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, 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`.
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