ScienceCBSE Class 10 · Board exam 2025, set 31/3/1 · 2 marks · Very short answer

CBSE Class 10 Science 2025: question 21(b)

Set in the CBSE Class 10 board exam of 2025, paper 31/3/1, section B, question 21(b), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q21(b) · 2 marks · Very short answerChemical Reactions and Equations

What is observed when hydrated ferrous sulphate crystals are heated in a dry boiling tube? Give balanced chemical equation(s) of the reaction(s) that occur(s).

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/3/1.

Hints

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

Hint 1

The crystals contain water of crystallisation; where does it go on heating?

Hint 2

Ferrous sulphate then breaks into a metal oxide and two oxides of sulphur.

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.

Observation: water droplets collect on the cooler upper part of the tube and the green crystals turn white (anhydrous $F e S O_{4}$ ).
½ mark
On strong heating, the white solid turns reddish-brown ( $F e_{2} O_{3}$ ) and a pungent smell of burning sulphur is given off ( $S O_{2}$ , $S O_{3}$ ).
½ mark
Loss of water of crystallisation: $F e S O_{4} \cdot 7 H_{2} O (s) Heat F e S O_{4} (s) + 7 H_{2} O (g)$
½ mark
Decomposition: $2 F e S O_{4} (s) Heat F e_{2} O_{3} (s) + S O_{2} (g) + S O_{3} (g)$
½ mark

Answer: Water droplets form, green crystals turn white then reddish-brown with a burning-sulphur smell; $F e S O_{4} \cdot 7 H_{2} O \to F e S O_{4} + 7 H_{2} O$ , $2 F e S O_{4} \to F e_{2} O_{3} + S O_{2} + S O_{3}$

Where the marks are

Any two correct observations (½ + ½ mark)
Balanced equation for loss of water of crystallisation (½ mark)
Balanced equation for decomposition of $F e S O_{4}$ (½ mark)

Revise the chapter

Chapter: Chemical Reactions and Equations (notes, formulas, where marks are lost and every past question)

Practise this chapterBoard-style questions from Chemical Reactions and Equations, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 31/3/1: every question, by section.

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CBSE Class 10 Science 2025: question 21(b)

Hints

Step-by-step solution

Revise the chapter

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