ScienceCBSE Class 10 · Board exam 2024, set 31/2/1 · 3 marks · Short answer

CBSE Class 10 Science 2024: question 27

Set in the CBSE Class 10 board exam of 2024, paper 31/2/1, section C, question 27, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q27 · 3 marks · Short answerChemical Reactions and Equations

It is observed that Calcium on reaction with water floats on its surface. Explain why it happens. Also write a balanced chemical equation for the reaction that occurs. What happens when the aqueous solution of the product of this reaction reacts with Carbon dioxide gas ? Write a balanced chemical equation for the reaction.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 31/2/1.

Hints

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

Hint 1

Which gas is released when a reactive metal reacts with water, and where do its bubbles go?

Hint 2

The product is lime water. Recall the lime-water test for $C O_{2}$ .

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.

Calcium reacts with water less vigorously than sodium. The bubbles of hydrogen gas formed stick to the surface of the calcium metal. This makes it lighter than water, so it floats.
½ mark
$Ca (s) + 2 H_{2} O (l) \to Ca (OH)_{2} (aq) + H_{2} (g)$
1 mark
When carbon dioxide is passed through the aqueous calcium hydroxide (lime water), the solution turns milky because insoluble calcium carbonate is formed.
½ mark
$Ca (OH)_{2} (aq) + C O_{2} (g) \to CaC O_{3} (s) + H_{2} O (l)$
1 mark

Answer: Hydrogen bubbles stick to calcium, so it floats: $Ca + 2 H_{2} O \to Ca (OH)_{2} + H_{2}$ . With $C O_{2}$ the lime water turns milky: $Ca (OH)_{2} + C O_{2} \to CaC O_{3} + H_{2} O$ .

Where the marks are

Reason: hydrogen bubbles stick to the calcium surface (½ mark)
Balanced equation for calcium with water (1 mark)
States that the solution turns milky (½ mark)
Balanced equation forming $CaC O_{3}$ (1 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 paper2024 board paper, set 31/2/1: every question, by section.

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CBSE Class 10 Science 2024: question 27

Hints

Step-by-step solution

Revise the chapter

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