NDA General Studies · Chemistry · 38 questions · 2017 to 2026

NDA Carbon, organic chemistry, polymers and fuels previous year questions with solutions

Carbon and its compounds give about one General Studies question in fifty-five, two or three a sitting. Nearly half are on the allotropes, asked as which statement is not correct; the rest are one-fact recall on fuels, bonding and everyday compounds. Every question UPSC has set on it since 2017, solved. Grouped by the method each needs, newest first; open a question for its answer and worked solution.

12.1 Carbon the element: tetravalency, catenation and covalent bonding; organic and inorganic compounds (5)12.2 Allotropes of carbon: diamond, graphite, fullerenes and graphene; the amorphous forms (16)12.3 Hydrocarbons: saturated and unsaturated, homologous series, molecular formulae and isomers (4)12.4 Functional groups and important carbon compounds: ethanol, ethanoic acid, esters, soaps; addition and substitution reactions; a box on polymers (5)12.5 Fuels: water gas and syngas, biogas and biomass, clean fuels, LPG, CNG and calorific value (8)

12.1 Carbon the element: tetravalency, catenation and covalent bonding; organic and inorganic compounds

1. NDA 2026 (I)

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(c) I and III only(d) I, II and III

Answer and solution

Answer: (c) I and III only

Method. Evaluate each statement as printed

I: soaps are sodium/⁠potassium salts of long-chain fatty (carboxylic) acids: correct. II as printed says catenation and isomerism are attributed to the existence of many organic compounds, which reverses cause and effect (the large number of compounds is due to catenation and isomerism): incorrect. III: CO₂ is an acidic oxide and CO is neutral: correct. So I and III only. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds).

2. NDA 2023 (II)

Which one of the following is NOT an example of organic compound?

(a) Ammonium cyanate(b) Marsh gas(c) Urea(d) Cane sugar

Answer and solution

Answer: (a) Ammonium cyanate

Method. Organic versus inorganic

Marsh gas is methane, urea is CH₄N₂O (organic), and cane sugar is sucrose (organic). Ammonium cyanate, NH₄OCN, is an inorganic salt (it rearranges on heating to urea in Wohler's synthesis). Source: Class 11 Chemistry ch 8 (organic chemistry introduction) and standard result.

3. NDA 2023 (II)

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(c) 1, 2 and 3(d) 2 and 4

Answer and solution

Answer: (c) 1, 2 and 3

Method. Properties of carbon

1, 2 and 3 are correct (basis of life, tetravalency and catenation, covalent bonding). 4 is false: carbon forms single, double and triple bonds; there are no 'tetra' bonds between carbon atoms. Hence 1, 2 and 3. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds).

4. NDA 2022 (I)

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.(c) Graphite is used as a lubricant.(d) Diamond is an allotrope of carbon.

Answer and solution

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

Method. Properties of carbon compounds

Most carbon compounds are covalent, so they do not conduct electricity (poor conductors). Hence a is not correct. b, c (graphite lubricant) and d are correct. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds).

5. NDA 2017 (I)

Which one of the following elements forms highest number of compounds?

(a) Oxygen(b) Hydrogen(c) Chlorine(d) Carbon

Answer and solution

Answer: (d) Carbon

Method. Catenation and tetravalency of carbon

Carbon bonds to itself in long chains, branches and rings (catenation) and has valency 4, giving millions of compounds, more than any other element. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds).

12.2 Allotropes of carbon: diamond, graphite, fullerenes and graphene; the amorphous forms

6. NDA 2026 (I)

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

(a) Coal(b) Graphite(c) Diamond(d) Fullerene

Answer and solution

Answer: (b) Graphite

Method. Thermodynamic stability of carbon

Graphite is the thermodynamically stable form of carbon under ordinary conditions; diamond is metastable (enthalpy about +1.9 kJ/⁠mol above graphite) and only kinetically stable. Standard result.

7. NDA 2025 (II)

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(c) Diamond is a good conductor of electricity(d) Graphite is used as a lubricant

Answer and solution

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

Method. Diamond properties

Diamond has no free electrons (every C uses all four valence electrons in sigma bonds) so it is an insulator. c is not correct; a, b (fullerene has pentagons and hexagons), d are correct. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds).

8. NDA 2024 (II)

The elemental composition of diamond is

(a) Carbon and Hydrogen(b) Carbon and Oxygen(c) Pure Carbon(d) Pure Silicon

Answer and solution

Answer: (c) Pure Carbon

Method. Composition of diamond

Diamond is a form of pure carbon. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds).

9. NDA 2024 (I)

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(c) 1 3 2 4(d) 1 2 3 4

Answer and solution

Answer: (a) 4 2 3 1

Method. Matching allotropes with properties

Graphite: soft and slippery (4); diamond: hardest natural substance (2); fullerene: very light and strong (3); graphene: thinnest and strongest material (1). Code 4 2 3 1. Standard result.

10. NDA 2023 (II)

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

(a) Phosphorus(b) Sulfur(c) Carbon(d) Tin

Answer and solution

Answer: (c) Carbon

Method. Fullerene

Buckminsterfullerene C₆₀, football-shaped, is an allotrope of carbon. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds) and standard result.

11. NDA 2023 (I)

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 and solution

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

Method. Hardness of carbon allotropes

Graphite is soft (layers slide), so it is not the second hardest substance. a, b, c are correct. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds).

12. NDA 2022 (I)

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

(a) Coke(b) Diamond(c) Graphite(d) Coal

Answer and solution

Answer: (b) Diamond

Method. Isomorphism: same crystal structure

Crystalline silicon has the diamond cubic structure, each atom tetrahedrally bonded to four others. Diamond is the isomorphous allotrope. Standard result.

13. NDA 2021 (I)

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

(a) Diamond(b) Graphene(c) Fly ash(d) Fullerene

Answer and solution

Answer: (c) Fly ash

Method. Allotropes of carbon

Diamond, graphene and fullerene are all pure carbon forms. Fly ash is a residue of coal combustion (silica, alumina, oxides), not an allotrope. Standard result.

14. NDA 2020 (I)

Which one of the following statements is NOT correct?

(a) Buckminsterfullerene is an allotrope of carbon(b) Diamond is a good conductor of electricity(c) Graphite is a good conductor of electricity(d) In graphite, each carbon atom is linked to three other carbon atoms

Answer and solution

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

Method. Electrical conduction of allotropes

Diamond has all four valence electrons in localised sigma bonds, so it is an insulator. Graphite conducts through delocalised electrons; each graphite carbon is bonded to three others; buckminsterfullerene is an allotrope. b is not correct. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds).

15. NDA 2019 (II)

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.(c) Graphite is soft but diamond is hard.(d) Graphite is a good conductor of electricity while diamond is not.

Answer and solution

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

Method. Allotropes: same chemistry, different physics

Diamond and graphite are both pure carbon, so their chemical properties are the same (each burns to give CO₂) while physical properties differ. Hence b is not correct. a, c, d are correct. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds).

16. NDA 2018 (II)

Which one of the following statements is not correct?

(a) All carbons in diamond are linked by carbon-carbon single bond.(b) Graphite is 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 and solution

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

Method. Structure of graphite

In graphite layers are held together by weak van der Waals forces, not covalent bonds, which is why graphite is soft. a, b, c are correct (diamond: all C-C single bonds). Statement d is false. Standard result.

17. NDA 2018 (I)

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(c) By burning hydrocarbons in a limited supply of air(d) By heating coal at high temperature in presence of air

Answer and solution

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

Method. Preparation of carbon black

Carbon black (soot) is made by incomplete combustion of hydrocarbons (natural gas, oil) in a limited supply of air. Heating wood in absence of air gives charcoal, and heating coal in absence of air gives coke. Standard result.

18. NDA 2018 (I)

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 delocalized over the whole sheet of atoms(d) Each layer is composed of hexagonal rings

Answer and solution

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

Method. Hybridisation in graphite

Each carbon in graphite is bonded to three others in a hexagonal sheet: sp², with one delocalised p electron per atom. sp³ is diamond. So a is NOT true; b, c, d are true. Standard result.

19. NDA 2018 (I)

Which one of the following is the purest form of Carbon?

(a) Charcoal(b) Coke(c) Fullerene(d) Carbon black

Answer and solution

Answer: (c) Fullerene

Method. Purity of allotropes

Charcoal, coke and carbon black are amorphous and impure (contain H, ash, volatile matter). Fullerene is a crystalline allotrope made only of carbon atoms, so it is the purest of the options. Standard result.

20. NDA 2017 (I)

Molecules of which of the following has cage like structure?
(1) Diamond
(2) Graphite
(3) Fullerenes
Select the correct answer using the code given below:

(a) 1, 2 and 3(b) 2 and 3 only(c) 2 only(d) 3 only

Answer and solution

Answer: (d) 3 only

Method. Cage structure belongs to fullerenes

Diamond is a 3-D network of tetrahedral carbons and graphite is stacked planar sheets; neither is a cage. Fullerenes (C₆₀, buckminsterfullerene) are closed hollow cages of 5- and 6-membered rings. Only 3, so d. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds) (allotropes); fullerene beyond Class 10: standard result.

21. NDA 2017 (I)

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

(a) Zinc(b) Lead(c) Carbon (Graphite)(d) Tin

Answer and solution

Answer: (c) Carbon (Graphite)

Method. Graphite is the soft, layered form of carbon

'Pencil lead' contains no lead: it is graphite (carbon) mixed with clay. Layers slide easily and leave a mark. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds); standard result.

12.3 Hydrocarbons: saturated and unsaturated, homologous series, molecular formulae and isomers

22. NDA 2023 (I)

Which of the hydrocarbons are arranged as per the increasing order of their boiling points?

(a) Methane, Butane, Propane, Heptane(b) Propane, Butane, Pentane, Octane(c) Propane, Butane, Heptane, Methane(d) Octane, Ethane, Methane, Propane

Answer and solution

Answer: (b) Propane, Butane, Pentane, Octane

Method. Boiling points rise with chain length

Boiling points: methane -162 C, propane -42 C, butane -1 C, pentane 36 C, heptane 98 C, octane 126 C. Only b (propane, butane, pentane, octane) is in increasing order; a puts methane before propane wrongly and c, d are not ordered. Standard result.

23. NDA 2022 (II)

The number of structural isomers of pentane is

(a) 5(b) 4(c) 2(d) 3

Answer and solution

Answer: (d) 3

Method. Isomers of C₅H₁₂

Chain isomers of pentane: n-pentane, 2-methylbutane (isopentane), 2,2-dimethylpropane (neopentane): 3 isomers. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds); Class 11 Chemistry ch 8, 9 (isomerism).

24. NDA 2022 (II)

C₄H₈ belongs to the homologous series of

(a) alkanes(b) alkenes(c) alkynes(d) cycloalkanes

Answer and solution

Answer: (b) alkenes

Method. General formula C_nH_2n

Alkanes are C_nH_2n+2 (C₄H₁₀) and alkynes C_nH_2n-2 (C₄H₆); C₄H₈ fits C_nH_2n, the alkene series (butene). Cycloalkanes also have C_nH_2n, but by the standard school reading of 'homologous series' with this formula the answer is alkenes. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds).

25. NDA 2020 (I)

How many covalent bonds are present in a Chloropropane molecule having molecular formula, C₃H₇Cl?

(a) 6(b) 8(c) 9(d) 10

Answer and solution

Answer: (d) 10

Method. Count bonds in C₃H₇Cl

Count for CH₃CH₂CH₂Cl: C-C = 2, C-H = 7, C-Cl = 1, total = 10. (Same for the isomer CH₃CHClCH₃.) Source: NCERT Class 10 Science ch 4 (Carbon and its compounds) (covalent bonding).

12.4 Functional groups and important carbon compounds: ethanol, ethanoic acid, esters, soaps; addition and substitution reactions; a box on polymers

26. NDA 2024 (I)

Which one among the following compounds has a sweet and fruity smell?

(a) Ethyl alcohol(b) Acetic acid(c) Ethyl acetate(d) Acetophenone

Answer and solution

Answer: (c) Ethyl acetate

Method. Esters

Esters such as ethyl acetate have a sweet fruity smell; ethanol and ethanoic acid have sharp odours. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds) (esterification).

27. NDA 2023 (II)

What is the action of litmus on ethanol?

(a) Litmus is neutral towards ethanol(b) Ethanol turns blue litmus to red; confirming acidic nature of ethanol(c) Ethanol turns red litmus to blue; confirming basic nature of ethanol(d) Ethanol decolorizes litmus through bleaching action

Answer and solution

Answer: (a) Litmus is neutral towards ethanol

Method. Acid-base nature of ethanol

Ethanol is neutral: it does not change the colour of litmus. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds).

28. NDA 2022 (II)

Liquid vegetable oils are converted to solid margarine by the use of

(a) hydrogen gas(b) chlorine gas(c) carbon dioxide gas(d) oxygen gas

Answer and solution

Answer: (a) hydrogen gas

Method. Hydrogenation of oils

Vegetable oils are unsaturated; H₂ gas with Ni catalyst saturates them to solid fat (vanaspati, margarine). Source: NCERT Class 10 Science ch 4 (Carbon and its compounds).

29. NDA 2021 (I)

Hydrogenation of vegetable oils using nickel catalyst is an example of

(a) Substitution reaction.(b) Elimination reaction.(c) Addition reaction.(d) Free-radical polymerization.

Answer and solution

Answer: (c) Addition reaction.

Method. Hydrogenation is addition

H₂ with Ni catalyst adds across the C=C bonds of unsaturated oils to give saturated fats: an addition reaction. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds).

30. NDA 2020 (I)

Vinegar is also known as

(a) ethanoic acid(b) nitric acid(c) sulphuric acid(d) tartaric acid

Answer and solution

Answer: (a) ethanoic acid

Method. Vinegar

Vinegar is a 5-8 percent solution of ethanoic (acetic) acid in water. Source: NCERT Class 10 Science ch 4 (Carbon and its compounds).

12.5 Fuels: water gas and syngas, biogas and biomass, clean fuels, LPG, CNG and calorific value

31. NDA 2025 (II)

Which one of the following is known as 'water gas'?

(a) H₂O + CO₂(b) NH₃(c) H₂ + N₂(d) CO + H₂

Answer and solution

Answer: (d) CO + H₂

Method. Water gas

Water gas is CO + H₂ (C + H₂O -> CO + H₂). Standard result.

32. NDA 2024 (I)

Water gas is a mixture of:

(a) CO₂ and H₂(b) CO and H₂(c) CO₂ and H₂O(d) CO and H₂O

Answer and solution

Answer: (b) CO and H₂

Method. Water gas

Water gas is the mixture of CO and H₂, made by passing steam over red-hot coke: C + H₂O -> CO + H₂. Standard result.

33. NDA 2021 (I)

Which one of the following is not a bio-mass energy source?

(a) Wood(b) Nuclear reactor(c) Gobar gas(d) Coal

Answer and solution

Answer: (b) Nuclear reactor

Method. Biomass energy sources

Wood and gobar gas come from biomass directly. A nuclear reactor uses fissile uranium and has no link to biomass, so it is clearly not a biomass source. Coal is a fossil fuel, formed from ancient plant matter and conventionally classed as fossil, not biomass; by the standard school reading b is the answer.

34. NDA 2020 (I)

Which one of the following statements is NOT correct?

(a) Biomass is a renewable source of energy(b) Gobar gas is produced when cow-dung, crop residues, vegetable waste and sewage are allowed to decompose in the absence of oxygen(c) Biogas generation reduces soil and water pollution(d) Heating capacity of biogas is very low

Answer and solution

Answer: (d) Heating capacity of biogas is very low

Method. Biogas as fuel

Biogas is mostly methane, so it has a high heating value and burns without smoke; the other statements are correct (biomass is renewable, anaerobic decomposition makes gobar gas, and the slurry is manure so it cuts pollution). d is not correct. Standard result (Class 10 Science).

35. NDA 2020 (I)

Which one of the following is the largest composition in biogas?

(a) Carbon dioxide(b) Methane(c) Hydrogen(d) Hydrogen sulphide

Answer and solution

Answer: (b) Methane

Method. Composition of biogas

Methane is the main constituent (about 65 percent); CO₂ is next, with traces of H₂ and H₂S. Standard result.

36. NDA 2019 (II)

Which one of the following is the major constituent of biogas?

(a) Carbon dioxide(b) Nitrous oxide(c) Methane(d) Oxygen

Answer and solution

Answer: (c) Methane

Method. Composition of biogas

Biogas is mainly methane (about 55-70 percent) with CO₂ and traces of H₂S. Source: standard result (Class 10 Science, sources of energy).

37. NDA 2019 (I)

Which one of the following is an example of a clean fuel?

(a) Coke(b) Propane(c) Petrol(d) Wax

Answer and solution

Answer: (b) Propane

Method. Clean fuel

Propane (LPG) burns almost completely with little smoke or ash and no sulphur; coke, petrol and wax give soot, CO or sulphur/⁠lead pollutants. Propane is the clean fuel. Standard result.

38. NDA 2018 (II)

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 and solution

Answer: (c) CO(g) + H₂(g)

Method. Synthesis gas

Syngas is the mixture of carbon monoxide and hydrogen, CO(g) + H₂(g), made from C + H₂O (water gas) or by steam reforming of methane. Standard result.

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