Carbon and Its Compounds – NDA Chemistry PYQs

Coastal States, Gulfs, Straits, Islands and Maritime Zones of India

Indian Geography • Coastal Geography • PYQs Included

Q1. 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 (c) 1 and 2 only (d) 1, 3 and 4 only

Answer: (b) 2, 3 and 4 only
Statement 1 is FALSE — diamond is an allotrope of CARBON, not silicon (though it is isomorphous with crystalline silicon, they are different substances). Statement 2 ✓ — diamond is a bad conductor of electricity (good thermal conductor but poor electrical). Statement 3 ✓ — hardest natural substance. Statement 4 ✓ — burns in oxygen to give CO₂.
[NDA 2009-I]

Q2. Assertion (A): Carbon can form more compounds than any other element.
Reason (R): Carbon can exist in various allotropes.
(a) Both true, R correctly explains A
(b) Both true, R does NOT correctly explain A
(c) A true, R false
(d) A false, R true

Answer: (b) Both true, R does NOT correctly explain A
A is true — carbon forms over 10 million compounds. R is also true — carbon has multiple allotropes (diamond, graphite, fullerene, graphene). However, the ability to form more compounds is due to catenation and tetravalency — NOT because carbon has allotropes. The allotropes explanation is irrelevant to compound formation ability.
[NDA 2009-I]

Q3. Which one among the following does NOT have an allotrope?
(a) Oxygen (b) Sulphur (c) Nitrogen (d) Carbon

Answer: (c) Nitrogen
Nitrogen exists only as N₂ — it has no known allotropes. Oxygen has two allotropes: O₂ and ozone (O₃). Sulphur has rhombic and monoclinic allotropes. Carbon has diamond, graphite, fullerene, and graphene.
[NDA 2012-II]

Q4. Consider the following statements about graphite and diamond:

  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 are correct?
    (a) 1 and 3 (b) 1 only (c) 2 and 3 (d) 1 and 4

Answer: (a) 1 and 3
Statement 1 ✓ — diamond is hard (3D rigid covalent network), graphite is soft (layers slide). Statement 3 ✓ — diamond has no free electrons (insulator), graphite has delocalised electrons (conductor). Statements 2 and 4 reverse these facts.
[NDA 2012-II]

Q5. The most stable form of carbon is:
(a) Diamond (b) Graphite (c) Fullerene (d) Coal

Answer: (b) Graphite
Graphite is the thermodynamically most stable allotrope of carbon. Diamond is metastable — it converts to graphite very slowly over geological timescales. Fullerene and coal are not allotropes in the pure crystalline sense.
[NDA 2014-I]

Q6. Which of the following statements about fullerenes are correct?

  1. Fullerenes have only six-membered carbon rings.
  2. Fullerenes are cage-like molecules.
  3. Diamond is thermodynamically the most stable allotrope of carbon.
  4. Graphite is slippery and hard, and is therefore used as a dry lubricant in machines.
    (a) 1 and 3 (b) 2 only (c) 1, 2 and 3 (d) 1 and 4

Answer: (b) 2 only
Statement 1 ✗ — fullerenes contain both five-membered and six-membered rings. Statement 2 ✓ — fullerenes are cage-like spherical molecules. Statement 3 ✗ — graphite is thermodynamically most stable, not diamond. Statement 4 ✗ — graphite is slippery and SOFT (not hard) — its softness is why it lubricates.
[NDA 2014-I]

Q7. Which one of the following is the purest form of Carbon?
(a) Charcoal (b) Coke (c) Fullerene (d) Carbon black

Answer: (c) Fullerene
Fullerene (C₆₀) consists entirely of pure carbon atoms arranged in a perfect molecular structure. Charcoal, coke, and carbon black are all impure, amorphous forms of carbon containing impurities.
[NDA 2013-I, NDA 2018-I]

Q8. Why is Graphite used in electrolytic cells?
(a) Graphite is soft and can be easily moulded
(b) Graphite is made of layers that can slide
(c) Graphite is inert to most chemicals and remains intact in electrolytic cells
(d) Graphite is a good conductor of electricity

Answer: (d) Graphite is a good conductor of electricity
The primary reason graphite is used as electrodes in electrolytic cells is its excellent electrical conductivity due to delocalised electrons. Its chemical inertness is also an advantage but conductivity is the essential requirement for an electrode.
[NDA 2015-I]

Q9. 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
(c) Electrons are delocalised over the whole sheet of atoms
(d) Each layer is composed of hexagonal rings

Answer: (a) Hybridisation of each carbon atom is sp³
In graphite, each carbon atom forms three sigma bonds with neighbours using sp² hybridised orbitals. The sp² hybridisation creates the flat hexagonal layer structure. sp³ hybridisation is what diamond has — in graphite it is sp².
[NDA 2018-I]

Q10. Which one of the following statements is NOT correct about graphite?
(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
(c) Graphite layers are formed by hexagonal rings of carbon atoms
(d) Graphite layers are held together by carbon-carbon single bond

Answer: (d) Graphite layers are held together by carbon-carbon single bond
This is false. Graphite layers are held together by WEAK VAN DER WAALS FORCES — not covalent bonds. If layers were held by covalent bonds, they would not slide and graphite would not be soft or a lubricant. Statements (b) and (c) are both correct.
[NDA 2018-II]

Q11. Which one of the following is called ‘syngas’?
(a) C(s) + H₂O(g)
(b) CO(g) + H₂O(g)
(c) CO(g) + H₂(g)
(d) NO₂(g) + H₂(g)

Answer: (c) CO(g) + H₂(g)
Syngas (synthesis gas) is a mixture of carbon monoxide and hydrogen. It is produced by reacting steam with hot coke or by steam reforming of natural gas. It is used as a raw material for making methanol and other chemicals.
[NDA 2018-II]

Q12. Water gas is a mixture of:
(a) CO and H₂ (b) CO and N₂ (c) CO₂ and H₂O (d) CO and H₂O

Answer: (a) CO and H₂
Water gas is made by passing steam over red-hot coke: C + H₂O → CO + H₂. Option (b) is producer gas (CO + N₂). Options (c) and (d) contain water or water vapour — not water gas.
[NDA 2016-I, NDA 2024-I]

Q13. Which one of the following carbon compounds will NOT give a sooty flame?
(a) Benzene (b) Hexane (c) Naphthalene (d) Anthracene

Answer: (b) Hexane
Hexane (C₆H₁₄) is a saturated hydrocarbon — it has only C-C single bonds. Saturated hydrocarbons have lower carbon-to-hydrogen ratios and burn more cleanly — blue flame, little soot. Benzene, naphthalene, and anthracene are aromatic/unsaturated — high carbon ratio — produce sooty flames.
[NDA 2016-I]

Q14. What is the number of moles of H₂ (g) required to saturate one mole of benzene?
(a) 1 (b) 2 (c) 3 (d) 4

Answer: (c) 3
Benzene (C₆H₆) has three double bonds in its ring structure. Each double bond requires one mole of H₂ to saturate. So 3 moles of H₂ saturate one mole of benzene: C₆H₆ + 3H₂ → C₆H₁₂ (cyclohexane).
[NDA 2016-I]

Q15. Which one of the following materials is NOT an allotrope of carbon?
(a) Diamond (b) Graphene (c) Fly ash (d) Fullerene

Answer: (c) Fly ash
Fly ash is fine particulate matter produced from burning coal in power plants — it is not a pure form of carbon and not an allotrope. Diamond, graphene, and fullerene are all genuine allotropes of carbon.
[NDA 2021-I]

Q16. Which one of the following allotropes of carbon is isomorphous with crystalline silicon?
(a) Coke (b) Diamond (c) Graphite (d) Coal

Answer: (b) Diamond
Diamond and crystalline silicon have the same crystal structure — a three-dimensional tetrahedral network where each atom is bonded to four others. This identical structural arrangement means they are isomorphous (same crystal form, different chemical composition).
[NDA 2022-I]

Q17. 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
(c) Fullerene is one of the allotropes of carbon
(d) Next to diamond, graphite is the second hardest known substance

Answer: (d) Next to diamond, graphite is the second hardest known substance
This is completely false — graphite is one of the softest materials known (hardness 1–2 on Mohs scale). Diamond is the hardest. The second hardest substances after diamond include cubic boron nitride and moissanite — not graphite. Statements (a), (b), and (c) are all correct.
[NDA 2023-I]

Q18. The elemental composition of diamond is:
(a) Carbon and Hydrogen (b) Carbon and Silicon (c) Carbon only (d) Silicon only

Answer: (c) Carbon only
Diamond is composed entirely of carbon atoms. Every atom in diamond is a carbon atom — no hydrogen, no silicon, no other element. It is a pure allotrope of carbon.
[NDA 2024-II]

Q19. Which one of the following is also known as Dry ice in its solid form?
(a) SiO₂ (b) CO₂ (c) CaO (d) MgO

Answer: (b) CO₂
Dry ice is solid carbon dioxide. It sublimes at −78°C — converting directly from solid to gas without melting. It is used as a refrigerant and does not produce water when it evaporates.
[NDA 2025-I]

Q20. Which one of the following is the major constituent of biogas?
(a) Carbon dioxide (b) Nitrous oxide (c) Methane (d) Oxygen

Answer: (c) Methane
Biogas is produced by anaerobic decomposition of organic waste (cow dung, crop residue, sewage). Its main combustible component is methane (CH₄) — approximately 55–65%. CO₂ makes up most of the remainder.
[NDA 2019-II, NDA 2013-II]

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