Refining of Metals, Corrosion Prevention, and Alloys for CBSE Class 10

Extracting a metal from its ore yields a crude metal that still contains residual impurities. To make the metal suitable for industrial and commercial use—such as high-conductivity electrical copper or corrosion-resistant structural steel—the crude metal must undergo final refining. Furthermore, once pure metals are isolated, they must be protected against destructive chemical degradation (corrosion) or modified by alloying to enhance their mechanical strength.

In CBSE Class 10 Science, the concluding section of Chapter 3 (Metals and Non-Metals) connects laboratory metallurgy with industrial metallurgy and materials science.

What You Will Learn

• The principle, apparatus, and reactions of Electrolytic Refining (copper refining model)
• Nature and commercial significance of Anode Mud
• The conditions necessary for the corrosion of iron (rusting) (NCERT Activity 3.14)
• Corrosion prevention techniques: painting, greasing, galvanisation, and chrome plating
• Alloys: definition, preparation, and modified properties
• Compositions and uses of vital alloys: steel, stainless steel, brass, bronze, solder, amalgam, and 22-carat gold
• Board examination questions and common misconceptions

1. Electrolytic Refining of Metals

The most widely used method for refining crude, impure metals (such as Copper, Zinc, Tin, Nickel, Silver, and Gold) is electrolytic refining.

                           Electrolytic Refining of Copper
                                          |
                +-------------------------+-------------------------+
                |                                                   |
      At the Anode (+)                                    At the Cathode (-)
   Thick block of IMPURE copper                        Thin strip of PURE copper
                |                                                   |
 Cu(impure) → Cu²⁺ + 2e⁻ (dissolves)                 Cu²⁺ + 2e⁻ → Cu(pure) (deposits)
                \                                                   /
                 +------ [ Acidified CuSO4 Electrolyte ] ----------+
                                          |
                          Insoluble Impurities Settle As
                                   "ANODE MUD"

Apparatus and Setup:

1. Anode (Positive Electrode): A thick slab of crude, impure copper.
2. Cathode (Negative Electrode): A thin strip of pure copper.
3. Electrolyte: An aqueous solution of acidified copper sulphate ($\text{CuSO}_4 + \text{dilute } \text{H}_2\text{SO}_4$).

Chemical Mechanism:

When an electric current is passed through the electrolyte:

1. Pure copper from the impure anode dissolves into the electrolyte solution as $\text{Cu}^{2+}$ ions: $\text{At Anode: } \text{Cu}(s) \text{ [Impure]} \longrightarrow \text{Cu}^{2+}(aq) + 2e^-$
2. An equivalent amount of pure copper from the electrolyte deposits onto the cathode strip: $\text{At Cathode: } \text{Cu}^{2+}(aq) + 2e^- \longrightarrow \text{Cu}(s) \text{ [Pure]}$
3. Over time, the impure anode block becomes thinner, while the pure cathode strip grows substantially thicker.

What is Anode Mud?

• Soluble impurities in the crude metal pass into the solution.
• Insoluble impurities (often containing precious noble metals like gold, silver, and platinum) fall to the bottom of the electrolytic tank, directly below the anode.
• This settled deposit is called anode mud.
• In commercial metallurgy, recovering gold and silver from anode mud often pays for the entire operating cost of the refining plant!

2. Corrosion of Metals and Rusting Conditions

Corrosion is the gradual deterioration of metals caused by chemical reactions with atmospheric gases and moisture.

NCERT Activity 3.14: Conditions Necessary for Rusting of Iron

Three test tubes containing clean iron nails are set up under different conditions:

1. Test Tube A (Water and Air): Iron nails are partially submerged in tap water exposed to air.
   • Result: Nails rust heavily.
2. Test Tube B (Water only, No Air): Iron nails are placed in boiled distilled water (boiling drives out dissolved air) and covered with a layer of oil to seal out atmospheric oxygen.
   • Result: Nails do NOT rust.
3. Test Tube C (Dry Air, No Moisture): Iron nails are placed in a dry test tube with anhydrous calcium chloride ($\text{CaCl}_2$, a drying agent that absorbs all moisture).
   • Result: Nails do NOT rust.

Important: <u>Both air (oxygen) and water (moisture) are simultaneously required for the rusting of iron. In the absence of either component, rusting cannot take place.</u>

$\text{Rusting Reaction: } 4\text{Fe}(s) + 3\text{O}_2(g) + 2x\text{H}_2\text{O}(l) \longrightarrow 2\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}(s) \text{ [Hydrated Ferric Oxide / Rust]}$

3. Corrosion Prevention Techniques

1. Barrier Protection (Painting, Oiling, Greasing): Forms a physical barrier that prevents air and moisture from contacting the metallic surface.
2. Galvanisation (CBSE High-Frequency Topic):

   Galvanisation is a method of protecting iron and steel from rusting by coating them with a thin protective layer of zinc ($\text{Zn}$).

   • Why is Galvanisation superior? Even if the zinc coating is scratched or chipped, the iron remains protected from rusting because zinc is more electropositive than iron and undergoes sacrificial oxidation first!
3. Chrome Plating and Anodising: Electroplating with chromium or building a thick oxide coat on aluminium.
4. Alloying: Changing the chemical composition of the metal to make it naturally corrosion-resistant.

4. Alloys: Composition and Applications

Definition

An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal.

Why are Alloys Made?

Pure metals often have undesirable properties (pure iron is too soft, pure gold bends easily). Alloying improves hardness, tensile strength, resistance to corrosion, and adjusts melting points or electrical conductivity.

High-Yield Alloys in CBSE Board Exams:

1. Steel:
   • Composition: Iron ($\approx 99.95\%$) $+$ Carbon ($0.05\%$).
   • Properties: Pure iron is soft and stretches easily when hot. Adding just $0.05\%$ carbon makes it hard and strong.
2. Stainless Steel:
   • Composition: Iron $+$ Nickel ($8\%$) $+$ Chromium ($18\%$).
   • Properties: Hard, strong, and does not rust at all. Used for surgical instruments, cutlery, and kitchen utensils.
3. Brass:
   • Composition: Copper ($70\% - 80\%$) $+$ Zinc ($20\% - 30\%$).
   • Properties: Malleable, decorative gold-like appearance. Unlike pure copper, it is a poor conductor of electricity.
4. Bronze:
   • Composition: Copper ($90\%$) $+$ Tin ($10\%$).
   • Properties: Hard, ductile, corrosion-resistant. Used for medals, statues, coins, and bells.
5. Solder:
   • Composition: Lead ($50\%$) $+$ Tin ($50\%$).
   • Properties: Has an exceptionally low melting point. Used for welding/soldering electrical wires together.
6. Amalgam:
   • Definition: An alloy in which mercury is one of the constituent metals (e.g., sodium amalgam, silver-tin amalgam used in dental fillings).
7. 24-Carat vs. 22-Carat Gold:
   • 24-Carat Gold: Pure gold ($100\%$ gold). It is very soft and pliable, making it unsuitable for crafting durable jewellery.
   • 22-Carat Gold: $22$ parts of pure gold alloyed with $2$ parts of either copper or silver. This hardens the gold and makes it durable for intricate jewellery.

5. Summary and Examination Tips

Exam Tip: In questions asking about electrolytic refining, always draw a neat labeled diagram showing the thick impure anode, thin pure cathode, acidified copper sulphate electrolyte, and the anode mud at the bottom.

Common Mistake: Writing that bronze contains zinc. Brass contains Zinc ($\text{Cu} + \text{Zn}$), whereas Bronze contains Tin ($\text{Cu} + \text{Sn}$). Remember the mnemonic: Bronze has a 'Z', so it does NOT have Zinc!