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Give reasons : (Class 10 Science 2026, Q27a)

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ScienceCBSE Class 10 · Board exam 2026, set 31/3/1 · 3 marks · Short answer

CBSE Class 10 Science 2026: question 27(a)

Set in the CBSE Class 10 board exam of 2026, paper 31/3/1, section B, question 27(a), for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q27(a) · 3 marks · Short answerAcids, Bases and Salts

Give reasons :

  1. (i)

    For dilution of an acid, acid is added into water and not water into acid.

    1 mark
  2. (ii)

    Dry HCl gas does not turn blue litmus red whereas dilute hydrochloric acid does.

    1 mark
  3. (iii)

    Blue colour of Copper(II) sulphate crystals turns white on heating.

    1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 31/3/1.

Hints

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

Hint 1

Mixing an acid with water gives out a lot of heat. Where would the heat build up in each order of mixing?

Hint 2

Acidic behaviour needs H⁺ ions, and blue vitriol contains water of crystallisation.

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) Dissolving an acid in water is highly exothermic. If acid is added slowly to a large amount of water, the heat is spread through the water, so there is no excessive local heating. If water is added to acid instead, the heat produced locally can make the mixture splash out and cause burns, or crack the glass container.

    1 mark

  2. (ii) Dry HCl gas does not ionise, so it produces no ions and cannot change the colour of litmus. In dilute hydrochloric acid, HCl ionises in water to give (as ) ions, which turn blue litmus red.

    1 mark

  3. (iii) Blue copper sulphate crystals are . On heating they lose their 5 molecules of water of crystallisation and become anhydrous , which is white: .

    1 mark

Answer: (i) Avoids excessive local heating and splashing; (ii) Dry HCl gives no H⁺ ions, but dilute HCl does; (iii) Loss of 5 molecules of water of crystallisation.

Where the marks are

  • Explains that adding acid to water avoids local heating and splashing, since the process is exothermic (1 mark)
  • H⁺ ions are formed only in the presence of water (1 mark)
  • Loss of water of crystallisation (5H₂O) (1 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 paper2026 board paper, set 31/3/1: 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.