Carbon and Its Allotropes – NDA Chemistry PYQs

Solve NDA Chemistry previous-year questions on Carbon and Its Allotropes with detailed solutions and explanations.

Chapter-wise PYQs • Concept-based explanations • Exam insights

NDA 2026-I

Q. 1. Which one of the following forms of carbon is thermodynamically most stable?

(a) Coal

(b) Graphite

(d) Fullerene

Answer: (b) Graphite

Explanation: Graphite is thermodynamically the most stable allotrope of carbon at standard conditions. Diamond is metastable and theoretically converts to graphite (extremely slowly at room temperature). Coal is amorphous and less stable. Concept Tested: Thermodynamically most stable carbon allotrope – graphite (repeated)

Q. 2. Consider the following statements :

I. Soaps are sodium or potassium salts of long-chain carboxylic acids.

II. The properties of catenation and isomerism in the element carbon are attributed to existence of large number of organic compounds than inorganic compounds.

III. Carbon dioxide is acidic but carbon monoxide is neutral molecule.

Which of the statements given above are correct?

(a) I and II only

(b) II and III only

(d) I, II and III

Answer: (d) I, II and III

Explanation: Statement I: Correct – soaps are Na or K salts of long-chain fatty acids. Statement II: Correct – the immense variety of organic compounds is due to catenation (C-C bonds) and isomerism. Statement III: Correct – CO₂ is acidic (forms H₂CO₃ with water); CO is a neutral oxide. All three statements are correct. Concept Tested: Properties of carbon compounds – soaps, catenation, CO₂ acidity

NDA 2025-II

Q. 3. Which one of the following statements is NOT correct?

(a) Diamond is hard due to extended covalent bonding in it

(b) Fullerene is composed of five and six membered rings

(d) Graphite is used as a lubricant

Answer: (c) Diamond is a good conductor of electricity

Explanation: Diamond is a poor conductor of electricity – all its electrons are in covalent bonds and there are no free electrons. Options (a), (b) and (d) are all correct. Concept Tested: Diamond – poor conductor of electricity

NDA 2024-II

Q. 4. The elemental composition of diamond is

(a) Carbon and Hydrogen

(b) Carbon and Oxygen

(d) Pure Silicon

Answer: (c) Pure Carbon

Explanation: Diamond is composed entirely of carbon atoms arranged in a tetrahedral covalent network. It contains no hydrogen, oxygen or silicon. Concept Tested: Diamond composition – pure carbon

NDA 2024-I

Q. 5. Match List-I with List-II and select the correct answer using the code given below the Lists:

List-I (Allotrope of Carbon)

A. Graphite

B. Diamond

C. Fullerene

D. Graphene

List-II (Property)

1. Thinnest and strongest material

2. Hardest natural substance

3. Very light and strong material

4. Soft and slippery material

Code :

A B C D

(a) 4 2 3 1

(b) 4 3 2 1

(d) 1 2 3 4

(a) 4 2 3 1

(b) 4 3 2 1

(d) 1 2 3 4

Answer: (a) A-4, B-2, C-3, D-1

Explanation: A. Graphite: soft and slippery = 4. B. Diamond: hardest natural substance = 2. C. Fullerene: very light and strong material (cage-like, low density) = 3. D. Graphene: thinnest (single atom thick) and strongest = 1. Code (a): A-4, B-2, C-3, D-1. Concept Tested: Properties of carbon allotropes – graphite, diamond, fullerene, graphene

NDA 2023-II

Q. 6. Buckminster fullerene, which looks like a football, is an allotropic form of

(a) Phosphorus

(b) Sulfur

(d) Tin

Answer: (c) Carbon

Explanation: Buckminsterfullerene (C₆₀) is an allotrope of carbon. It consists of 60 carbon atoms arranged in a sphere with hexagonal and pentagonal faces, resembling a football (soccer ball). Concept Tested: Buckminsterfullerene C₆₀ – allotrope of carbon

Q. 7. Which among the following statements with respect to carbon is/are correct ?

1. Carbon forms the basis for all living organisms and many things we use

2. Carbon shows tetra-valency and the property of catenation

3. Carbon forms covalent bonds with itself and other elements

4. Carbon forms compounds containing triple and tetra bonds between carbon atoms

Select the correct answer using the code given below :

(a) 1 only

(b) 1 and 2 only

(d) 2 and 4

Answer: (c) 1, 2 and 3

Explanation: Statement 1: Correct – all organic life is carbon-based. Statement 2: Correct – carbon has a valency of 4 (tetravalency) and can form chains with other carbon atoms (catenation). Statement 3: Correct – carbon forms covalent bonds. Statement 4: Incorrect – carbon forms single, double and TRIPLE bonds, but not “tetra bonds.” Carbon-carbon quadruple bonds do not exist (maximum bond order between C atoms is triple, C≡C). Concept Tested: Properties of carbon – tetravalency, catenation, covalent bonding

NDA 2023-I

Q. 8. Which among the following statements is not correct with respect to allotropes of Carbon ?

(a) Graphite is a good conductor of electricity.

(b) Diamond is the hardest substance known.

(d) Next to diamond, graphite is the second hardest known substance.

Answer: (d) Next to diamond, graphite is the second hardest known substance.

Explanation: Graphite is soft (hardness ~1–2 on Mohs scale). It is NOT the second hardest substance. The second hardest substance is actually cubic boron nitride or corundum (Al₂O₃), not graphite. Concept Tested: Hardness of carbon allotropes – graphite is soft, not second hardest

NDA 2022-I

Q. 9. Which one of the following allotropes of carbon is isomorphous with crystalline silicon?

(a) Coke

(b) Diamond

(d) Coal

Answer: (b) Diamond

Explanation: Diamond and crystalline silicon have the same tetrahedral covalent network structure – both are isomorphous. In both, each atom is bonded to four others in a tetrahedral arrangement. This is why silicon’s structure is called a diamond-cubic structure. Concept Tested: Diamond and silicon – isomorphous crystal structures

Q. 10. Which one of the following statements is not correct?

(a) Most carbon compounds are good conductors of electricity.

(b) Bonding in organic compounds is covalent.

(d) Diamond is an allotrope of carbon.

Answer: (a) Most carbon compounds are good conductors of electricity.

Explanation: Most organic/carbon compounds are covalent and are poor conductors of electricity (non-electrolytes). The exception is graphite. So (a) is NOT correct. (b), (c), (d) are all correct. Concept Tested: Carbon compounds – mostly poor conductors of electricity

NDA 2021-I

Q. 11. Which one of the following materials is not an allotrope of carbon?

(a) Diamond

(b) Graphene

(d) Fullerene

Answer: (c) Fly ash

Explanation: Fly ash is the fine ash produced from burning coal – it is a mixture of various compounds and is NOT an allotrope of carbon. Diamond, graphene and fullerene are all recognised allotropes of carbon. Concept Tested: Non-allotrope of carbon – fly ash

NDA 2019-II

Q. 12. Which one of the following statements about diamond and graphite is not correct?

(a) Diamond has a tetrahedral structure, whereas graphite has a hexagonal planar structure.

(b) Both physical and chemical properties of diamond and graphite are different.

(d) Graphite is a good conductor of electricity while diamond is not.

Answer: (b) Both physical and chemical properties of diamond and graphite are different.

Explanation: Diamond and graphite are both allotropes of carbon – they have the same chemical composition and same chemical properties (both burn in O₂ to give CO₂). Only their physical properties differ. So (b) is NOT correct. Concept Tested: Allotropes share chemical properties – diamond and graphite

NDA 2018-II

Q. 13. Which one of the following statements is not correct?

(a) All carbons in diamond are linked by carbon-carbon single bond.

(b) Graphite is a layered structure in which layers are held together by weak van der Waals forces.

(d) Graphite layers are held together by carbon-carbon single bond.

Answer: (d) Graphite layers are held together by carbon-carbon single bond.

Explanation: Graphite layers are held together by weak van der Waals forces, NOT by covalent C-C bonds. Within each layer, carbons are bonded by covalent bonds. Options (a), (b) and (c) are all correct. Concept Tested: Graphite interlayer forces – van der Waals, not covalent bonds

NDA 2018-I

Q. 14. How is carbon black obtained?

(a) By heating wood at high temperature in absence of air

(b) By heating coal at high temperature in absence of air

(d) By heating coal at high temperature in presence of air

Answer: (c) By burning hydrocarbons in a limited supply of air

Explanation: Carbon black is produced by incomplete combustion (burning hydrocarbons in a limited supply of air). It is used in tyres, inks and pigments. Option (a) gives charcoal. Option (b) gives coke. Concept Tested: Carbon black – obtained by incomplete combustion of hydrocarbons

Q. 15. Which one of the following properties is NOT true for graphite?

(a) Hybridisation of each carbon atom is sp³

(b) Hybridisation of each carbon atom is sp²

(d) Each layer is composed of hexagonal rings

Answer: (a) Hybridisation of each carbon atom is sp³

Explanation: In graphite, each carbon is sp² hybridised, not sp³. sp³ hybridisation belongs to diamond (tetrahedral bonding). Options (b), (c) and (d) are all correct for graphite. Concept Tested: Graphite structure – sp² hybridisation, not sp³

Q. 16. Which one of the following is the purest form of Carbon?

(a) Charcoal

(b) Coke

(d) Carbon black

Answer: (c) Fullerene

Explanation: Among these options, fullerene (C₆₀) is the purest form of carbon – it is made entirely of carbon atoms in a perfectly defined molecular structure. Charcoal, coke and carbon black are amorphous and contain impurities. Concept Tested: Purest form of carbon from options – fullerene

Q. 17. The Poisonous nature of Carbon monoxide (CO) is due to its

(a) insolubility in water

(b) ability to form a complex with haemoglobin

(d) property of having one sigma bond

Answer: (b) ability to form a complex with haemoglobin

Explanation: CO is toxic because it binds strongly to haemoglobin to form carboxyhaemoglobin, preventing haemoglobin from carrying oxygen. CO has ~250 times greater affinity for haemoglobin than O₂, causing oxygen deprivation. Concept Tested: Toxicity of CO – binds to haemoglobin forming carboxyhaemoglobin

★ JOVIK Exam Insight: CO is poisonous because it occupies the oxygen-binding site on haemoglobin. CO₂ is NOT poisonous – it is CO.

NDA 2017-I

Q. 18. Which one of the following elements forms highest number of compounds?

(a) Oxygen

(b) Hydrogen

(d) Carbon

Answer: (d) Carbon

Explanation: Carbon forms the highest number of compounds of any element because of its ability to form strong C-C bonds (catenation) and four covalent bonds (tetravalency). This is the basis of the entire field of organic chemistry. Concept Tested: Carbon – forms the most compounds due to catenation and tetravalency

Q. 19. Which one of the following elements is used in pencil-lead?

(a) Zinc

(b) Lead

(d) Tin

Answer: (c) Carbon (Graphite)

Explanation: “Pencil lead” is a misnomer – it contains no lead. It is made of graphite mixed with clay. Graphite is the allotrope of carbon used because it leaves marks on paper when rubbed. Concept Tested: Pencil lead = graphite (carbon), not lead metal

NDA 2015-II

Q. 20. Graphite is a much better conductor of heat and electricity than diamond. This is due to the fact that each carbon atom in graphite:

(a) undergoes sp² hybridization and forms three sigma bonds with three neighbouring carbon atoms

(b) undergoes sp³ hybridization

(d) is free from van der Waals force

Answer: (a) undergoes sp² hybridization and forms three sigma bonds with three neighbouring carbon atoms

Explanation: In graphite, each carbon is sp² hybridised, bonding to three other carbons in a hexagonal layer. The fourth electron (unhybridised p orbital) forms a delocalised π system across each layer, allowing electrical conduction. Diamond is sp³ hybridised with all electrons in bonds – no free electrons for conduction. Concept Tested: Graphite conductivity – sp² hybridisation and delocalised electrons

NDA 2015-I

Q. 21. Statement I : The granules of modern gunpowder (also called black powder) are typically coated with Graphite. Statement II : Graphite prevents the build-up of electrostatic charge.

(a) Both the statements are individually true and Statement II is the correct explanation of Statement I

(b) Both the statements are individually true but Statement II is not the correct explanation of Statement I

(d) Statement I is false but Statement II is true

Answer: (a) Both the statements are individually true and Statement II is the correct explanation of Statement I

Explanation: Gunpowder granules are coated with graphite to prevent static electricity build-up. Graphite is an electrical conductor, so it dissipates electrostatic charge that could otherwise cause accidental ignition. Statement II directly and correctly explains Statement I. Concept Tested: Graphite coating on gunpowder – prevents electrostatic charge

Q. 22. Statement I : Diamond is very bright. Statement II : Diamond has very low refractive index.

(a) Both the statements are individually true and Statement II is the correct explanation of Statement I

(b) Both the statements are individually true but Statement II is not the correct explanation of Statement I

(d) Statement I is false but Statement II is true

Answer: (c) Statement I is true but Statement II is false

Explanation: Diamond is very bright (Statement I correct) because of its extremely high refractive index (~2.42) which causes total internal reflection. But Statement II says it has a very LOW refractive index – this is false. Diamond’s high refractive index is precisely why it is bright. Concept Tested: Diamond’s brilliance – due to HIGH refractive index

NDA 2014-I

Q. 23. The most stable form of carbon is

(a) diamond

(b) graphite

(d) coal

Answer: (b) graphite

Explanation: Graphite is the thermodynamically most stable allotrope of carbon at standard temperature and pressure. Diamond is metastable (it slowly converts to graphite at high temperatures, though the process is extremely slow at room temperature). Fullerene is less stable. Concept Tested: Thermodynamically most stable carbon allotrope – graphite

★ JOVIK Exam Insight: Diamond is the hardest, but graphite is the most thermodynamically stable allotrope of carbon. NDA tests both distinctions.

Q. 24. Which of the following statements is correct?

(a) Fullerenes have only six-membered carbon rings

(b) Fullerenes are cage-like molecules

(d) Graphite is slippery and hard, and is therefore used as a dry lubricant in machines

Answer: (b) Fullerenes are cage-like molecules

Explanation: Fullerenes (like C₆₀, Buckminsterfullerene) have a cage-like spherical structure. Option (a) is wrong – fullerenes contain both five-membered and six-membered rings. Option (c) is wrong – graphite is the most stable. Option (d) is wrong – graphite is slippery and soft. Concept Tested: Fullerene structure – cage-like molecules with 5 and 6 membered rings

NDA 2013-I

Q. 25. The pure form of carbon is

(a) diamond

(b) graphite

(d) fullerene

Answer: (a) diamond

Explanation: Diamond is the purest form of carbon. Graphite, charcoal and fullerene may contain impurities or other forms of carbon. Diamond is nearly 100% pure crystalline carbon. Concept Tested: Purest form of carbon – diamond

NDA 2012-II

Q. 26. Which one among the following does not have an allotrope?

(a) Oxygen

(b) Sulphur

(d) Carbon

Answer: (c) Nitrogen

Explanation: Nitrogen (N₂) does not have well-established stable allotropes under normal conditions. Oxygen has O₂ and O₃ (ozone). Sulphur has rhombic, monoclinic and amorphous forms. Carbon has diamond, graphite, fullerene, graphene. Concept Tested: Allotropy – nitrogen has no common allotrope

Q. 27. Consider the following statements:

1. Diamond is hard and graphite is soft.

2. Diamond is soft and graphite is hard.

3. Diamond is a bad conductor but graphite is a good conductor.

4. Diamond is a good conductor but graphite is a bad conductor.

Which of the statements given above is/are correct?

(a) 1 and 3

(b) 1 only

(d) 1 and 4

Answer: (a) 1 and 3

Explanation: Statement 1 is correct: diamond is hard, graphite is soft. Statement 3 is correct: diamond does not conduct electricity (bad conductor); graphite conducts electricity (good conductor). Statements 2 and 4 are the reverse. Concept Tested: Comparison of diamond and graphite – hardness and conductivity

NDA 2011-I

Q. 28. Which of the following statements about diamond are correct ?

1. It is used as a gem in jewellery because of its ability to reflect light

2. It is a good conductor of electricity

3. It is used for cutting glass, marble stones and other hard materials

4. It is used for drilling of rocks

Select the correct answer using the code given below :

(a) 1, 3 and 4

(b) 2, 3 and 4

(d) 2 and 4 only

Answer: (a) 1, 3 and 4

Explanation: Statement 1: Correct – diamond reflects and refracts light brilliantly (total internal reflection). Statement 2: Wrong – diamond is a poor conductor of electricity. Statement 3: Correct – diamond is the hardest substance, used for cutting. Statement 4: Correct – diamond-tipped drills are used in rock drilling. Concept Tested: Uses of diamond – gem, cutting, drilling

NDA 2010-II

Q. 29. Which one among the following statements is not correct about graphite?

(a) It is the most stable allotrope of carbon

(b) It is an electrically conducting material

(d) It is the highest grade of coal

Answer: (d) It is the highest grade of coal

Explanation: Graphite is not coal – it is a distinct crystalline allotrope of carbon. Coal is an amorphous form. Graphite is the most stable allotrope (a correct), an electrical conductor (b correct), and has lubricating properties due to its layered structure (c correct, though lubrication works best in humid conditions). Concept Tested: Graphite – allotrope of carbon, not a grade of coal

NDA 2009-II

Q. 30. Diamond is a

(a) good conductor and soft

(b) non-conductor and soft

(d) good conductor and hard

Answer: (c) non-conductor and hard

Explanation: Diamond is the hardest known natural substance. It does not conduct electricity (it has no free electrons; all electrons are in covalent bonds). Therefore diamond is a non-conductor of electricity and hard. Concept Tested: Properties of diamond – electrical non-conductor, hardest substance

Q. 31. Following statements are made in connection with carbon dioxide (CO₂):

1. CO₂ is a poisonous gas.

2. CO₂ is an acidic oxide.

3. CO₂ turns limewater milky.

Which of the statements given above is/are correct?

(a) 1 and 2

(b) 2 and 3

(d) 1 and 3

Answer: (b) 2 and 3

Explanation: Statement 1 is wrong: CO₂ is not a poisonous gas – it is non-toxic at normal concentrations (though it can cause suffocation in high concentrations). CO is the poisonous gas. Statement 2 is correct: CO₂ is an acidic oxide (forms H₂CO₃ with water). Statement 3 is correct: CO₂ turns lime water milky (CaCO₃ precipitate). Concept Tested: Properties of CO₂ – acidic oxide, lime water test; CO₂ vs CO toxicity

★ JOVIK Exam Insight: CO₂ is NOT poisonous (it is CO that is poisonous). CO₂ is an acidic oxide and turns lime water milky.

NDA 2009-I

Q. 32. Oxygen and ozone are

(a) allotropes

(b) isomers

(d) isobars

Answer: (a) allotropes

Explanation: Allotropes are different structural forms of the same element. Oxygen (O₂) and ozone (O₃) are both forms of elemental oxygen – they are allotropes of oxygen. Concept Tested: Allotropy – oxygen and ozone as allotropes

Q. 33. Consider the following statements regarding diamond:

1. It is an allotrope of silicon.

2. It is a bad conductor of heat and electricity.

3. It is the hardest substance.

4. It burns to produce carbon dioxide.

Which of the statements given above are correct?

(a) 1, 2, 3 and 4

(b) 2, 3 and 4 only

(d) 1, 3 and 4 only

Answer: (b) 2, 3 and 4 only

Explanation: Statement 1 is wrong: diamond is an allotrope of CARBON, not silicon. Statement 2 is correct: diamond is a bad conductor of electricity (but is a good conductor of heat). Statement 3 is correct: diamond is the hardest known natural substance. Statement 4 is correct: diamond burns in O₂ to give CO₂. So 2, 3, 4 are correct. Concept Tested: Properties of diamond – allotrope of carbon, hardest, burns to CO₂