Class 10 CBSE Science Long Answer Questions – Carbon and Its Compounds Question with Answers

Class 10 CBSE Science – Carbon and Its Compounds as per the NCERT curriculum:

36. Diamond

(a) What is diamond? Of what substance is diamond made?
Answer:
Diamond is an allotrope of carbon. It is a crystalline form in which each carbon atom is strongly bonded to four other carbon atoms in a tetrahedral structure. Thus, diamond is made up entirely of carbon atoms.

(b) Describe the structure of diamond. Draw a simple diagram to show the arrangement of carbon atoms in diamond.
Answer:
In diamond, each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement. This structure extends in all directions, forming a rigid 3D network. This tight bonding makes diamond very hard and gives it a high melting point.

Simple Diagram:

      C
     /|\
    C C C
     |
     C

(Note: In actual diagrams, it is shown as a 3D tetrahedral lattice.)

(c) Explain why diamond has a high melting point.
Answer:
Diamond has a high melting point because of the strong covalent bonds between carbon atoms. A large amount of energy is required to break this rigid 3D covalent network, which results in a very high melting point.

(d) State any two uses of diamond.
Answer:

Used in cutting and drilling tools due to its hardness.
Used in jewellery for its brilliance and appearance.

37. Graphite

(a) What is graphite? Of what substance is graphite made?
Answer:
Graphite is an allotrope of carbon. It is made entirely of carbon atoms, arranged in layers of hexagonal rings.

(b) Describe the structure of graphite with the help of a labelled diagram.
Answer:
In graphite, each carbon atom is covalently bonded to three other carbon atoms in a planar hexagonal arrangement. These layers are held together by weak van der Waals forces, allowing them to slide over each other.

Simple Diagram:

Layer 1:   C — C — C  
             \ / \  
Layer 2:   C — C — C (stacked below)

(Each hexagon represents a layer of carbon atoms.)

(c) Why is graphite a good conductor of electricity but diamond is a non-conductor of electricity?
Answer:
In graphite, each carbon atom uses only three of its four valence electrons for bonding. The fourth electron is free to move, allowing graphite to conduct electricity. In diamond, all four electrons are involved in bonding, so no free electrons are available, making it a non-conductor.

(d) State any two uses of graphite.
Answer:

Used in making pencil leads.
Used as an electrode in electric arc furnaces and dry cells.

38. Isomers

(a) Explain the term 'isomers'. Give one example of isomers.
Answer:
Isomers are compounds that have the same molecular formula but different structural arrangements of atoms.
Example: Butane (C₄H₁₀) and isobutane are isomers.

(b) Write (i) structural formula, and (ii) electron-dot structure, of any one isomer of n-heptane (C₇H₁₆)
Answer:
(i) Structural Formula of n-heptane:
CH₃–CH₂–CH₂–CH₂–CH₂–CH₂–CH₃

(ii) Electron-dot structure (simplified for CH₄ as example; full version is long):
H
|
H–C–H
|
H

(For full C₇H₁₆, extend accordingly showing bonding electrons.)

(c) Write IUPAC name of the compound having the formula n-C₄H₁₀.
Answer:
n-Butane

(d) Give the IUPAC names for the following:
(i) CH₃–CH=CH₂ → Propene
(ii) CH₃–CH₂–CH₃ → Propane
(iii) CH₂=CH–CH₃ → Propene (same as i)
(iv) CH₃–C(CH₃)₂–CH₃ → 2,2-Dimethylpropane

39. Hydrocarbons

(a) What are hydrocarbons? Explain with examples.
Answer:
Hydrocarbons are organic compounds made up of only carbon and hydrogen atoms.
Examples: Methane (CH₄), Ethylene (C₂H₄)

(b) Explain the meaning of saturated and unsaturated hydrocarbons with two examples each.
Answer:

Saturated hydrocarbons: Contain only single bonds between carbon atoms.
Examples: Ethane (C₂H₆), Propane (C₃H₈)
Unsaturated hydrocarbons: Contain at least one double or triple bond between carbon atoms.
Examples: Ethene (C₂H₄), Ethyne (C₂H₂)

(c) Give the names and structural formulae of one saturated cyclic hydrocarbon and one unsaturated cyclic hydrocarbon.
Answer:

Saturated cyclic hydrocarbon: Cyclohexane – C₆H₁₂
Structural:
A hexagon with single bonds.
Unsaturated cyclic hydrocarbon: Cyclohexene – C₆H₁₀
Structural:
A hexagon with one double bond.

(d) Give one example of a hydrocarbon, other than pentane, having more than three isomers.
Answer:
Butane (C₄H₁₀) – It has two isomers, but hexane (C₆H₁₄) has 5 isomers, which is more than three.

(e) How many isomers of the following hydrocarbons are possible?

(i) C₃H₈ → 1 isomer
(ii) C₄H₁₀ → 2 isomers
(iii) C₅H₁₂ → 3 isomers
(iv) C₆H₁₄ → 5 isomers

Here are the Multiple Choice Questions (MCQs) with correct answers and detailed explanations:

40. Buckminsterfullerene is an allotropic form of the element:

(a) phosphorus
(b) fluorine
(c) ✅ carbon
(d) sulphur

Explanation: Buckminsterfullerene (C₆₀) is a spherical molecule made of 60 carbon atoms. It is an allotrope of carbon, like graphite and diamond.

41. Out of the following pairs of compounds, the unsaturated compounds are:

(a) C₂H₆ and C₄H₆
(b) C₆H₁₂ and C₅H₁₂
(c) ✅ C₄H₆ and C₆H₁₂
(d) C₂H₆ and C₄H₁₀

Explanation: Unsaturated hydrocarbons contain double or triple bonds. C₄H₆ (an alkyne) and C₆H₁₂ (an alkene) are both unsaturated.

42. The number of covalent bonds in pentane (C₅H₁₂) is:

(a) 5
(b) 12
(c) 17
(d) ✅ 16

Explanation: Pentane has 12 C–H bonds and 4 C–C bonds → total = 16 covalent bonds.

43. The property of self-combination of atoms of the same element to form chains is known as:

(a) protonation
(b) carbonation
(c) coronation
(d) ✅ catenation

Explanation: Catenation is the ability of atoms (like carbon) to form long chains by bonding to themselves.

44. A cyclic hydrocarbon having single and double bonds is:

(a) C₆H₁₂
(b) C₆H₁₄
(c) C₆H₆
(d) ✅ C₆H₁₀

Explanation: C₆H₁₀ is a cyclic hydrocarbon (like cyclohexene) containing both single and double bonds.

45. The hydrocarbon 2-methylbutane is an isomer of:

(a) ✅ pentane
(b) n-butane
(c) propane
(d) iso-butane

Explanation: 2-methylbutane and pentane have the same molecular formula (C₅H₁₂) but different structures → they are isomers.

46. An alkyne with a triple bond has 50 hydrogen atoms. Number of carbon atoms = ?

(a) 24
(b) 25
(c) ✅ 26
(d) 28

Explanation: For alkynes, the formula is CₙH₂ₙ₋₂.
2n − 2 = 50 → 2n = 52 → n = 26

47. An alkyne has 75 carbon atoms. Hydrogen atoms = ?

(a) 150
(b) ✅ 148
(c) 152
(d) 146

Explanation: Alkynes follow CₙH₂ₙ₋₂ → H = 2×75 − 2 = 148

48. A diamond-toothed saw is usually used for cutting:

(a) steel girders
(b) logs of wood
(c) ✅ marble slabs
(d) asbestos sheets

Explanation: Diamond, being very hard, is used to cut tough materials like marble.

49. The organic compound Wohler prepared from ammonium cyanate was:

(a) glucose
(b) ✅ urea
(c) uric acid
(d) vinegar

Explanation: Friedrich Wöhler synthesized urea from ammonium cyanate, proving organic compounds could be made from inorganic sources.

50. One of the following is not an allotrope of carbon:

(a) diamond
(b) graphite
(c) ✅ cumene
(d) buckminsterfullerene

Explanation: Cumene is an organic compound, not an allotrope of carbon. The others are carbon allotropes.

51. Number of carbon atoms in 2,2-dimethylpropane is:

(a) two
(b) ✅ five
(c) three
(d) four

Explanation: The main chain has 3 carbon atoms + 2 methyl branches → total = 5 carbon atoms.

52. Pair of elements showing catenation:

(a) sodium and silicon
(b) chlorine and carbon
(c) carbon and sodium
(d) ✅ silicon and carbon

Explanation: Carbon and silicon both can form long chains with themselves → catenation.

53. A saturated hydrocarbon has 50 hydrogen atoms. Carbon atoms = ?

(a) 25
(b) ✅ 24
(c) 26
(d) 27

Explanation: Alkane formula: CₙH₂ₙ₊₂
2n + 2 = 50 → 2n = 48 → n = 24

54. A hydrocarbon with one double bond has 100 carbon atoms. Hydrogen atoms = ?

(a) ✅ 200
(b) 198
(c) 202
(d) 196

Explanation: Alkenes: CₙH₂ₙ → C₁₀₀H₂₀₀

55. The hydrocarbon with alternating single and double bonds in a ring is:

(a) cyclobutane
(b) ✅ benzene
(c) butene
(d) hexene

Explanation: Benzene (C₆H₆) has a ring with alternating single and double bonds → called aromatic.

56. Which compound cannot show isomerism?

(a) C₄H₁₀
(b) C₅H₁₂
(c) ✅ C₃H₈
(d) C₆H₁₄

Explanation: C₃H₈ (propane) is too small to have structural isomers.

57. Pencil leads are mainly made of:

(a) lithium
(b) charcoal
(c) lead
(d) ✅ graphite

Explanation: Graphite is a soft allotrope of carbon used in pencils, often wrongly called “lead”.

58. Number of isomers of C₅H₁₂ is:

(a) 2
(b) 5
(c) ✅ 3
(d) 4

Explanation: C₅H₁₂ has 3 isomers: n-pentane, isopentane (methylbutane), neopentane (dimethylpropane).

59. Number of carbon atoms in buckminsterfullerene molecule is:

(a) fifty
(b) ✅ sixty
(c) seventy
(d) ninety

Explanation: Buckminsterfullerene (C₆₀) has a spherical structure of 60 carbon atoms arranged like a soccer ball.

60. The general formula for alkenes is:

(a) CₙH₂ₙ₊₂
(b) CₙH₂ₙ
(c) CₙH₂ₙ₋₂
(d) CₙHₙ

Answer: ✅ (b) CₙH₂ₙ
Explanation: Alkenes are unsaturated hydrocarbons containing one double bond and follow the formula CₙH₂ₙ.

61. Which of the following hydrocarbons is an alkyne?

(a) C₂H₆
(b) C₂H₄
(c) ✅ C₂H₂
(d) C₃H₈

Explanation: Alkynes contain triple bonds. Ethyne (C₂H₂) is the simplest alkyne.

62. Graphite conducts electricity due to the presence of:

(a) protons
(b) free neutrons
(c) free ions
(d) ✅ free electrons

Explanation: In graphite, each carbon atom forms three bonds, leaving one delocalized electron free to conduct electricity.

63. Which of the following is a saturated hydrocarbon?

(a) ethene
(b) ethyne
(c) ✅ ethane
(d) benzene

Explanation: Saturated hydrocarbons contain only single bonds. Ethane (C₂H₆) is an alkane and fully saturated.

64. Which of the following statements about diamond is true?

(a) It conducts electricity
(b) It is soft and slippery
(c) It contains delocalized electrons
(d) ✅ It is the hardest natural substance

Explanation: Diamond has a strong 3D covalent bond network, making it the hardest known natural material.

65. Which of the following is not a hydrocarbon?

(a) butane
(b) propane
(c) ✅ ethanol
(d) ethene

Explanation: Hydrocarbons contain only carbon and hydrogen. Ethanol (C₂H₅OH) contains oxygen, so it's not a pure hydrocarbon.

66. The molecular formula C₃H₆ represents which type of hydrocarbon?

(a) alkane
(b) ✅ alkene
(c) alkyne
(d) aromatic

Explanation: C₃H₆ fits the general formula for alkenes (CₙH₂ₙ), indicating the presence of a double bond.

67. Which hydrocarbon is most likely to undergo addition reactions?

(a) ethane
(b) propane
(c) ✅ ethene
(d) butane

Explanation: Unsaturated hydrocarbons like ethene (with a double bond) undergo addition reactions easily.

68. Which hydrocarbon is commonly used in welding torches?

(a) methane
(b) propane
(c) ✅ ethyne
(d) ethane

Explanation: Ethyne (acetylene) burns with a very hot flame and is used in oxy-acetylene welding.

69. Which of the following is an aromatic compound?

(a) cyclopentane
(b) ethyne
(c) ✅ benzene
(d) butane

Explanation: Benzene is the simplest aromatic compound, with a ring structure and delocalized π-electrons.

70. Which of the following alkanes has the lowest boiling point?

(a) butane
(b) propane
(c) hexane
(d) ✅ methane

Explanation: Methane (CH₄), being the smallest alkane, has the lowest molecular weight and hence the lowest boiling point.

Class 10 CBSE Board-style Higher Order Thinking Skills (HOTS) / Case-Based Questions (Q60–Q70) with detailed answers, crafted as per the latest CBSE exam pattern:

60. A solid element X has four electrons in the outermost shell of its atom. An allotrope Y of this element is used as a dry lubricant in machinery and also in making pencil leads.

(a) What is element X?
✅ Carbon
Explanation: Carbon has 4 valence electrons and exists in various allotropic forms.

(b) Name the allotrope Y.
✅ Graphite
Explanation: Graphite is soft, slippery, and used in lubricants and pencils.

(c) State whether allotrope Y is a good conductor or non-conductor of electricity.
✅ Good conductor
Explanation: Graphite has free electrons that allow it to conduct electricity.

(d) Name one use of allotrope Y (other than lubrication and pencil leads).
✅ Used as electrodes in batteries and arc lamps

(e) Name two other allotropes of element X.
✅ Diamond and Buckminsterfullerene

61. Two organic compounds A and B have the same molecular formula C₆H₁₂.

(a) If A is a cyclic compound:
✅ Name: Cyclohexane
✅ Structure:
CH₂–CH₂–CH₂–CH₂–CH₂–CH₂ (in a ring)

(b) If B is an open chain compound:
✅ Name: Hexene (an alkene)
✅ Structure: CH₂=CH–CH₂–CH₂–CH₂–CH₃

(c) Which compound contains single bonds as well as a double bond?
✅ B (Hexene)

(d) Which compound contains only single bonds?
✅ A (Cyclohexane)

62. The solid element A exhibits the property of catenation. It is also present in the form of a gas B in the air which is used in photosynthesis. An allotrope C of this element is used in glass cutters.

(a) What is element A?
✅ Carbon

(b) What is gas B?
✅ Carbon dioxide (CO₂)

(c) Name the allotrope C.
✅ Diamond

(d) Another use of allotrope C (other than glass cutters):
✅ Jewellery making

(e) Allotrope that exists as spherical molecules:
✅ Buckminsterfullerene

(f) Allotrope that conducts electricity:
✅ Graphite

63. An element E exists in three allotropic forms A, B, and C.

(a) Name the element E.
✅ Carbon

(b) Allotrope A (spherical molecules):
✅ Buckminsterfullerene

(c) Allotrope B (sheet-like structure):
✅ Graphite

(d) Allotrope C (rigid 3D structure):
✅ Diamond

(e) Allotrope used in jewellery:
✅ Diamond

(f) Allotrope used as anode in dry cell:
✅ Graphite

64. You are given the following hydrocarbons:

C₅H₈, C₇H₁₄, C₆H₆, C₅H₁₀, C₇H₁₂, C₆H₁₂

(a) Formula that represents cyclohexane and hexene:
✅ C₆H₁₂
(Used for both a cyclic alkane and an open-chain alkene)

(b) Formula that represents benzene:
✅ C₆H₆

(c) Three formulas for open-chain unsaturated hydrocarbons with double bonds:
✅ C₅H₁₀, C₇H₁₂, C₆H₁₂

(d) Two formulas for hydrocarbons with triple bonds:
✅ C₅H₈ and C₇H₁₂

(e) Three formulas that represent cyclic hydrocarbons:
✅ C₆H₆ (benzene), C₅H₁₀ (cyclopentane), C₆H₁₂ (cyclohexane)

65. Which of the following compounds can have a triple bond?

C₂H₄ ❌
✅ C₃H₄
C₃H₆ ❌

Explanation: C₃H₄ fits the formula of an alkyne (CₙH₂ₙ₋₂), hence can have a triple bond.

66. Write the molecular and structural formula of a cyclic hydrocarbon whose molecule contains 8 carbon atoms.

Answer:
✅ Molecular formula: C₈H₁₆
✅ Name: Cyclooctane
✅ Structure: A ring of 8 CH₂ groups:
CH₂–CH₂–CH₂–CH₂–CH₂–CH₂–CH₂–CH₂ (closed loop)

67. What is the molecular and structural formula of a cyclic hydrocarbon with 8 hydrogen atoms?

Answer:
✅ Molecular formula: C₄H₈
✅ Name: Cyclobutene
✅ Structure: A 4-carbon ring with one double bond. Example:

       CH2
      /   \
    CH — CH
      \   /
       CH2

68. Write the molecular formula of the following hydrocarbons with 20 carbon atoms:

(i) Alkane: ✅ C₂₀H₄₂
(ii) Alkene: ✅ C₂₀H₄₀
(iii) Alkyne: ✅ C₂₀H₃₈

Explanation:

Alkanes: CₙH₂ₙ₊₂
Alkenes: CₙH₂ₙ
Alkynes: CₙH₂ₙ₋₂

69. Which of the following compounds can have a double bond?

C₄H₃₀ ❌ (not valid)
✅ C₅H₈
✅ C₅H₁₀

Explanation: C₅H₈ and C₅H₁₀ can be unsaturated hydrocarbons with double bonds.

70. Which of the following hydrocarbons is unsaturated?

CH (Incomplete)
✅ C₂H₆ ❌ (Saturated)
✅ C₂H₄ (Correct answer if intended)

Correct Answer (Assumed intent):
✅ C₂H₄ (an alkene with a double bond)