ScienceCBSE Class 10 · Sample paper, session 2025-26 · 1 mark · Assertion-reason

CBSE Class 10 Science sample paper 2025-26: question 24

From the official CBSE sample question paper for 2025-26, section B, question 24, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q24 · 1 mark · Assertion-reasonCompetency-basedCarbon and its Compounds

The following question consists of two statements – Assertion (A) and Reason (R). Answer these questions by selecting the appropriate option given below:
A. Both A and R are true, and R is the correct explanation of A.
B. Both A and R are true, and R is not the correct explanation of A.
C. A is true but R is false.
D. A is false but R is true.

Assertion (A): $C_{4} H_{8}$ , $C_{4} H_{6}$ and $C_{4} H_{10}$ are members of the same homologous series.
Reason (R): $C_{4} H_{8}$ , $C_{4} H_{6}$ , $C_{3} H_{4}$ , $C_{3} H_{6}$ , $C_{2} H_{4}$ , $C_{2} H_{2}$ are unsaturated hydrocarbons.

(A)Both A and R are true, and R is the correct explanation of A.
(B)Both A and R are true, and R is not the correct explanation of A.
(C)A is true but R is false.
(D)A is false but R is true.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

Open one hint at a time and try again before the next.

Hint 1

Match each formula to $C_{n} H_{2 n + 2}$ , $C_{n} H_{2 n}$ or $C_{n} H_{2 n - 2}$ .

Hint 2

A homologous series needs one common general formula. Check the Reason separately.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

Assertion: $C_{4} H_{8}$ fits $C_{n} H_{2 n}$ (alkene), $C_{4} H_{6}$ fits $C_{n} H_{2 n - 2}$ (alkyne) and $C_{4} H_{10}$ fits $C_{n} H_{2 n + 2}$ (alkane). Members of one homologous series share the same general formula, so these do not form one series. A is false.
Reason: $C_{4} H_{8}$ , $C_{3} H_{6}$ and $C_{2} H_{4}$ are alkenes, and $C_{4} H_{6}$ , $C_{3} H_{4}$ and $C_{2} H_{2}$ are alkynes. All contain double or triple bonds, so all are unsaturated. R is true.
A is false but R is true, option (D).
1 mark

Answer: Option (D)

Where the marks are

Recognises A false (different general formulae) and R true (all unsaturated), option (D) (1 mark)

Revise the chapter

Chapter: Carbon and its Compounds (notes, formulas, where marks are lost and every past question)
Topic: Homologous Series and Nomenclature

Practise this chapterBoard-style questions from Carbon and its Compounds, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2025-26: every question, by section.

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CBSE Class 10 Science sample paper 2025-26: question 24

Hints

Step-by-step solution

Revise the chapter

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