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Identification of… (Class 12 Chemistry SQP 2026-27, Q30)

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ChemistryCBSE Class 12 · Sample paper, session 2026-27 · 4 marks · Case-based

CBSE Class 12 Chemistry sample paper 2026-27: question 30

From the official CBSE sample question paper for 2026-27, section D, question 30, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q30 · 4 marks · Case-basedCompetency-basedBiomolecules

Identification of carbohydrate
Students of class 12 are given a sample of carbohydrate obtained from a plant extract. They conducted the following tests to identify the carbohydrate.
The students find that the given carbohydrate is soluble in water. They take about 1-2 mL of aqueous solution of the carbohydrate in a test tube and add 2 mL of Benedict's reagent. The test tube is heated in water bath for 3-5 minutes. A brick red precipitate is obtained.
The sample of the given carbohydrate on hydrolysis with dilute acid, gives only one type of monosaccharide.

Read the passage and answer the following questions:

  1. (I)

    The sample gives a positive Benedict’s test. We can conclude that the given carbohydrate is a
    A. monosaccharide
    B. reducing sugar
    C. non-reducing sugar
    D. polysaccharide

    1 mark
  2. (II)

    Which of the following is plant based carbohydrate?
    (i) sucrose (ii) lactose (iii) starch (iv) glycogen
    A. (i) and (ii)
    B. (i) and (iii)
    C. (iii) and (iv)
    D. (i) and (iv)

    1 mark
  3. (IIIA)

    Considering all observations reported in the passage the sample is most likely (i) glucose (ii) fructose (iii) sucrose (iv) lactose (v) maltose. Justify your answer.

    2 marks

  4. OR

    (IIIB)

    Ravi reports that the given sample is starch. What could be his justification, based on the observations in the passage? Do you agree with Ravi? Why or why not?

    2 marks

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

Benedict's test detects a free carbonyl group.

Hint 2

Use each observation to rule out options: reducing, hydrolysable, one monosaccharide.

Hint 3

Recall the solubility and reducing nature of starch.

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) A brick-red precipitate with Benedict's reagent shows the sugar has a free aldehyde/ketone group, i.e. it is a reducing sugar. Option B.

    1 mark

  2. (II) Sucrose (cane sugar) and starch are made by plants. Lactose is milk sugar and glycogen is animal starch. Option B.

    1 mark

  3. (IIIA) The sample is maltose. It gives a positive Benedict's test, so it is reducing, which rules out sucrose. It undergoes hydrolysis, so it is not a monosaccharide, which rules out glucose and fructose.

    1 mark

  4. (IIIA) Hydrolysis gives only one type of monosaccharide. Lactose gives glucose + galactose, while maltose gives two α-D-glucose units. Maltose also comes from plants (germinating grain). Hence maltose.

    1 mark

  5. (IIIB) Ravi's justification: the sample is plant-derived and on hydrolysis gives only one monosaccharide (glucose), which is also true of starch.

    1 mark

  6. (IIIB) I do not agree. Starch is insoluble in (cold) water, but the sample dissolved. Starch is also a non-reducing polysaccharide and does not give a brick-red precipitate with Benedict's reagent.

    1 mark

Answer: (I) B; (II) B; (IIIA) Maltose; (IIIB) Ravi is wrong: starch is insoluble and non-reducing.

Where the marks are

  • (I) Option B (1 mark)
  • (II) Option B (1 mark)
  • (IIIA) Excludes sucrose (non-reducing) and monosaccharides (hydrolysable) (1 mark)
  • (IIIA) Single monosaccharide on hydrolysis means maltose, not lactose (1 mark)
  • (IIIB) Justification: plant origin, one monosaccharide (glucose) on hydrolysis (1 mark)
  • (IIIB) Disagrees: starch is insoluble and non-reducing (1 mark)

Revise the chapter

  • Chapter: Biomolecules (notes, formulas, where marks are lost and every past question)
Practise this chapterBoard-style questions from Biomolecules, 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.