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Class 10 Science: Carbon and its Compounds, 8 practice questions with answers

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These are 8 practice questions on NCERT Class 10 Science Chapter 4, Carbon and its Compounds, each with a worked answer and the textbook page it comes from. They cover counting covalent bonds, the homologous series, balancing a combustion equation, the addition reaction used on vegetable oils, ethanoic acid, and why soap does not lather well in hard water. Try each one on paper before you read the answer.

These questions are our own, drafted with AI help. Every number and equation was re-checked by a test script, but no practising teacher has reviewed them yet. If you find a mistake, please tell us.

What the chapter says

Facts from the NCERT textbook (Science, Textbook for Class X, Reprint 2026-27, Chapter 4, printed pages 58 to 78):

Bonds and the homologous series

Question 1. Propane has the formula C₃H₈, and all its bonds are single bonds. How many covalent bonds does one propane molecule have?

Answer: 10. The three carbon atoms are joined in a chain by 2 carbon-carbon bonds, and each of the 8 hydrogen atoms is joined to a carbon by one bond. 2 + 8 = 10 covalent bonds. Each carbon then has four bonds, as its valency of four needs. (Building ethane and propane step by step with single bonds, page 63; tetravalency, page 60.)

Question 2. Methane (CH₄) and ethane (C₂H₆) are successive members of a homologous series. By how much do their molecular masses differ? Take C = 12 u and H = 1 u.

Answer: 14 u. Methane: 12 + 4 × 1 = 16 u. Ethane: 2 × 12 + 6 × 1 = 30 u. The difference is 30 − 16 = 14 u, which is the mass of the –CH₂– unit (12 + 2 = 14). The next pair, ethane and propane (44 u), also differs by 14 u. (Homologous series, page 66, where the textbook asks this question for you to work out.)

Question 3. Which of these belongs to the same homologous series as ethene (C₂H₄): C₂H₆, C₃H₆ or C₄H₁₀?

Answer: C₃H₆ (propene). Alkenes follow CₙH₂ₙ. For C₃H₆, n = 3 and 2n = 6, so it fits. C₂H₆ and C₄H₁₀ have 2n + 2 hydrogens (6 and 10): they are alkanes, not alkenes. (General formula of alkenes, page 67.)

Reactions of carbon compounds

Question 4. Balance the equation for the burning of methane: CH₄ + O₂ → CO₂ + H₂O.

Answer: CH₄ + 2O₂ → CO₂ + 2H₂O. Carbon: 1 on each side. Hydrogen: 4 on the left, so 2H₂O on the right. Oxygen: 2 (in CO₂) + 2 (in 2H₂O) = 4 on the right, so 2O₂ on the left. (The textbook gives this reaction unbalanced and asks you to balance it, page 69.)

Question 5. Vegetable oils are hydrogenated: hydrogen is added to them in the presence of nickel. What kind of reaction is this, and what does the nickel do?

Answer: an addition reaction (hydrogenation). Vegetable oils have long unsaturated carbon chains; hydrogen adds across the double bonds and makes them saturated. Nickel is a catalyst: a substance that makes a reaction occur or proceed at a different rate without itself being affected by the reaction. (Addition reaction, page 71.)

Question 6. Pure ethanoic acid is called glacial acetic acid. Its melting point is 290 K. What is that in degrees Celsius, and why does it explain the name?

Answer: about 17 °C. 290 − 273 = 17 °C. Below about 17 °C pure ethanoic acid becomes solid, so in cold winters it often freezes. That is how it got the name glacial acetic acid. (Page 73.)

Question 7. Dilute ethanoic acid is added to sodium hydrogencarbonate. A gas bubbles out. Which gas is it, and how would you test it?

Answer: carbon dioxide. CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂. The test is lime water: carbon dioxide turns it milky, because it forms a white precipitate of calcium carbonate. (Ethanoic acid with hydrogencarbonates, the equation on page 74; the book's Activity 4.9 uses sodium carbonate; the lime-water test, Chapter 2 "Acids, Bases and Salts", page 20.)

Soaps and hard water

Question 8. While bathing with water from a hand-pump, soap forms very little foam and leaves a white scum. Why, and what cleans better in such water?

Answer: The water is hard: it contains calcium and magnesium salts, and soap reacts with them to form an insoluble scum, so more soap is needed. Detergents work better, because their charged ends do not form insoluble precipitates with the calcium and magnesium ions in hard water. (Soaps and detergents, page 76.)

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Sources

All opened and read on 21 October 2026. Page numbers are the printed page numbers of the textbook.

The facts and reactions are from the textbook pages above. The questions, worked answers and checks are our own, and every number and equation was re-checked by script. This guide is general information; follow your teacher and the textbook.