Board Previous Year Questions

Board PYQs — Carbon and its Compounds

A curated collection of CBSE Board Previous Year Questions from 2018 to 2024.

Coverage: 1-mark MCQs, 2-mark short, 3-mark medium, 5-mark long.

This chapter typically contributes 8-10 marks to the Class 10 Science board paper.

Topic-wise Question Distribution (Last 5 Years)

Topic	Frequency	Typical Marks
Bonding & Allotropes	High	1-3
Hydrocarbons (saturated/unsaturated)	Very High	2-3
Functional Groups & Homologous Series	Very High	2-3
IUPAC Nomenclature	High	1-2
Combustion & Oxidation	High	2-3
Ethanol & Acetic Acid	Very High	3-5
Soaps & Detergents	High	3-5
Esterification	Medium	2-3

Strategy

1. Master Frequently-Asked Topics:

• Differences (saturated vs unsaturated, soap vs detergent).
• Reactions of ethanol and acetic acid.
• Cleaning action of soap.
• IUPAC naming of simple compounds.

2. Diagram-based questions:

• Practice drawing structural formulae.
• Soap micelle diagram.

3. Equations:

• Memorize key reactions (combustion, esterification, hydrogenation).
• Always balance equations.

Key insight: Acetic acid + ethanol-related questions almost always appear!

PYQ 1: CBSE Board 2024 — 1 mark

Q. Which of the following is the IUPAC name of acetic acid? (a) Methanoic acid (b) Ethanoic acid (c) Propanoic acid (d) Butanoic acid

Answer: (b) Ethanoic acid

Explanation: Acetic acid: CH₃COOH. 2 carbons (eth) + -COOH (oic acid) = ethanoic acid.

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PYQ 2: CBSE Board 2023 — 1 mark

Q. What is the test to distinguish saturated and unsaturated hydrocarbons?

Answer: Bromine water test.

Explanation: Saturated: No reaction with bromine water (remains red-brown). Unsaturated: Decolourises bromine water. Reaction: $CH_2=CH_2 + Br_2 \rightarrow CH_2Br-CH_2Br$ (colourless).

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PYQ 3: CBSE Board 2023 — 2 marks

Q. Define homologous series. Give two examples.

Answer: Homologous series — a series of organic compounds with:

• Same general formula.
• Same functional group.
• Differ by CH₂.
• Similar chemical properties.

Examples: 1. Alkanes: $C_nH_{2n+2}$ — methane, ethane, propane, butane. 2. Alcohols: $C_nH_{2n+1}OH$ — methanol, ethanol, propanol.

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PYQ 4: CBSE Board 2022 — 3 marks

Q. What are isomers? Draw the structural formulae of three isomers of pentane (C₅H₁₂).

Answer: Isomers — compounds with same molecular formula but different structural formulae.

1. n-Pentane: CH₃-CH₂-CH₂-CH₂-CH₃ 2. Iso-pentane: CH₃-CH(CH₃)-CH₂-CH₃ 3. Neo-pentane: C(CH₃)₄

PYQ 5: CBSE Board 2024 — 3 marks

Q. What is esterification? Write the chemical equation for the reaction of ethanol with ethanoic acid. Mention one use of esters.

Answer: Esterification — reaction of carboxylic acid with alcohol to form ester and water.

Equation: $$CH_3COOH + C_2H_5OH \xrightarrow{\text{conc. } H_2SO_4} CH_3COOC_2H_5 + H_2O$$

Conditions: Concentrated sulphuric acid as catalyst, gentle heating.

Use of esters:

• Used as artificial flavouring agents in food (banana, apple flavours).
• Used in perfumes for sweet smell.

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PYQ 6: CBSE Board 2023 — 5 marks

Q. (a) What are detergents? How do they differ from soaps? (b) Why do soaps not work well in hard water? (c) Explain the cleaning action of soap.

Answer:

(a) Detergents: Sodium salts of long-chain sulphonic acids. Have head -SO₃⁻Na⁺. Soaps: Sodium salts of long-chain fatty acids. Have head -COO⁻Na⁺.

(b) Hard water contains Ca²⁺ and Mg²⁺ ions. Soap reacts: $2RCOO^-Na^+ + Ca^{2+} \rightarrow (RCOO)_2Ca\downarrow + 2Na^+$ Forms insoluble scum that wastes soap and stains clothes.

(c) Cleaning action — Micelle formation: Soap molecule has hydrophilic head and hydrophobic tail. 1. Tails attach to oily dirt. 2. Heads remain in water. 3. Micelles form (dirt inside, water outside). *4. Washing with water removes micelles → clean.

PYQ 7: CBSE Board 2022 — 3 marks

Q. (a) Why is carbon tetravalent? (b) Define catenation. (c) Give two reasons why carbon forms a large number of compounds.

Answer:

(a) Carbon tetravalency: Carbon has electronic configuration 2,4 (4 valence electrons). It needs 4 more to complete octet → forms 4 covalent bonds.

(b) Catenation: Property of an element to form bonds with atoms of the same element to form long chains.

(c) Two reasons for large number of compounds: 1. Catenation — strong C-C bonds (348 kJ/mol) → long chains, branches, rings. 2. Tetravalency — bonds with H, O, N, S, halogens → diverse compounds.

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PYQ 8: CBSE Board 2024 — 2 marks

Q. Write the chemical equations for combustion of: (a) Methane (b) Ethanol

Answer:

(a) $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$

(b) $C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O + \text{heat}$

Both burn with blue flame; release heat used as fuel.

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PYQ 9: CBSE Board 2021 — 3 marks

Q. (a) What happens when ethanol is heated with conc. H₂SO₄ at 443 K? Write equation. (b) State the role of conc. H₂SO₄.

Answer:

(a) Dehydration: Ethanol loses water to form ethene. $$C_2H_5OH \xrightarrow[\text{443 K}]{\text{conc. } H_2SO_4} CH_2=CH_2 + H_2O$$

(b) Role of conc. H₂SO₄: Acts as a dehydrating agent — removes water from ethanol.

PYQ 10: CBSE Board 2020 — 3 marks

Q. What is hydrogenation? Mention its industrial application.

Answer: Hydrogenation — addition of hydrogen across multiple bond (C=C or C≡C) in presence of catalyst (Ni, Pt, Pd).

Reaction: $$\text{Vegetable oil (with C=C)} + H_2 \xrightarrow{Ni} \text{Vanaspati (C-C)}$$

Industrial application: Manufacture of vanaspati ghee from vegetable oils. Solid fat — easier to store and transport. Note: Trans fats produced — unhealthy in excess.

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PYQ 11: CBSE Board 2024 — 2 marks

Q. Why does ethanol have higher boiling point than ethane?

Answer: Ethanol (C₂H₅OH) has -OH group → forms hydrogen bonds between molecules. Ethane (C₂H₆) has only weak van der Waals forces. Hydrogen bonds are stronger → higher b.p. of ethanol (78°C) compared to ethane (-89°C).

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PYQ 12: CBSE Board 2023 — 5 marks

Q. (a) Distinguish ethanol from ethanoic acid by 2 chemical tests. (b) Why is ethanoic acid called a weak acid? (c) What are the uses of ethanol?

Answer:

(a) Tests: 1. Litmus paper: Ethanoic acid turns blue → red. Ethanol → no change. 2. With NaHCO₃: Ethanoic acid effervesces (CO₂ released). Ethanol → no change.

(b) Weak acid because: Only partially dissociates in water. pKa = 4.76. $CH_3COOH \rightleftharpoons CH_3COO^- + H^+$ (equilibrium far to left).

(c) Uses of ethanol: 1. Alcoholic beverages. 2. Solvent in cosmetics, drugs. 3. Fuel additive (E10, E20). 4. Disinfectant in sanitisers. 5. Industrial chemical.

PYQ 13: CBSE Board 2022 — 5 marks

Q. (a) Compare structures of diamond and graphite. (b) Why is diamond hard but graphite soft? (c) Why does graphite conduct electricity but diamond does not?

Answer:

(a) Structures: Diamond: 3D tetrahedral network. Each C bonded to 4 others. Graphite: 2D hexagonal layers. Each C bonded to only 3 others.

(b) Hardness: Diamond: Strong 3D covalent bonds in all directions → rigid, hardest. Graphite: Layers held by weak van der Waals forces → layers slide → soft, slippery.

(c) Conductivity: Diamond: All 4 electrons of each C used in bonding → no free electrons → no conduction. Graphite: Each C uses only 3 electrons in bonding → 4th electron free → conducts electricity.

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PYQ 14: CBSE Board 2021 — 3 marks

Q. Give IUPAC names of: (a) HCHO (b) CH₃-CHO (c) CH₃-CO-CH₃ (d) CH₃CH₂Cl (e) CH₃-COOH

Answer:

Compound	IUPAC Name
HCHO	Methanal
CH₃-CHO	Ethanal
CH₃-CO-CH₃	Propanone
CH₃CH₂Cl	Chloroethane
CH₃-COOH	Ethanoic acid

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PYQ 15: CBSE Board 2024 — 3 marks

Q. What is meant by saponification? Write the equation for the reaction.

Answer: Saponification — process of preparing soap by hydrolysis of fats/oils with NaOH.

Equation: $$\text{Fat/Oil} + 3NaOH \rightarrow \text{Soap} + \text{Glycerol}$$

Specifically: $$CH_3COOC_2H_5 + NaOH \rightarrow CH_3COONa + C_2H_5OH$$

By-product glycerol — used in cosmetics.

This is the reverse of esterification.