A few crystals of ferrous sulphate were taken in a dry boiling tube and heated. Tiny water droplets were observed in the tube after some time. (i) From where did these water droplets appear? Explain. (ii) What colour change will be observed during heating? (iii) How many molecules of water are attached per molecule of FeSO_4 crystal? Write the molecular formula of crystalline forms of (I) Copper sulphate, and (II) Sodium carbonate. (iv) State how is Plaster of Paris obtained from gypsum. Write two uses of Plaster of Paris.

(i) From the water of crystallisation lost on heating; (ii) green to white; (iii) seven, CuSO_4·5H_2O, Na_2CO_3·10H_2O; (iv) heating gypsum at 373 K gives CaSO_4·12H_2O; used for plastering fractured bones and making toys/decorative items.

ScienceCBSE Class 10 · Board exam 2024, set 31/5/1 · 5 marks · Long answer

CBSE Class 10 Science 2024: question 34(a)

Set in the CBSE Class 10 board exam of 2024, paper 31/5/1, 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 answerAcids, Bases and Salts

A few crystals of ferrous sulphate were taken in a dry boiling tube and heated. Tiny water droplets were observed in the tube after some time.

(i)
From where did these water droplets appear? Explain.
1 mark
(ii)
What colour change will be observed during heating?
1 mark
(iii)
How many molecules of water are attached per molecule of $FeS O_{4}$ crystal? Write the molecular formula of crystalline forms of (I) Copper sulphate, and (II) Sodium carbonate.
2 marks
(iv)
State how is Plaster of Paris obtained from gypsum. Write two uses of Plaster of Paris.
1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 31/5/1.

Hints

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

Hint 1

The tube was dry, so the water must have been locked inside the crystals themselves.

Hint 2

Recall the formulas $F e S O_{4} \cdot x H_{2} O$ , blue vitriol and washing soda. For PoP, think of what heating at 373 K does to $C a S O_{4} \cdot 2 H_{2} O$ .

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.

(i) The droplets come from the water of crystallisation in ferrous sulphate crystals ( $FeS O_{4} \cdot 7 H_{2} O$ ). On heating, this water is driven off as vapour and condenses on the cooler upper walls of the tube.
1 mark
(ii) The light green crystals turn white (anhydrous $FeS O_{4}$ ) as they lose water. On strong heating they finally form brown $F e_{2} O_{3}$ .
½ mark
(iii) Seven molecules of water are attached per molecule: $FeS O_{4} \cdot 7 H_{2} O$ .
½ mark
(I) Crystalline copper sulphate: $CuS O_{4} \cdot 5 H_{2} O$
½ mark
(II) Crystalline sodium carbonate (washing soda): $N a_{2} C O_{3} \cdot 10 H_{2} O$
½ mark
(iv) Gypsum is heated at 373 K, where it loses part of its water of crystallisation and forms Plaster of Paris:
$CaS O_{4} \cdot 2 H_{2} O 373 K CaS O_{4} \cdot \frac{1}{2} H_{2} O + 1 \frac{1}{2} H_{2} O$
1 mark
Uses of Plaster of Paris: (1) supporting fractured bones in the right position; (2) making toys and materials for decoration, and smoothing surfaces.
1 mark

Answer: (i) From the water of crystallisation lost on heating; (ii) green to white; (iii) seven, $C u S O_{4} \cdot 5 H_{2} O$ , $N a_{2} C O_{3} \cdot 10 H_{2} O$ ; (iv) heating gypsum at 373 K gives $C a S O_{4} \cdot \frac{1}{2} H_{2} O$ ; used for plastering fractured bones and making toys/decorative items.

Where the marks are

Explains that the water comes from the water of crystallisation (1 mark)
States the colour change green to white (½ mark)
States seven water molecules (½ mark)
Writes $C u S O_{4} \cdot 5 H_{2} O$ (½ mark)
Writes $N a_{2} C O_{3} \cdot 10 H_{2} O$ (½ mark)
Describes heating gypsum at 373 K / gives the equation (1 mark)
Gives two uses of Plaster of Paris (½ + ½ marks)

Revise the chapter

Chapter: Acids, Bases and Salts (notes, formulas, where marks are lost and every past question)
Topic: Baking Soda, Washing Soda and Plaster of Paris

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 paper2024 board paper, set 31/5/1: every question, by section.

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CBSE Class 10 Science 2024: question 34(a)

Hints

Step-by-step solution

Revise the chapter

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