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(i) "Keerthi thinks… (Class 10 Science SQP 2024-25, Q34A)

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ScienceCBSE Class 10 · Sample paper, session 2024-25 · 5 marks · Long answer

CBSE Class 10 Science sample paper 2024-25: question 34(A)

From the official CBSE sample question paper for 2024-25, section D, question 34(A), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q34(A) · 5 marks · Long answerCarbon and its Compounds

(i) "Keerthi thinks that Substitution reaction occurs in saturated Hydrocarbons, on the contrary Krishi thinks, it occurs in unsaturated Hydrocarbons." Justify with valid reasoning whose thinking is correct.
(ii) "Methane and Propane and their Isomers are used as fuels" Comment. Draw the electron dot structure of the immediate lower homologue of Propane. Give any two characteristics of homologues of a given homologous series.
(iii) A mixture of oxygen and ethyne is burnt for welding. Can you predict why a mixture of ethyne and air is not used?

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

Saturated compounds have only single bonds. Can anything be added across them?

Hint 2

The homologue just below propane has one carbon less.

Hint 3

Compare the amount of oxygen in pure oxygen and in air: what happens to combustion when oxygen is limited?

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. (i) Keerthi is correct. Saturated hydrocarbons have only single bonds, so they react by substitution: an H atom is replaced by another atom such as Cl, e.g. . Unsaturated hydrocarbons have double or triple bonds, so they undergo addition across the multiple bond instead.

    1 mark

  2. (ii) Methane, propane and their isomers burn in oxygen with a clean flame, giving , water and a large amount of heat. This makes them good fuels, e.g. (CNG); propane is a component of LPG.

    1 mark

  3. The immediate lower homologue of propane is **ethane, **. Electron dot structure: the two C atoms share one pair of electrons (C–C), and each C shares one electron pair with each of three H atoms. Written in a line: , with a shared pair (:) between every C and each of its H atoms. That gives 7 shared pairs in all, an octet on each C and a duet on each H.

    1 mark

  4. Two characteristics of a homologous series: (1) successive members differ by a unit (14 u in molecular mass); (2) all members have the same general formula and functional group, and hence similar chemical properties. Physical properties show a gradual change.

    1 mark

  5. (iii) With pure oxygen, ethyne undergoes complete combustion and gives a very hot, clean blue flame suitable for welding. Air contains only about 21% oxygen, so ethyne burns incompletely in air, giving a sooty flame with less heat that is not hot enough for welding.

    1 mark

Answer: (i) Keerthi: substitution in saturated, addition in unsaturated hydrocarbons; (ii) they burn releasing much heat; ethane C₂H₆ dot structure with 7 shared pairs; homologues differ by –CH₂– and have similar chemical properties; (iii) air gives incomplete combustion and a sooty, less hot flame.

Where the marks are

  • Keerthi correct, with reasons (substitution vs addition) (1 mark)
  • Methane/propane burn with release of large heat, so they are fuels (1 mark)
  • Correct electron dot structure of ethane (1 mark)
  • Any two characteristics of a homologous series (1 mark)
  • Ethyne + O₂ gives complete combustion and a hot clean flame; air gives a sooty, incomplete flame (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 2024-25: 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.