus, copper, being very less reactive than steel, is used to make hot water tanks but steel (an alloy of iron) is not.

(b) Most of the metals are reactive towards air. They react with oxygen present in the air to form metal oxides. The metal oxide formation appears on the surface of the metals as a thin layer. This thin layer formation of metal oxide on the metallic surface causes the dull appearance of the metals. Thus, the surface of some metals acquires a dull appearance when exposed to air for a long time.