Corrosion - Class 10 Science

Corrosion

Class 10 Science | Metals and Non-metals | CBSE + Foundation + Competitive

🧲 Corrosion / संक्षारण

1. What is Corrosion? / संक्षारण क्या है?

English:
Corrosion is the gradual destruction or deterioration of a metal due to chemical or electrochemical reactions with substances present in the environment, such as oxygen, moisture, acids or salts. Common examples include:

• Rusting of iron
• Green coating on copper
• Black coating on silver

हिन्दी:
पर्यावरण में उपस्थित ऑक्सीजन, नमी, अम्ल या लवण आदि के साथ रासायनिक अथवा विद्युत-रासायनिक अभिक्रिया के कारण धातु का धीरे-धीरे नष्ट या खराब होना संक्षारण (Corrosion) कहलाता है। Examples / उदाहरण:

• लोहे में जंग लगना
• तांबे पर हरी परत बनना
• चांदी पर काली परत बनना

2. Rusting of Iron / लोहे में जंग लगना

English:
The corrosion of iron is called rusting. When iron is exposed to both oxygen and moisture for a sufficiently long time, a reddish-brown coating called rust develops on its surface. Rust is mainly a hydrated form of iron(III) oxide and is commonly represented as:

Fe₂O₃ · xH₂O

Hindi:
लोहे के संक्षारण को जंग लगना (Rusting) कहते हैं। जब लोहा पर्याप्त समय तक ऑक्सीजन और नमी दोनों के संपर्क में रहता है, तो उसकी सतह पर लाल-भूरे रंग की परत बनती है, जिसे जंग कहते हैं। जंग मुख्यतः जलयोजित आयरन(III) ऑक्साइड का मिश्रण होती है।

3. Conditions Necessary for Rusting / जंग लगने के लिए आवश्यक परिस्थितियाँ

Rusting of iron requires mainly two conditions: 1. Oxygen / ऑक्सीजन
Iron must be exposed to oxygen from air.

2. Water or Moisture / जल या नमी
Moisture is necessary for the electrochemical processes involved in rust formation.

Therefore:

Iron + Oxygen + Water/Moisture → Rust

Important: Both oxygen and water/moisture are essential for ordinary rusting. Dry air alone or water without dissolved oxygen does not produce rusting in the usual way.

4. Electrochemical Nature of Rusting / जंग लगने की विद्युत-रासायनिक प्रकृति

Rusting is an electrochemical process. Small regions on the iron surface behave as anode and cathode. At anodic region: Iron loses electrons.

Fe → Fe²⁺ + 2e⁻

At cathodic region: Oxygen is reduced in the presence of water.

O₂ + 2H₂O + 4e⁻ → 4OH⁻

The Fe²⁺ ions can further undergo oxidation and form iron(III) compounds, which ultimately form hydrated iron(III) oxide, commonly called rust.

5. Formation of Rust / जंग बनने की प्रक्रिया

Step 1: Iron atoms lose electrons.

Fe → Fe²⁺ + 2e⁻

Step 2: Oxygen gets reduced in the presence of water.

O₂ + 2H₂O + 4e⁻ → 4OH⁻

Step 3: Fe²⁺ combines with hydroxide ions to form iron(II) hydroxide.

Fe²⁺ + 2OH⁻ → Fe(OH)₂

Step 4: Further oxidation produces iron(III) compounds, which eventually form hydrated iron(III) oxide.

Fe₂O₃ · xH₂O → Rust

6. Why is Rusting Harmful? / जंग लगना हानिकारक क्यों है?

Rust is porous and does not form a protective, tightly adhering layer on iron. Therefore, oxygen and moisture can continue reaching fresh iron underneath. As a result, corrosion can continue and gradually weaken the metal. Examples:

• Bridges may become weak.
• Vehicles may develop structural damage.
• Iron tools and machines may deteriorate.
• Pipelines and metal structures may be damaged.

7. Factors Affecting Rusting / जंग लगने को प्रभावित करने वाले कारक

Factor / कारक	Effect / प्रभाव
Moisture / नमी	Generally increases rusting.
Oxygen / ऑक्सीजन	Required for ordinary rusting.
Salt / लवण	Salt solution increases electrical conductivity and can accelerate corrosion.
Acidic conditions / अम्लीय वातावरण	Can accelerate corrosion of iron.

8. Corrosion of Copper and Silver / तांबा और चांदी का संक्षारण

Copper:
Copper develops a green coating when exposed to moist air containing carbon dioxide over a long period. The green layer mainly contains basic copper carbonate. Silver:
Silver develops a black coating due mainly to the formation of silver sulphide when exposed to sulphur-containing substances in air.

Copper → Green coating

Silver → Black coating

9. Corrosion vs Rusting / संक्षारण और जंग लगना

Corrosion	Rusting
General deterioration of metals.	Specific corrosion of iron.
Can occur in many metals.	Occurs in iron and iron-based materials.
May produce different corrosion products.	Produces hydrated iron oxides.

10. 30 MCQs / बहुविकल्पीय प्रश्न

Q1. What is corrosion?
संक्षारण क्या है?

A. Gradual deterioration of a metal due to environmental reactions

B. Melting of a metal

C. Cutting of a metal

D. Polishing of a metal

Answer: A

Q2. Corrosion of iron is commonly called:

A. Tarnishing

B. Rusting

C. Galvanising

D. Alloying

Answer: B

Q3. Which two conditions are essential for ordinary rusting?

A. Nitrogen and sunlight

B. Carbon dioxide and nitrogen

C. Oxygen and moisture

D. Hydrogen and sunlight

Answer: C

Q4. Rust is mainly:

A. Iron sulphide

B. Hydrated iron(III) oxide

C. Iron chloride

D. Pure iron oxide only

Answer: B

Q5. The commonly used representation of rust is:

A. FeCl₃

B. FeS

C. Fe₂O₃·xH₂O

D. FeCO₃

Answer: C

Q6. Which gas is necessary for ordinary rusting?

A. Oxygen

B. Hydrogen

C. Nitrogen

D. Helium

Answer: A

Q7. Which substance supplies moisture for rusting?

A. Water

B. Nitrogen

C. Carbon monoxide

D. Hydrogen

Answer: A

Q8. Rusting of iron is mainly:

A. A nuclear process

B. An electrochemical process

C. A physical process

D. A melting process

Answer: B

Q9. At an anodic region of iron during corrosion:

A. Iron gains electrons

B. Iron loses electrons

C. Oxygen disappears

D. Water freezes

Answer: B

Q10. Which reaction represents oxidation of iron?

A. Fe → Fe²⁺ + 2e⁻

B. Fe²⁺ + 2e⁻ → Fe

C. O₂ + 4e⁻ → 2O²⁻

D. Fe + H₂O → H₂

Answer: A

Q11. At the cathodic region, oxygen is generally:

A. Oxidised

B. Reduced

C. Sublimed

D. Evaporated

Answer: B

Q12. Which coating develops on iron during rusting?

A. Reddish-brown

B. Bright yellow

C. Pure white

D. Blue

Answer: A

Q13. Rust is generally:

A. Porous

B. Completely impermeable

C. Metallic

D. Liquid

Answer: A

Q14. Why can rusting continue under a rust layer?

A. Rust is porous

B. Rust is a pure metal

C. Rust prevents all contact with air

D. Rust is liquid

Answer: A

Q15. Which metal develops a green coating on prolonged exposure to moist air?

A. Copper

B. Sodium

C. Calcium

D. Aluminium

Answer: A

Q16. The green coating on copper mainly contains:

A. Basic copper carbonate

B. Copper sulphide only

C. Iron oxide

D. Sodium carbonate

Answer: A

Q17. Silver generally develops a:

A. Green coating

B. Black coating

C. Blue coating

D. Red coating

Answer: B

Q18. Black coating on silver is mainly associated with:

A. Silver sulphide

B. Iron oxide

C. Copper carbonate

D. Sodium oxide

Answer: A

Q19. Which condition generally accelerates corrosion?

A. Completely dry air

B. Moist environment

C. Vacuum

D. Absence of oxygen

Answer: B

Q20. Salt water can accelerate corrosion because it:

A. Reduces electrical conductivity

B. Increases ionic conductivity

C. Removes oxygen completely

D. Prevents electrochemical reactions

Answer: B

Q21. Which process occurs at the anodic region?

A. Oxidation

B. Reduction

C. Neutralisation

D. Precipitation only

Answer: A

Q22. Which process occurs at the cathodic region?

A. Oxidation

B. Reduction

C. Combustion

D. Sublimation

Answer: B

Q23. Which of the following is NOT normally necessary for ordinary rusting?

A. Oxygen

B. Moisture

C. Water

D. Sunlight

Answer: D

Q24. Which material is more likely to rust?

A. Iron exposed to moist air

B. Gold in dry air

C. Platinum in vacuum

D. Glass in dry air

Answer: A

Q25. The Fe²⁺ ions formed during corrosion can be further:

A. Oxidised

B. Sublimed

C. Frozen

D. Distilled

Answer: A

Q26. Which statement about rust is correct?

A. It is always protective

B. It is porous and allows corrosion to continue

C. It is pure iron

D. It stops all contact with moisture

Answer: B

Q27. Corrosion is:

A. Always beneficial

B. Generally harmful to metals

C. A method of alloying

D. A method of refining metals

Answer: B

Q28. Which equation represents oxygen reduction during rusting?

A. O₂ + 2H₂O + 4e⁻ → 4OH⁻

B. Fe → Fe²⁺ + 2e⁻

C. Cu → Cu²⁺ + 2e⁻

D. H₂ → 2H⁺

Answer: A

Q29. Which of these is an example of corrosion other than rusting?

A. Green coating on copper

B. Melting of ice

C. Boiling water

D. Dissolving sugar

Answer: A

Q30. The best description of rusting is:

A. Physical melting of iron

B. Electrochemical corrosion of iron in the presence of oxygen and moisture

C. Formation of an alloy

D. Purification of iron

Answer: B

11. 30 Subjective Questions with Answers / व्यक्तिपरक प्रश्न

1 Mark
Q1. What is corrosion?
संक्षारण क्या है?

Answer: Corrosion is the gradual deterioration of a metal due to chemical or electrochemical reactions with the environment.

1 Mark
Q2. What is rusting?

Answer: The corrosion of iron in the presence of oxygen and moisture is called rusting.

1 Mark
Q3. Name the two main conditions necessary for rusting.

Answer: Oxygen and moisture/water.

1 Mark
Q4. What is the colour of rust?

Answer: Reddish-brown.

1 Mark
Q5. Write the commonly used representation of rust.

Answer: Fe₂O₃·xH₂O.

2 Marks
Q6. What conditions are necessary for rusting of iron?

Answer: Rusting requires both oxygen from air and water/moisture.

2 Marks
Q7. Why does iron rust faster in moist air?

Answer: Moisture acts as an electrolyte and allows the electrochemical reactions involved in corrosion to occur more readily.

2 Marks
Q8. Why does rusting not occur readily in completely dry air?

Answer: Water/moisture is required for the electrochemical processes of ordinary rusting. Completely dry air lacks this necessary moisture.

2 Marks
Q9. What happens to iron at the anodic region during rusting?

Answer:
Iron loses electrons and forms Fe²⁺ ions:

Fe → Fe²⁺ + 2e⁻

2 Marks
Q10. Why is rusting considered an electrochemical process?

Answer: Different regions of the iron surface act as anode and cathode, where oxidation and reduction reactions occur respectively.

3 Marks
Q11. Explain rusting of iron.

Answer:
1. Iron reacts electrochemically in the presence of moisture and oxygen.
2. Iron loses electrons to form Fe²⁺ ions.
3. Oxygen is reduced in the presence of water.
4. Iron hydroxides and oxides are formed, eventually producing hydrated iron(III) oxide, called rust.

3 Marks
Q12. Write the anodic and cathodic reactions involved in rusting.

Anodic reaction:
Fe → Fe²⁺ + 2e⁻

Cathodic reaction:
O₂ + 2H₂O + 4e⁻ → 4OH⁻

3 Marks
Q13. Why is rusting faster in coastal areas?

Answer: Coastal air contains high moisture and dissolved salts. Salt solution increases ionic conductivity, which can accelerate electrochemical corrosion.

3 Marks
Q14. Differentiate between corrosion and rusting.

Corrosion	Rusting
General deterioration of metals.	Specific corrosion of iron.
Occurs in many metals.	Occurs in iron and iron-based materials.
Products vary with metal.	Produces hydrated iron oxides.

3 Marks
Q15. Why is rusting harmful to iron structures?

Answer: Rust is porous and corrosion continues underneath it. Continuous loss of iron weakens structures such as bridges, vehicles, pipelines and machines.

4 Marks
Q16. Explain the formation of rust step by step.

Answer:
1. Iron loses electrons:
Fe → Fe²⁺ + 2e⁻

2. Oxygen is reduced:
O₂ + 2H₂O + 4e⁻ → 4OH⁻

3. Fe²⁺ reacts with OH⁻ to form iron hydroxide.
4. Further oxidation produces iron(III) compounds.
5. Hydrated iron(III) oxide forms as rust.

4 Marks
Q17. Explain why both oxygen and water are required for rusting.

Answer: Oxygen participates in the cathodic reduction reaction, while water provides the medium necessary for ionic movement and electrochemical reactions. Hence ordinary rusting requires both oxygen and moisture.

4 Marks
Q18. What is the role of moisture in rusting?

Answer: Moisture provides a conducting medium for ions on the iron surface and permits the anodic and cathodic electrochemical reactions to proceed.

4 Marks
Q19. Why does salt water accelerate corrosion?

Answer: Salt dissolves into ions in water. These ions increase the electrical conductivity of the water film on the metal surface, facilitating electrochemical corrosion. Therefore iron often corrodes faster in salty environments.

4 Marks
Q20. Explain why rust is not a protective layer.

Answer: Rust is porous and loosely adherent. It allows water and oxygen to reach the underlying iron, so corrosion can continue rather than being stopped by the rust layer.

5 Marks
Q21. Explain rusting of iron with suitable chemical equations.

Answer:
At anodic regions:
Fe → Fe²⁺ + 2e⁻

At cathodic regions:
O₂ + 2H₂O + 4e⁻ → 4OH⁻

Then:
Fe²⁺ + 2OH⁻ → Fe(OH)₂

Further oxidation produces iron(III) compounds which ultimately form hydrated iron(III) oxide:
Fe₂O₃·xH₂O

This hydrated iron oxide is commonly called rust.

5 Marks
Q22. Explain the conditions required for rusting of iron.

Answer:
1. Iron must be exposed to oxygen.
2. Water or moisture must be present.
3. Moisture provides an electrolyte-like medium for ionic movement.
4. The anodic and cathodic reactions occur on different regions of the iron surface.
5. These reactions ultimately produce hydrated iron oxides.

5 Marks
Q23. Explain corrosion with three examples.

Answer:
Corrosion is the gradual deterioration of a metal due to reactions with the environment. Examples:
1. Iron develops reddish-brown rust.
2. Copper develops a green coating.
3. Silver develops a black coating.

5 Marks
Q24. Explain the electrochemical nature of rusting.

Answer:
Different regions of an iron surface act as anodic and cathodic areas. At the anode, iron is oxidised:
Fe → Fe²⁺ + 2e⁻

At the cathode, oxygen is reduced:
O₂ + 2H₂O + 4e⁻ → 4OH⁻

The resulting iron compounds undergo further reactions and ultimately form hydrated iron oxide, or rust.

5 Marks
Q25. Why does iron rust faster in salty and acidic environments?

Answer: Salt solutions increase electrical conductivity, while acidic conditions can facilitate electrochemical reactions and metal dissolution. Therefore corrosion of iron can be accelerated in such environments.

6 Marks
Q26. Describe the complete mechanism of rusting of iron.

Answer:
Step 1: Moisture forms a thin water layer on iron.

Step 2: Some regions of iron become anodic:
Fe → Fe²⁺ + 2e⁻

Step 3: Electrons travel through the metal to cathodic regions.

Step 4: Oxygen is reduced at cathodic regions:
O₂ + 2H₂O + 4e⁻ → 4OH⁻

Step 5: Iron ions react with hydroxide ions.

Step 6: Further oxidation forms iron(III) compounds.

Step 7: Hydrated iron(III) oxide forms as reddish-brown rust.

6 Marks
Q27. A student keeps three iron nails under different conditions. Predict the rusting.

Condition	Expected Result
Dry air	Little or no ordinary rusting
Boiled water covered with oil	Very little/no rusting because oxygen availability is greatly reduced
Water exposed to air	Rusting occurs

Conclusion: Oxygen and moisture together favour rusting.

6 Marks
Q28. Why does rusting continue even after a layer of rust has formed?

Answer:
Rust is porous and does not form a compact protective layer. Water and oxygen can pass through the pores and reach fresh iron underneath. Therefore new anodic and cathodic regions continue to develop, causing further corrosion.

6 Marks
Q29. Compare rusting of iron with corrosion of copper and silver.

Metal	Corrosion Product	Appearance
Iron	Hydrated iron oxides	Reddish-brown
Copper	Basic copper carbonate mainly	Green
Silver	Silver sulphide mainly	Black

6 Marks
Q30. Explain why corrosion is considered an important problem in daily life.

Answer:
Corrosion causes gradual loss of useful metals and weakens structures. It can damage bridges, vehicles, pipelines, machines, tools and buildings. Rusting of iron is particularly important because rust is porous and allows corrosion to continue. Corrosion also causes economic losses because damaged components require repair or replacement. Therefore prevention of corrosion is important for safety, durability and economy.

12. Quick Revision / त्वरित पुनरावृत्ति

Corrosion = Gradual deterioration of a metal

Rusting = Corrosion of Iron

Rusting requires → Oxygen + Moisture

Anode: Fe → Fe²⁺ + 2e⁻

Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻

Rust ≈ Fe₂O₃·xH₂O

Iron → Reddish-Brown Rust

Copper → Green Coating

Silver → Black Coating

⭐ CBSE Most Important:
याद रखें:

Rusting = Iron + Oxygen + Water/Moisture → Rust

और rusting में:
Anode → Oxidation
Cathode → Reduction

Electrolytic Refining - Class 10 Science

Electrolytic Refining

Class 10 Science | Metals and Non-metals | CBSE + Foundation + Competitive

⚡ Electrolytic Refining / विद्युत अपघटनी परिष्करण

1. What is Electrolytic Refining? / विद्युत अपघटनी परिष्करण क्या है?

English:
Electrolytic refining is a method used to obtain a very pure metal from an impure metal by using electrolysis. In this process, the impure metal is made the anode, a thin sheet of pure metal is made the cathode, and a suitable solution of the metal salt is used as the electrolyte. When electric current is passed through the electrolyte, metal from the impure anode dissolves into the solution and an equal amount of pure metal gets deposited on the cathode.

हिन्दी:
विद्युत अपघटनी परिष्करण वह विधि है जिसमें विद्युत अपघटन द्वारा अशुद्ध धातु से बहुत अधिक शुद्धता वाली धातु प्राप्त की जाती है। इस प्रक्रिया में अशुद्ध धातु को एनोड, शुद्ध धातु की पतली पट्टी को कैथोड तथा धातु के उपयुक्त लवण के विलयन को विद्युत अपघट्य (Electrolyte) बनाया जाता है। जब विद्युत धारा प्रवाहित की जाती है, तो अशुद्ध एनोड से धातु विलयन में चली जाती है और उतनी ही मात्रा में शुद्ध धातु कैथोड पर जमा होती है।

2. Electrolytic Refining of Copper / तांबे का विद्युत अपघटनी परिष्करण

English:
Copper obtained from extraction processes contains impurities such as iron, zinc, silver, gold and other substances. To obtain highly pure copper, electrolytic refining is carried out. The arrangement consists of:

• Anode: Impure copper plate
• Cathode: Thin sheet of pure copper
• Electrolyte: Acidified copper sulphate (CuSO₄) solution
• Power supply: Direct current (DC)

हिन्दी:
निष्कर्षण द्वारा प्राप्त तांबे में लोहे, जिंक, चांदी, सोना आदि की अशुद्धियाँ हो सकती हैं। अधिक शुद्ध तांबा प्राप्त करने के लिए विद्युत अपघटनी परिष्करण किया जाता है। इसमें:

• एनोड: अशुद्ध तांबे की प्लेट
• कैथोड: शुद्ध तांबे की पतली पट्टी
• विद्युत अपघट्य: अम्लीकृत CuSO₄ विलयन
• विद्युत धारा: Direct Current (DC)

3. Working of Copper Refining / तांबे के परिष्करण की कार्यविधि

जब विद्युत धारा प्रवाहित की जाती है, तो अशुद्ध तांबे के एनोड से Cu²⁺ आयन विलयन में जाते हैं। At the anode:

Cu → Cu²⁺ + 2e⁻

कैथोड पर Cu²⁺ आयन इलेक्ट्रॉन ग्रहण करके शुद्ध तांबे के रूप में जमा होते हैं। At the cathode:

Cu²⁺ + 2e⁻ → Cu

इस प्रकार एनोड धीरे-धीरे घुलता जाता है और कैथोड पर शुद्ध तांबे की परत मोटी होती जाती है। Thus: Impure copper anode becomes smaller, while the pure copper cathode becomes thicker.

4. Role of Anode, Cathode and Electrolyte / एनोड, कैथोड एवं विद्युत अपघट्य की भूमिका

Part / भाग	Role / भूमिका
Anode / एनोड	Impure copper is connected to the positive terminal. Copper atoms lose electrons and enter the electrolyte as Cu²⁺ ions.
Cathode / कैथोड	Pure copper sheet is connected to the negative terminal. Cu²⁺ ions gain electrons and deposit as pure copper.
Electrolyte / विद्युत अपघट्य	Acidified CuSO₄ solution provides Cu²⁺ ions and allows electric current to pass through the solution.

5. What Happens to Impurities? / अशुद्धियों का क्या होता है?

Electrolytic refining separates impurities from the metal. Less reactive impurities such as silver and gold do not dissolve easily. They settle at the bottom of the electrolytic tank as anode mud. More reactive impurities such as iron and zinc may dissolve into the electrolyte but generally do not get deposited on the cathode under the refining conditions.

Hindi:
कम क्रियाशील अशुद्धियाँ जैसे चांदी और सोना आसानी से विलयन में नहीं जातीं और नीचे बैठकर anode mud बनाती हैं। अधिक क्रियाशील अशुद्धियाँ जैसे आयरन और जिंक विलयन में जा सकती हैं, लेकिन सामान्यतः कैथोड पर जमा नहीं होतीं।

6. Important Equations / महत्वपूर्ण समीकरण

At Anode: Cu → Cu²⁺ + 2e⁻

At Cathode: Cu²⁺ + 2e⁻ → Cu

Overall: Copper is transferred from impure anode to pure cathode.

7. Flow of Copper During Refining / तांबे का प्रवाह

Impure Copper Anode → Cu²⁺ ions in Electrolyte → Pure Copper Cathode

Exam Point:
In electrolytic refining of copper:
Anode = Impure copper
Cathode = Pure copper
Electrolyte = Acidified CuSO₄ solution
Impurities like Ag and Au → Anode mud

8. 30 MCQs / बहुविकल्पीय प्रश्न

Q1. What is the main purpose of electrolytic refining?
विद्युत अपघटनी परिष्करण का मुख्य उद्देश्य क्या है?

A. To purify a metal

B. To convert metal into ore

C. To make an alloy

D. To produce non-metal

Answer: A — Electrolytic refining is used to obtain highly pure metal.

Q2. In copper refining, the anode is made of:
तांबे के परिष्करण में एनोड किसका बना होता है?

A. Pure copper

B. Impure copper

C. Zinc

D. Iron

Answer: B

Q3. The cathode in copper refining is:
तांबे के परिष्करण में कैथोड होता है:

A. Impure copper

B. Graphite

C. Thin sheet of pure copper

D. Zinc sheet

Answer: C

Q4. Which electrolyte is used in electrolytic refining of copper?

A. NaCl solution

B. Acidified CuSO₄ solution

C. NaOH solution

D. HCl only

Answer: B

Q5. At the anode, copper undergoes:

A. Reduction

B. Oxidation

C. Neutralisation

D. Combustion

Answer: B — Cu loses electrons at the anode.

Q6. Which reaction occurs at the cathode?

A. Cu → Cu²⁺ + 2e⁻

B. Cu²⁺ + 2e⁻ → Cu

C. Cu + 2e⁻ → Cu²⁺

D. Cu²⁺ → Cu³⁺

Answer: B

Q7. The anode is connected to the:

A. Negative terminal

B. Positive terminal

C. Neutral terminal

D. Earth terminal

Answer: B

Q8. The cathode is connected to the:

A. Positive terminal

B. Neutral terminal

C. Negative terminal

D. AC terminal only

Answer: C

Q9. Which impurities may form anode mud?

A. Gold and silver

B. Sodium and potassium

C. Calcium and magnesium

D. Hydrogen and oxygen

Answer: A

Q10. Copper ions in the electrolyte move towards the:

A. Anode

B. Cathode

C. Salt bridge

D. Container wall

Answer: B

Q11. During copper refining, the mass of the anode generally:

A. Increases

B. Remains unchanged

C. Decreases

D. Becomes zero instantly

Answer: C

Q12. During copper refining, the mass of the cathode:

A. Increases

B. Decreases

C. Remains zero

D. Always remains unchanged

Answer: A

Q13. Copper atoms at the anode lose:

A. Protons

B. Neutrons

C. Electrons

D. Nuclei

Answer: C

Q14. Cu²⁺ ions gain electrons at the:

A. Anode

B. Cathode

C. Electrolyte surface

D. Salt bridge

Answer: B

Q15. The process at the cathode is:

A. Oxidation

B. Reduction

C. Combustion

D. Sublimation

Answer: B

Q16. What is anode mud?

A. Pure copper deposited at cathode

B. Insoluble valuable impurities collected below the anode

C. Electrolyte

D. Copper sulphate crystals

Answer: B

Q17. Which metal is commonly refined electrolytically?

A. Copper

B. Sodium chloride

C. Sulphur

D. Carbon

Answer: A

Q18. Which ion is mainly responsible for deposition of copper?

A. Na⁺

B. Cl⁻

C. Cu²⁺

D. SO₄²⁻

Answer: C

Q19. Acidified CuSO₄ is used as:

A. Anode

B. Cathode

C. Electrolyte

D. Insulator

Answer: C

Q20. Which statement is correct?

A. Pure copper is used as anode

B. Impure copper is used as anode

C. Copper ions are oxidised at cathode

D. Gold dissolves completely into solution

Answer: B

Q21. The reaction Cu → Cu²⁺ + 2e⁻ represents:

A. Oxidation

B. Reduction

C. Neutralisation

D. Precipitation

Answer: A

Q22. The reaction Cu²⁺ + 2e⁻ → Cu represents:

A. Oxidation

B. Reduction

C. Combustion

D. Roasting

Answer: B

Q23. Which of the following is a valuable impurity recovered from anode mud?

A. Gold

B. Sodium

C. Magnesium

D. Aluminium

Answer: A

Q24. Electrolytic refining is based on:

A. Electrolysis

B. Filtration

C. Distillation

D. Evaporation only

Answer: A

Q25. What happens to Cu²⁺ ions at the cathode?

A. They lose electrons

B. They gain electrons

C. They become sulphate ions

D. They form oxygen

Answer: B

Q26. Which terminal is the cathode connected to in an electrolytic cell?

A. Positive

B. Negative

C. Both terminals

D. No terminal

Answer: B

Q27. Which process occurs at the anode during copper refining?

A. Deposition of pure copper

B. Dissolution of copper

C. Formation of sodium

D. Formation of chlorine only

Answer: B

Q28. Why is the cathode made of pure copper?

A. To collect pure copper

B. To dissolve impurities

C. To produce oxygen

D. To stop current

Answer: A

Q29. Which combination is correct?

A. Anode–pure copper; Cathode–impure copper

B. Anode–impure copper; Cathode–pure copper

C. Both electrodes–zinc

D. Both electrodes–gold

Answer: B

Q30. The purity of copper increases mainly because:

A. Impurities are deposited preferentially

B. Copper selectively transfers from anode to cathode

C. Electrolyte evaporates

D. Copper burns

Answer: B

9. 30 Subjective Questions with Answers / व्यक्तिपरक प्रश्न

1 Mark
Q1. What is electrolytic refining?
विद्युत अपघटनी परिष्करण क्या है?

Answer: It is the process of purifying an impure metal using electrolysis.
यह विद्युत अपघटन द्वारा अशुद्ध धातु को शुद्ध करने की प्रक्रिया है।

1 Mark
Q2. What is used as the anode in copper refining?

Answer: Impure copper is used as the anode.

1 Mark
Q3. What is used as the cathode in copper refining?

Answer: A thin sheet of pure copper.

1 Mark
Q4. Name the electrolyte used in copper refining.

Answer: Acidified copper sulphate (CuSO₄) solution.

1 Mark
Q5. What is anode mud?

Answer: Insoluble impurities such as silver and gold that settle below the anode are called anode mud.

2 Marks
Q6. State the roles of anode and cathode in copper refining.

Answer:
Anode: Impure copper dissolves and releases Cu²⁺ ions.
Cathode: Cu²⁺ ions gain electrons and deposit as pure copper.

2 Marks
Q7. Why is acidified CuSO₄ solution used as electrolyte?

Answer: It provides Cu²⁺ ions required for copper deposition and conducts electric current through the solution.

2 Marks
Q8. Write the electrode reactions during copper refining.

At anode:
Cu → Cu²⁺ + 2e⁻

At cathode:
Cu²⁺ + 2e⁻ → Cu

2 Marks
Q9. What happens to the mass of the anode and cathode?

Answer: The mass of the impure copper anode decreases, while the mass of the pure copper cathode increases.

2 Marks
Q10. What happens to gold and silver impurities?

Answer: They do not dissolve readily and settle at the bottom as anode mud.

3 Marks
Q11. Explain electrolytic refining of copper.

Answer:
1. Impure copper is used as anode.
2. Pure copper sheet is used as cathode.
3. Acidified CuSO₄ solution is used as electrolyte.
4. On passing current, copper dissolves from anode and deposits on cathode.

3 Marks
Q12. Explain the role of anode, cathode and electrolyte.

Answer:
Anode: Impure copper dissolves into the electrolyte.
Cathode: Pure copper is deposited on it.
Electrolyte: Acidified CuSO₄ provides Cu²⁺ ions and conducts current.

3 Marks
Q13. Differentiate between anode and cathode in copper refining.

Anode	Cathode
Impure copper	Pure copper
Connected to positive terminal	Connected to negative terminal
Copper dissolves	Copper deposits

3 Marks
Q14. Why does copper get deposited on the cathode?

Answer: Cu²⁺ ions in the electrolyte move towards the negatively charged cathode. They gain two electrons and are converted into copper atoms:

Cu²⁺ + 2e⁻ → Cu

3 Marks
Q15. What is the importance of anode mud?

Answer: Anode mud contains valuable less-reactive metals such as silver and gold. These metals can be recovered separately and have economic value.

4 Marks
Q16. Describe the complete electrolytic refining process of copper.

Answer:
1. A large impure copper plate is connected to the positive terminal.
2. A thin pure copper sheet is connected to the negative terminal.
3. Acidified CuSO₄ solution is used as electrolyte.
4. Copper atoms from anode form Cu²⁺ ions.
5. Cu²⁺ ions move towards cathode.
6. They gain electrons and form pure copper.
7. Insoluble impurities settle as anode mud.

4 Marks
Q17. Write the reactions occurring at the two electrodes.

Anode:
Cu → Cu²⁺ + 2e⁻

Cathode:
Cu²⁺ + 2e⁻ → Cu

Copper is therefore transferred from the impure anode to the pure cathode.

4 Marks
Q18. Explain the fate of impurities during electrolytic refining.

Answer: Less reactive impurities such as gold and silver settle as anode mud. More reactive impurities such as iron and zinc may dissolve in the electrolyte but generally remain in solution instead of depositing on the cathode.

4 Marks
Q19. Why is electrolytic refining better than simple physical separation for copper?

Answer: Electrolytic refining separates copper from chemically mixed impurities and produces very high-purity copper suitable for electrical applications.

4 Marks
Q20. Explain why the cathode becomes thicker during refining.

Answer: Cu²⁺ ions from the electrolyte gain electrons at the cathode:

Cu²⁺ + 2e⁻ → Cu

The newly formed copper atoms deposit on the cathode, increasing its thickness and mass.

5 Marks
Q21. Explain electrolytic refining of copper with a labelled arrangement.

Answer:
The arrangement consists of an impure copper anode, pure copper cathode and acidified CuSO₄ electrolyte. Connections:
Anode → Positive terminal
Cathode → Negative terminal Anode reaction:
Cu → Cu²⁺ + 2e⁻ Cathode reaction:
Cu²⁺ + 2e⁻ → Cu Thus pure copper gets deposited on the cathode and impurities are separated.

5 Marks
Q22. Explain the role of all three components used in copper electrolytic refining.

Anode: Impure copper plate. It supplies Cu²⁺ ions to the solution.

Cathode: Thin pure copper sheet. It receives Cu²⁺ ions and becomes coated with pure copper.

Electrolyte: Acidified CuSO₄ solution. It provides Cu²⁺ ions and permits ionic conduction.

5 Marks
Q23. Explain oxidation and reduction during electrolytic refining.

Answer:
At the anode, copper loses electrons:
Cu → Cu²⁺ + 2e⁻
This is oxidation. At the cathode, Cu²⁺ ions gain electrons:
Cu²⁺ + 2e⁻ → Cu
This is reduction. Therefore oxidation occurs at the anode and reduction occurs at the cathode.

5 Marks
Q24. How does electrolytic refining help in obtaining pure copper?

Answer: Copper from the impure anode dissolves as Cu²⁺ ions. These ions are selectively deposited as copper on the pure cathode. Less reactive impurities settle as anode mud, while many more reactive impurities remain in solution. Hence high-purity copper is obtained.

5 Marks
Q25. Give five important points about electrolytic refining of copper.

Answer:
1. Impure copper is the anode.
2. Pure copper is the cathode.
3. Acidified CuSO₄ is the electrolyte.
4. Copper dissolves from the anode and deposits on the cathode.
5. Gold and silver may be collected as anode mud.

6 Marks
Q26. Explain the complete mechanism of electrolytic refining of copper with equations.

Answer:
Step 1: Impure copper is made the anode.
Step 2: Pure copper sheet is made the cathode.
Step 3: Acidified CuSO₄ is used as electrolyte.
Step 4: At anode:
Cu → Cu²⁺ + 2e⁻
Step 5: Cu²⁺ ions move through the electrolyte towards the cathode.
Step 6: At cathode:
Cu²⁺ + 2e⁻ → Cu
Step 7: Pure copper accumulates on the cathode while impurities are separated.

6 Marks
Q27. Describe the electrolytic refining of copper and explain the fate of impurities.

Answer:
Impure copper is connected to the positive terminal and pure copper to the negative terminal. Acidified CuSO₄ is used as electrolyte. Copper from the anode oxidises to Cu²⁺ ions. These ions migrate towards the cathode and are reduced to copper atoms. Less reactive impurities such as silver and gold settle at the bottom as anode mud. More reactive impurities such as iron and zinc may remain in the electrolyte. Thus high-purity copper is obtained at the cathode.

6 Marks
Q28. A student says that the anode should be pure copper and cathode should be impure copper. Is this correct? Explain.

Answer:
No, the statement is incorrect. In electrolytic refining of copper:

Anode = Impure copper
Cathode = Pure copper Copper from the impure anode dissolves into the electrolyte as Cu²⁺ ions. These ions are deposited as pure copper on the cathode. Therefore the arrangement must be reversed from the student's statement.

6 Marks
Q29. Why are gold and silver recovered from anode mud during copper refining?

Answer:
Gold and silver are less reactive than copper. They do not readily oxidise and dissolve into the electrolyte during refining. Therefore they separate from the anode and settle at the bottom as anode mud. Since gold and silver are valuable metals, they can be recovered economically from the anode mud.

6 Marks
Q30. Compare electrolytic refining at the anode and cathode.

Feature	Anode	Cathode
Material	Impure copper	Pure copper
Terminal	Positive	Negative
Process	Oxidation	Reduction
Reaction	Cu → Cu²⁺ + 2e⁻	Cu²⁺ + 2e⁻ → Cu
Mass	Decreases	Increases

Conclusion: Copper is transferred from the impure anode to the pure cathode, producing highly pure copper.

10. Quick Revision / त्वरित पुनरावृत्ति

Electrolytic Refining = Purification of Metal by Electrolysis

Anode = Impure Copper = Positive

Cathode = Pure Copper = Negative

Electrolyte = Acidified CuSO₄

Anode: Cu → Cu²⁺ + 2e⁻

Cathode: Cu²⁺ + 2e⁻ → Cu

Ag + Au + other insoluble impurities → Anode Mud

⭐ CBSE Most Important:
Remember the sequence:

Impure Copper → Anode → Cu²⁺ → Electrolyte → Cathode → Pure Copper

Extraction Based on Reactivity - Class 10 Science

⚙️ Extraction Based on Reactivity

क्रियाशीलता के आधार पर धातुओं का निष्कर्षण

Class 10 Science • CBSE • Foundation • Competitive Level

⚙️ Extraction Based on Reactivity / क्रियाशीलता के आधार पर निष्कर्षण

1. Basic Concept / मूल अवधारणा

English: The method used to extract a metal from its ore depends mainly on the reactivity of the metal. More reactive metals form more stable compounds and therefore require stronger methods of extraction.

हिन्दी: किसी धातु को उसके अयस्क से निकालने की विधि मुख्यतः उसकी क्रियाशीलता पर निर्भर करती है। अधिक क्रियाशील धातुएँ अधिक स्थायी यौगिक बनाती हैं, इसलिए इनके निष्कर्षण के लिए अधिक शक्तिशाली विधियों की आवश्यकता होती है।

2. Reactivity Series / क्रियाशीलता श्रेणी

K > Na > Ca > Mg > Al
↓
Highly Reactive Metals

Zn > Fe > Pb
↓
Moderately Reactive Metals

H > Cu > Hg > Ag > Au
↓
Low Reactivity / Less Reactive Metals

3. Classification Based on Reactivity / क्रियाशीलता के आधार पर वर्गीकरण

Group	Examples	General Extraction Method
Highly reactive	K, Na, Ca, Mg, Al	Electrolytic reduction
Moderately reactive	Zn, Fe, Pb	Roasting/Calcination followed by reduction
Low-reactivity	Cu, Hg, Ag, Au	Heating or chemical reduction; some may occur native

4. Highly Reactive Metals / अत्यधिक क्रियाशील धातुएँ

English: Highly reactive metals such as potassium, sodium, calcium, magnesium and aluminium have a strong tendency to combine with oxygen and other elements. Their compounds are difficult to reduce using ordinary reducing agents such as carbon.

हिन्दी: potassium, sodium, calcium, magnesium तथा aluminium जैसी अत्यधिक क्रियाशील धातुओं की ऑक्सीजन और अन्य तत्वों के साथ संयोजन की प्रवृत्ति बहुत अधिक होती है। इनके यौगिकों का carbon जैसे सामान्य अपचायक द्वारा अपचयन कठिन होता है।

Main method / मुख्य विधि: Electrolytic Reduction / वैद्युत अपघटनी अपचयन

5. Electrolytic Reduction / वैद्युत अपघटनी अपचयन

English: In electrolytic reduction, electricity is passed through the molten ionic compound. Metal ions gain electrons at the cathode and form the metal.

हिन्दी: वैद्युत अपघटनी अपचयन में पिघले हुए आयनिक यौगिक में विद्युत धारा प्रवाहित की जाती है। धातु आयन cathode पर इलेक्ट्रॉन ग्रहण करके धातु बनाते हैं।

Na⁺ + e⁻ → Na

Mg²⁺ + 2e⁻ → Mg

6. Extraction of Aluminium / Aluminium का निष्कर्षण

English: Aluminium is highly reactive and cannot generally be extracted by reducing aluminium oxide with carbon. It is obtained by electrolytic reduction of molten alumina.

हिन्दी: Aluminium अत्यधिक क्रियाशील है और aluminium oxide का carbon द्वारा सामान्यतः अपचयन नहीं किया जा सकता। इसे molten alumina के electrolytic reduction द्वारा प्राप्त किया जाता है।

Al³⁺ + 3e⁻ → Al

7. Moderately Reactive Metals / मध्यम क्रियाशील धातुएँ

English: Metals such as zinc, iron and lead are moderately reactive. Their ores are first concentrated and generally converted into oxides by roasting or calcination. The metal oxide is then reduced using a suitable reducing agent such as carbon or carbon monoxide.

हिन्दी: zinc, iron और lead जैसी धातुएँ मध्यम क्रियाशील होती हैं। इनके अयस्कों को पहले सांद्रित किया जाता है और roasting या calcination द्वारा ऑक्साइड में बदला जाता है। इसके बाद धातु ऑक्साइड का carbon या carbon monoxide जैसे अपचायक द्वारा अपचयन किया जाता है।

8. Example: Extraction of Zinc / उदाहरण: जिंक का निष्कर्षण

Step 1: Roasting of zinc sulphide

2ZnS + 3O₂ → 2ZnO + 2SO₂

Step 2: Reduction of zinc oxide

ZnO + C → Zn + CO

Thus zinc is obtained by reducing its oxide.

9. Example: Extraction of Iron / उदाहरण: आयरन का निष्कर्षण

English: Iron is extracted from iron ores in a blast furnace. Iron oxide is reduced mainly by carbon monoxide.

Fe₂O₃ + 3CO → 2Fe + 3CO₂

हिन्दी: आयरन का निष्कर्षण blast furnace में किया जाता है। Iron oxide का मुख्यतः carbon monoxide द्वारा अपचयन होता है।

10. Low-Reactivity Metals / कम क्रियाशील धातुएँ

English: Metals such as copper, mercury, silver and gold are relatively less reactive. Their compounds are comparatively less stable and therefore can often be reduced more easily.

हिन्दी: copper, mercury, silver और gold जैसी धातुएँ अपेक्षाकृत कम क्रियाशील होती हैं। इनके यौगिक अपेक्षाकृत कम स्थायी होते हैं, इसलिए इनका अपचयन आसानी से किया जा सकता है।

11. Example: Extraction of Mercury / उदाहरण: पारे का निष्कर्षण

Mercury is obtained from cinnabar (HgS). The sulphide ore is heated in air.

2HgS + 3O₂ → 2HgO + 2SO₂

The mercury(II) oxide is then decomposed by heating.

2HgO → 2Hg + O₂

12. Example: Extraction of Copper / उदाहरण: कॉपर का निष्कर्षण

English: Copper is less reactive than zinc and iron. Copper sulphide ores undergo roasting and subsequent reduction/conversion steps to obtain copper.

हिन्दी: कॉपर, zinc और iron से कम क्रियाशील है। Copper sulphide ores को roasting तथा आगे की उपयुक्त प्रक्रियाओं द्वारा कॉपर में बदला जाता है।

13. Metals Found in Native State / मुक्त अवस्था में पाए जाने वाली धातुएँ

English: Very low-reactivity metals such as gold and platinum may occur in the Earth's crust in the free or native state.

हिन्दी: बहुत कम क्रियाशील धातुएँ जैसे gold और platinum पृथ्वी की पर्पटी में मुक्त या native state में पाई जा सकती हैं।

Low reactivity → less tendency to form compounds → possibility of occurrence in free state.

14. Why Carbon Cannot Reduce Highly Reactive Metals? / Carbon अत्यधिक क्रियाशील धातुओं का अपचयन क्यों नहीं कर सकता?

English: Highly reactive metals have a greater affinity for oxygen than carbon. Their oxides are therefore very stable and cannot generally be reduced by carbon.

हिन्दी: अत्यधिक क्रियाशील धातुओं की oxygen के प्रति affinity carbon से अधिक होती है। इसलिए इनके oxides बहुत स्थायी होते हैं और carbon द्वारा सामान्यतः उनका अपचयन नहीं किया जा सकता।

15. Quick Revision / त्वरित पुनरावृत्ति

Metal Group	Examples	Extraction Method
Highly Reactive	K, Na, Ca, Mg, Al	Electrolysis
Moderately Reactive	Zn, Fe, Pb	Roasting/Calcination → Reduction
Low Reactive	Cu, Hg, Ag	Heating / Chemical Reduction
Very Low Reactive	Au, Pt	May occur in native state

16. 30 MCQs / 30 बहुविकल्पीय प्रश्न

17. 30 Subjective Questions with Answers / 30 वर्णनात्मक प्रश्न