Occurrence and Extraction of Metals for CBSE Class 10 Science

Most metals do not occur in nature in a pure, uncombined metallic state. Because of their natural tendency to react with atmospheric oxygen, moisture, carbon dioxide, and sulphur, metals are found in the Earth's crust as chemical compounds such as oxides, sulphides, carbonates, and halides. The science and technology of extracting pure metals from these naturally occurring minerals is called metallurgy.

In CBSE Class 10 Science, Chapter 3 (Metals and Non-Metals) explains the sequential metallurgical workflow used to isolate metals based on their position in the Reactivity Series.

What You Will Learn

• Definitions of minerals, ores, and gangue
• The multi-stage metallurgical workflow: Concentration $\to$ Extraction $\to$ Refining
• Extracting metals low in the reactivity series (cinnabar, copper glance) by heating alone
• Extracting metals in the middle of the reactivity series: Roasting vs. Calcination
• Reduction using carbon and displacement by reactive metals (The Thermite Process)
• Extracting highly reactive metals by electrolytic reduction
• High-yield board exam chemical equations and common mistakes

1. Minerals, Ores, and Gangue

Before extracting any metal, understanding geological raw materials is essential:

1. Minerals: Naturally occurring inorganic substances or compounds found in the Earth's crust are called minerals.
2. Ores: Minerals that contain a very high percentage of a particular metal, from which the metal can be extracted profitably and conveniently, are called ores.

   All ores are minerals, but not all minerals are ores.

3. Gangue: Ores mined from the earth are naturally contaminated with large amounts of unwanted earthly impurities such as soil, sand, clay, and rocky matter. These unwanted impurities are collectively known as gangue.

2. Overview of the Metallurgical Process

The steps involved in extracting a pure metal from its ore depend on its chemical reactivity:

                                  Crude Ore from Earth
                                           |
                                  Enrichment of Ore
                             (Removal of unwanted Gangue)
                                           |
       +-----------------------------------+-----------------------------------+
       |                                   |                                   |
Metals of High Reactivity           Metals of Medium Reactivity         Metals of Low Reactivity
(K, Na, Ca, Mg, Al)                 (Zn, Fe, Pb)                        (Hg, Cu)
       |                                   |                                   |
Electrolysis of Molten Chloride    Carbonate Ore       Sulphide Ore     Sulphide Ores
       |                                   |                   |               |
Pure Metal at Cathode                 Calcination          Roasting         Roasting
                                           |                   |               |
                                           +---------+---------+        Reduction to Metal
                                                     |                         |
                                                Oxide of Metal              Refining
                                                     |
                                            Reduction by Carbon
                                                     |
                                              Purification

3. Extracting Metals Low in the Reactivity Series

Metals at the bottom of the reactivity series (such as Mercury and Copper) are very unreactive. The oxides of these metals can be reduced to pure metallic form by heating alone.

1. Extraction of Mercury from Cinnabar ($ext{HgS}$)

• Cinnabar ($\text{HgS}$) is the primary sulphide ore of mercury.
• When heated strongly in air, it first oxidizes into mercuric oxide ($\text{HgO}$): $2\text{HgS}(s) + 3\text{O}_2(g) \xrightarrow{\quad \Delta \quad} 2\text{HgO}(s) + 2\text{SO}_2(g) \uparrow$
• On continued heating, mercuric oxide decomposes into liquid mercury metal: $2\text{HgO}(s) \xrightarrow{\quad \Delta \quad} 2\text{Hg}(l) + \text{O}_2(g) \uparrow$

2. Extraction of Copper from Copper Glance ($ext{Cu}_2 ext{S}$)

• Heating copper(I) sulphide ore in air converts a portion of it into copper(I) oxide: $2\text{Cu}_2\text{S}(s) + 3\text{O}_2(g) \xrightarrow{\quad \Delta \quad} 2\text{Cu}_2\text{O}(s) + 2\text{SO}_2(g) \uparrow$
• The supply of air is then stopped, and the remaining $\text{Cu}_2\text{S}$ reacts with $\text{Cu}_2\text{O}$ to produce pure copper: $2\text{Cu}_2\text{O}(s) + \text{Cu}_2\text{S}(s) \xrightarrow{\quad \Delta \quad} 6\text{Cu}(s) + \text{SO}_2(g) \uparrow$

4. Extracting Metals in the Middle of the Reactivity Series

Metals like zinc, iron, and lead are moderately reactive. They usually occur as sulphides or carbonates. Because it is much easier to obtain a metal from its oxide than from sulphides or carbonates, the ore must first be converted into an oxide.

Roasting vs. Calcination (CBSE Core Distinction)

Important: <u>Roasting applies to Sulphide ores (excess air); Calcination applies to Carbonate ores (limited/no air). Both processes yield the metal oxide!</u>

Reduction of the Metal Oxide:

Once the metal oxide is formed, it is reduced to free metal:

1. Using Carbon (Coke) as Reducing Agent: $\text{ZnO}(s) + \text{C}(s) \longrightarrow \text{Zn}(s) + \text{CO}(g)$
2. Using Highly Reactive Metals as Reducing Agents (Displacement): Highly reactive metals like aluminium, sodium, or calcium can displace less reactive metals from their oxides.
   • The Thermite Reaction (CBSE High-Frequency Question): The reaction of iron(III) oxide with aluminium powder is known as the Thermite reaction: $\text{Fe}_2\text{O}_3(s) + 2\text{Al}(s) \longrightarrow 2\text{Fe}(l) \text{ [Molten Iron]} + \text{Al}_2\text{O}_3(s) + \text{Tremendous Heat}$ Application: The reaction is so highly exothermic that the iron produced is in the molten liquid state. This molten iron is poured into gaps to weld cracked railway tracks and broken heavy machine parts on-site!

5. Extracting Metals High in the Reactivity Series

Metals at the top of the reactivity series—Sodium ($\text{Na}$), Potassium ($\text{K}$), Calcium ($\text{Ca}$), Magnesium ($\text{Mg}$), Aluminium ($\text{Al}$)—are extremely reactive.

Why Can Carbon Not Reduce Their Oxides?

These metals have a far higher chemical affinity for oxygen than carbon does. Therefore, heating oxides of sodium, magnesium, or aluminium with carbon cannot reduce them.

Electrolytic Reduction

These metals are extracted by the electrolysis of their molten chlorides or oxides:

• Extraction of Sodium from Molten $\text{NaCl}$: $2\text{NaCl}(l) \xrightarrow{\quad \text{Electric Current} \quad} 2\text{Na}(s) + \text{Cl}_2(g)$
  • At Cathode (Negative Electrode): Sodium ions gain electrons (reduction) to deposit pure sodium metal: $\text{Na}^+ + e^- \longrightarrow \text{Na}$
  • At Anode (Positive Electrode): Chloride ions lose electrons (oxidation) to evolve chlorine gas: $2\text{Cl}^- \longrightarrow \text{Cl}_2 + 2e^-$

6. Summary and Examination Tips

Exam Tip: In questions asking to distinguish between roasting and calcination, always write their defining conditions alongside balanced equations for $\text{ZnS}$ and $\text{ZnCO}_3$ for full 3-mark credit.

Common Mistake: Writing that sodium is obtained by electrolysis of "aqueous NaCl solution". Electrolysis of aqueous $\text{NaCl}$ produces sodium hydroxide and hydrogen gas (the Chlor-Alkali process), NOT sodium metal! To obtain sodium metal, the salt must be molten (fused)!