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Which of the following… (Class 10 Science SQP 2026-27, Q21)

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ScienceCBSE Class 10 · Sample paper, session 2026-27 · 1 mark · MCQ

CBSE Class 10 Science sample paper 2026-27: question 21

From the official CBSE sample question paper for 2026-27, section B, question 21, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q21 · 1 mark · MCQCarbon and its Compounds

Which of the following pair of compounds are isomers?

  1. (A)Pair of structures A (a straight five-carbon chain fully saturated, and a branched chain of four carbons with a side carbon, both fully saturated)
  2. (B)Pair of structures B (a straight six-carbon saturated chain, and a branched saturated chain of five carbons with a side carbon)
  3. (C)Pair of structures C (a straight saturated five-carbon chain, and a straight five-carbon chain with one double bond)
  4. (D)Pair of structures D (a straight saturated five-carbon chain, and a straight saturated five-carbon chain drawn as a second structure)

Figure in the paper: Each option shows two Lewis-type structural formulas of carbon compounds with all H atoms shown. Option A: a straight five-carbon chain beside a branched chain with a carbon branch. Option B: a straight six-carbon chain beside a branched chain. Option C: a straight five-carbon saturated chain beside a five-carbon chain containing a C=C double bond. Option D: a straight five-carbon chain beside another five-carbon straight chain. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

Count the C and H atoms in each structure of a pair.

Hint 2

Isomers need the same formula but a different arrangement of atoms.

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.

  1. Isomers have the same molecular formula but different structures. In pair A, both compounds are : n-pentane and a branched isomer (isopentane).

    ½ mark

  2. Pair B has different carbon counts (6 C and 5+1 = 6 C is possible, but the description shows different formulae). Pair C differs in H atoms ( and ). Pair D shows the same compound drawn twice. So the answer is (A).

    ½ mark

Answer: Option (A)

Where the marks are

  • Recognises that the pair has the same molecular formula but a different structure; chooses (A) (1 mark)

Revise the chapter

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 2026-27: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.