Write the principle of the following refining methods :
(a) vapour phase refining
(b) chromatography
OR
Write chemical equations involved to obtain :
(i) `Cu` from `Cu_(2) S`
(ii) `Ag` from `[Ag(CN)_(2) ]^(-)` complex

Explanation

(a) Vapour phase refining : In this method, the metal is converted into its volatile compound which is collected and decomposed to give pure metal.
The metal should form a volatile compound with an available reagent and the volatile compound should be easily decomposable.
(b) Chromatography : This method is based on the principle that different components of a mixture are adsorbed to different extent on an adsorbent. Different components of a mixture get separated gradually, as the moving phase moves.
OR
(i) `Cu` from `Cu_(2) S`
`2 Cu_(2) S + 3O_(2) to 2 Cu_(2) O+ 2SO_(2)`
`Cu_(2) O+ Cu to 2 Cu + CO`
(ii) `Ag` from `[Ag (CN)_(2)]^(-)` complex
`2[Ag(CN)_(2) ]^(-) + Zn to [Zn (CN)_(4)]^(2-) + 2 Ag`