Extracting Mercury and Copper by Heating | Class 10 Science

Quick answer: Metals low in the reactivity series, such as mercury and copper, can be obtained from their sulphide ores by heating alone. Cinnabar is roasted: $\mathrm{2HgS + 3O_2 \to 2HgO + 2SO_2}$, then $\mathrm{2HgO \to 2Hg + O_2}$. Copper glance is partly roasted to $\mathrm{Cu_2O}$, which then reacts with the remaining $\mathrm{Cu_2S}$: $\mathrm{2Cu_2O + Cu_2S \to 6Cu + SO_2}$. This is called auto-reduction (self-reduction).

Metals low in the reactivity series

Metals low in the series are not very reactive, so their oxides break down or are reduced easily on heating.

Extraction of mercury from cinnabar

Cinnabar is mercury(II) sulphide, $\mathrm{HgS}$.

 HgS (cinnabar)
   ↓ roast in air
 HgO + SO₂
   ↓ heat further
 Hg (liquid) + O₂

Step 1: Roasting. Cinnabar is heated strongly in air, forming mercury(II) oxide and sulphur dioxide:

$\mathrm{2HgS(s) + 3O_2(g) \xrightarrow{heat} 2HgO(s) + 2SO_2(g)}$

Step 2: Reduction by heat. On further heating, mercury(II) oxide decomposes to mercury:

$\mathrm{2HgO(s) \xrightarrow{heat} 2Hg(l) + O_2(g)}$

Key point: No carbon or other reducing agent is needed. Heat alone breaks down $\mathrm{HgO}$.

Extraction of copper from copper glance

Copper glance is copper(I) sulphide, $\mathrm{Cu_2S}$.

Step 1: Partial roasting. The ore is heated in air. Part of the $\mathrm{Cu_2S}$ is converted to copper(I) oxide:

$\mathrm{2Cu_2S(s) + 3O_2(g) \xrightarrow{heat} 2Cu_2O(s) + 2SO_2(g)}$

Step 2: Auto-reduction. The air supply is reduced, and the copper(I) oxide formed reacts with the remaining copper(I) sulphide:

$\mathrm{2Cu_2O(s) + Cu_2S(s) \xrightarrow{heat} 6Cu(s) + SO_2(g)}$

Why "auto-reduction"? The sulphide in the ore itself acts as the reducing agent. No external reducing agent such as carbon is added.

Balancing check: Left side: 4 Cu from $\mathrm{2Cu_2O}$ + 2 Cu from $\mathrm{Cu_2S}$ = 6 Cu; 2 O; 1 S. Right side: 6 Cu; 2 O in $\mathrm{SO_2}$; 1 S. ✓

Because the second step uses 1 $\mathrm{Cu_2S}$ for every 2 $\mathrm{Cu_2O}$, about two-thirds of the ore needs to be roasted in Step 1.

Blister copper and refining (extra)

The copper obtained is impure. As it solidifies, escaping $\mathrm{SO_2}$ gas leaves blister-like marks on the surface, so it is called blister copper. It is then purified by electrolytic refining, which NCERT describes: impure copper is made the anode, a thin strip of pure copper the cathode, and acidified copper sulphate solution the electrolyte.

Summary table

Exam tips

• Write all four equations balanced, with "heat" over the arrows.
• Name the ores correctly: cinnabar (HgS) and copper glance (Cu₂S).
• State clearly that these metals are obtained by heating alone, because they are low in the reactivity series.
• For copper, explain that $\mathrm{Cu_2S}$ itself acts as the reducing agent in the second step.

Common mistakes

• Writing $\mathrm{HgS + O_2 \to Hg + SO_2}$ as the only step, or not balancing the roasting equation.
• Writing $\mathrm{CuO}$ instead of $\mathrm{Cu_2O}$ in the copper extraction.
• Writing 4Cu instead of 6Cu in the auto-reduction equation.
• Saying carbon is used to reduce mercury or copper oxides in this process.

Frequently Asked Questions

How is mercury extracted from cinnabar?

Cinnabar (HgS) is roasted in air to form mercury(II) oxide, 2HgS + 3O₂ → 2HgO + 2SO₂. On further heating, the oxide decomposes: 2HgO → 2Hg + O₂.

What is auto-reduction?

Auto-reduction is the reduction of a metal oxide by the metal's own sulphide, without adding any external reducing agent. In copper extraction, 2Cu₂O + Cu₂S → 6Cu + SO₂.

How is copper obtained from copper glance?

Copper glance (Cu₂S) is partly roasted to Cu₂O, and the Cu₂O then reacts with the remaining Cu₂S on heating to give copper metal and sulphur dioxide.

Why can mercury and copper be extracted by heating alone?

They are low in the reactivity series, so their compounds break down or are reduced easily on heating without a reducing agent like carbon.

What is blister copper?

Blister copper is impure copper whose surface has blister-like marks left by escaping sulphur dioxide gas. It is purified by electrolytic refining.