(a) Which of the following metals would give hydrogen when added to dilute hydrochloride acid (i) iron (ii) copper (iii) magnesium ?
(b) Explain why do surfaces of some metals acquire a dull appearance when exposed to air for a long time.

Explanation

(a) Both iron (Fe) and magnesium (Mg) will evolve hydrogen on reacting with dilute hydrochloric acid. These are active metals and are placed above hydrogen in the activity series. As copper is placed below hydrogen in the series, it will not evolve hydrogen.
(b) Surfaces of some metals acquire a dull appearance when exposed to air for a long time and they lose their lustre. This is due to the formation of layer of oxides, hydroxides, carbonates etc. on the surface. For example, surface of aluminium metal becomes dull white due to the formation of coating of aluminium oxide `(Al_(2)O_(3))`. Similarly, the surface of copper acquires a greenish colour since a layer of basic copper carbonate with the formula `Cu(OH)_(2)CuCO_(3)` is deposited on the surface.