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Seawater contains many salts… (Class 10 Science 2025, Q37)

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ScienceCBSE Class 10 · Board exam 2025, set 31/1/1 · 4 marks · Case-based

CBSE Class 10 Science 2025: question 37

Set in the CBSE Class 10 board exam of 2025, paper 31/1/1, section E, question 37, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q37 · 4 marks · Case-basedCompetency-basedAcids, Bases and Salts

Seawater contains many salts dissolved in it. Common salt is separated from these salts. Deposits of solid salt are also found in several parts of the world. These large crystals are often brown due to impurities. This is called rock salt and is mined like coal. The common salt is an important raw material for chemicals of daily use.

  1. (a)

    Write balanced chemical equations to show the products formed during electrolysis of brine.

    1 mark
  2. (b)

    List two uses of any one product obtained during electrolysis of brine.

    1 mark
  3. (c)(i)

    A mild non-corrosive basic salt 'A', used for faster cooking, is strongly heated to produce a compound 'B', that is used for removing permanent hardness of water. Identify A and B and also write the equation for the reaction that occurs when A is heated.

    2 marks

  4. OR

    (c)(ii)

    Define water of crystallisation. Give two examples of salts that have water of crystallisation.

    2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/1/1.

Hints

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

Hint 1

Electrolysis of brine gives one product at each electrode and one in the solution.

Hint 2

The salt used in baking and cooking releases CO₂ when heated.

Hint 3

Think of salts whose formulas have a dot followed by H₂O.

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. (a) Electrolysis of brine (chlor-alkali process): . Chlorine forms at the anode, hydrogen at the cathode, and NaOH near the cathode.

    1 mark

  2. (b) Two uses of sodium hydroxide: (1) making soaps and detergents; (2) paper making / artificial fibres / degreasing metals. (Hydrogen: fuel, making margarine or ammonia. Chlorine: water treatment, PVC, bleaching powder.)

    1 mark

  3. (c)(i) A mild, non-corrosive base used for faster cooking is **A = sodium hydrogencarbonate (baking soda), **.

    ½ mark

  4. On strong heating it gives **B = sodium carbonate, **. Sodium carbonate (washing soda after recrystallisation) removes permanent hardness of water.

    ½ mark

  5. Equation: .

    1 mark

  6. (c)(ii) Water of crystallisation is the fixed number of water molecules chemically present in one formula unit of a salt in its crystals.

    1 mark

  7. Examples: copper sulphate pentahydrate (blue vitriol), ; washing soda, . (Gypsum is also correct.)

    1 mark

Answer: (a) 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂; (b) NaOH: soaps/detergents, paper making; (c)(i) A = NaHCO₃, B = Na₂CO₃; 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂; (c)(ii) fixed number of water molecules in one formula unit of a salt, e.g. CuSO₄·5H₂O, Na₂CO₃·10H₂O

Where the marks are

  • Balanced equation for electrolysis of brine (1 mark)
  • Any two uses of one product (½ + ½ mark)
  • A identified as NaHCO₃ (½ mark)
  • B identified as Na₂CO₃ (½ mark)
  • Equation for heating NaHCO₃ (1 mark)
  • Definition of water of crystallisation (1 mark)
  • Two correct examples of hydrated salts (½ + ½ mark)

Revise the chapter

Practise this chapterBoard-style questions from Acids, Bases and Salts, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 31/1/1: 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.