Give reasons for the following :
(i) Zinc can displace copper from copper sulphate solution.
(ii) Silver articles become black after sometime when exposed to air.
(iii) A metal sulphide is converted to its oxide to extract the metal from sulphide ore.

Explanation

(i) Zinc is placed above copper in the activity series. Therefore, it can easily displace copper from copper sulphate solution
`Zn(s) + CuSO_(4)(aq) rarrZnSO_(4)(aq) + Cu(s)`
(ii) Air contains traces of hydrogen sulphide `(H_(2)S)`gas. Silver has a tendency to combine with `H_(2)S` to form silver sulphide `(Ag_(2)S)` which is black in colour. On account of this, silver articles become black after some time when kept exposed to air.
(iii) A metal sulphide is normally converted into oxide by heating with excess of air or oxygen. This process is called roasting. Actually, the oxide of metal which is formed can be easily reduced to the metallic form by reduction with carbon or some electropositive element.
`underset("Zinc sulphide")(2ZnS + 3O_(2)) overset("heat")rarr2ZnO + 2SO_(2)`
`ZnO + C overset("heat")rarrZn + CO`