Write the role of :
(i) NaCn in the extraction of gold from its ore.
(ii) Cryolite in the extraction of aluminium from pure alumina.
(iii) Co in the purification of Nickel.

Explanation

(i) Leaching of gold ore occurs in the presence of air from which the gold is obtained later by displaacement method.
`4Au(s)+8CN^(-)(aq)+2H_2O(aq)+O_2(g) to 4[Au(CN)_2]^(-)+4OH^(-)(aq)`
`2[Au(CN_2]^(-)Zn(s) to [Zn(CN_4]^(2-)(aq)+2Au(s)`
(ii) In order to decrease the melting point of mixture and to increase the conductivity of alumina `(Al_2O_3)`, cryolite is added in the metallurgy of Al.
(iii) Impure Nickel reacts with CO to form volatile tetracarbonyl nickel (o) which on heating forms pure nickel.