Cinnabar `(HgS)` and galena `(PbS)` on roasting often give their respective metals, but zinc blende `(ZnS)` does not. Give reason.

Explanation

On roasting all these sulphide ores are partly converted into their respective oxides . Since the oxides of mercury and lead are unstable while that of zinc is stable , therefore , oxides of mercury and lead bring about the reduction of their respective sulphides to the corresponding metals but zinc oxide does not reduce ZnS to Zn metal
`HgS + underset("(unstable)")(2 HgO) overset(Delta)(to) 3 Hg + SO_(2) ,`
`PbS + underset("(unstable)")(2PbO) overset(Delta)(to) 3 Pb + SO_(2)`
`ZnS + underset("(stable)") (2ZnO) to 3 Zn + SO_(2)`