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On heating 'X' at 373 K, it… (Class 10 Science 2023, Q21a)

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ScienceCBSE Class 10 · Board exam 2023, set 31/5/1 · 2 marks · Very short answer

CBSE Class 10 Science 2023: question 21(a)

Set in the CBSE Class 10 board exam of 2023, paper 31/5/1, section B, question 21(a), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q21(a) · 2 marks · Very short answerAcids, Bases and Salts

On heating 'X' at 373 K, it loses water molecules and becomes 'Y'. 'Y' is a substance which doctors use for supporting fractured bones in the right position.

  1. (i)

    Identify 'X' and 'Y'.

    0 marks
  2. (ii)

    How can 'X' be reobtained from 'Y' ?

    0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 31/5/1.

Hints

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

Hint 1

Which white powder is used to make plaster casts for broken bones?

Hint 2

That powder is made by partly removing the water of crystallisation from a calcium salt.

Hint 3

Think about what happens to that powder when it is mixed with water and left to set.

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) 'X' is gypsum, (calcium sulphate dihydrate).

    ½ mark

  2. 'Y' is Plaster of Paris, (calcium sulphate hemihydrate). Doctors use it to hold fractured bones in place. On heating at 373 K:

    ½ mark

  3. (ii) 'X' is obtained again by mixing 'Y' with water. Plaster of Paris takes up water and sets into a hard mass of gypsum:

    1 mark

Answer: (i) X = Gypsum, CaSO₄·2H₂O; Y = Plaster of Paris, CaSO₄·½H₂O; (ii) by adding water to Y: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O

Where the marks are

  • Identifies X as gypsum, CaSO₄·2H₂O (½ mark)
  • Identifies Y as Plaster of Paris, CaSO₄·½H₂O (½ mark)
  • States that adding water to Y gives X, with the equation (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 paper2023 board paper, set 31/5/1: 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.