The correct option is Roast to HgO, then heat to Hg.
Explanation
The method used for extracting a metal from its ore depends on the metal's position in the reactivity series. Mercury (Hg) is located low in the reactivity series, meaning it is relatively unreactive. Consequently, its oxides are thermally unstable and can be reduced to the metal by heating alone, without the need for reducing agents like carbon.Step-by-Step Extraction Process:
- Ore Identification: The primary ore of mercury is Cinnabar, which is Mercury(II) sulfide (HgS).
- Step 1 (Roasting): Cinnabar is heated in the presence of excess air. During this process, the sulfide is oxidized to mercuric oxide (HgO), and sulfur is eliminated as sulfur dioxide.
Reaction: 2HgS + 3O2 → 2HgO + 2SO2 - Step 2 (Thermal Reduction): The mercuric oxide formed is unstable at high temperatures. Upon further heating, it decomposes (reduces) directly into liquid mercury and oxygen.
Reaction: 2HgO → 2Hg + O2
Analysis of Incorrect Options:
- Dissolve in water, then electrolyse: HgS is insoluble in water; therefore, it cannot be dissolved to form an electrolyte for electrolysis.
- Reduce with carbon directly: Carbon reduction is typically employed for metals in the middle of the reactivity series (e.g., Zinc, Iron). Since mercury oxide reduces simply by heating, using carbon is unnecessary.
- React with chlorine to form HgCl2: Chlorination is not the standard metallurgical process for extracting mercury from cinnabar.
Key Takeaway:
Metals low in the activity series (like Mercury) are extracted from their sulfide ores by roasting (heating in air) to form the oxide, followed by thermal decomposition of the oxide to obtain the metal.