ScienceCBSE Class 10 · Board exam 2025, set 31/3/1 · 5 marks · Long answer

CBSE Class 10 Science 2025: question 34(a)

Set in the CBSE Class 10 board exam of 2025, paper 31/3/1, 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 answerMetals and Non-metals

(i) Consider the following metals: K, Ca, Al, Cu, Ag, Fe. Select from the above metals, a metal which
I. does not react with oxygen even at high temperature.
II. reacts with oxygen at ordinary temperature and forms a protective oxide layer which prevents the metal from further oxidation.
III. catches fire when kept in the open.
IV. does not burn in oxygen but the hot metal is coated with a black coloured oxide layer.

(ii) What are amphoteric oxides? With the help of balanced chemical equations show that aluminium oxide is an amphoteric oxide.

(iii) What are alkalis? Give one example.

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

Use the reactivity series: least reactive metals do not react with oxygen.

Hint 2

An amphoteric oxide behaves as both acidic and basic oxide – show one reaction of each type.

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) I. Silver (Ag) does not react with oxygen even at high temperature.
½ mark
II. Aluminium (Al) forms a protective oxide layer at ordinary temperature.
½ mark
III. Potassium (K) catches fire when kept in the open, so it is stored in kerosene.
½ mark
IV. Copper (Cu) does not burn but gets coated with black copper(II) oxide when heated.
½ mark
(ii) Amphoteric oxides are metal oxides that react with both acids and bases to form salt and water.
1 mark
With acid: $A l_{2} O_{3} + 6 H C l \to 2 A l C l_{3} + 3 H_{2} O$
½ mark
With base: $A l_{2} O_{3} + 2 N a O H \to 2 N a A l O_{2} + H_{2} O$ (sodium aluminate). Since it reacts with both, $A l_{2} O_{3}$ is amphoteric.
½ mark
(iii) Alkalis are bases that are soluble in water.
½ mark
Example: sodium hydroxide (NaOH) or potassium hydroxide (KOH).
½ mark

Answer: (i) I Ag, II Al, III K, IV Cu; (ii) oxides reacting with both acids and bases; $A l_{2} O_{3} + 6 H C l \to 2 A l C l_{3} + 3 H_{2} O$ , $A l_{2} O_{3} + 2 N a O H \to 2 N a A l O_{2} + H_{2} O$ ; (iii) water-soluble bases, e.g. NaOH

Where the marks are

Ag, Al, K, Cu (½ × 4 marks)
Definition of amphoteric oxides (1 mark)
Equations with HCl and NaOH (½ + ½ mark)
Definition of alkali (½ mark) and one example (½ mark)

Revise the chapter

Chapter: Metals and Non-metals (notes, formulas, where marks are lost and every past question)

Practise this chapterBoard-style questions from Metals and Non-metals, 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 34(a)

Hints

Step-by-step solution

Revise the chapter

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