Write the principal of the following refining methods:
(a) vapour phase refining
(b) chromatography.
OR
Write chemical equations involved to obtain:
(a) $\mathrm{Cu}$ from ${\mathrm{Cu}}_2\mathrm{S}$
(b) Ag from ${\left[\mathrm{Ag}{\left(\mathrm{CN}\right)}_2\right]}^{-}$complex